Soulajule Reservoir Mercury Occurrence and Bioaccumulation Study

Transcription

FINAL STUDY PLAN ◦ SEPTEMBER 2015
Soulajule Reservoir & Arroyo Sausal Methylmercury
Control Study
P R E P A R E D
F O R
Marin Municipal Water District
220 Nellen Ave.
Corte Madera, CA 94925
P R E P A R E D
B Y
Stillwater Sciences
2855 Telegraph Avenue, Suite 400
Berkeley, CA 94705
Brown and Caldwell
201 North Civic Drive, Suite 115
Walnut Creek, CA 94596
Stillwater Sciences
Final Study Plan
Soulajule and Arroyo Sausal MeHg Control Study
Additional report preparers: Dr. Alex Horne, University of California, Berkeley, and Dr. Marc
Buetel, Washington State University, Pullman, Washington.
Suggested citation:
Stillwater Sciences. 2015. Soulajule Reservoir and Arroyo Sausal Methylmercury Control Study
Final Study Plan. Prepared by Stillwater Sciences, Berkeley, and Brown and Caldwell, Walnut
Creek, California for the Marin Municipal Water District, Corte Madera, California.
Cover photos: Soulajule Reservoir and Arroyo Sausal, 2012. Stillwater Sciences and Marin
Municipal Water District.
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Table of Contents
1 INTRODUCTION .................................................................................................... 1
1.1
Project Goals ..................................................................................................... 3
2 BACKGROUND ...................................................................................................... 4
2.1
2.2
2.3
2.4
2.5
Watershed Context ........................................................................................... 4
Regulatory Context ........................................................................................... 4
Reservoir Conditions ......................................................................................... 6
Methylmercury in Reservoirs.......................................................................... 13
Overview of factors controlling methylmercury production and
bioaccumulation in lakes and reservoirs .................................................. 13
Relevant case studies ................................................................................ 15
Climate Change ............................................................................................... 37
3 POTENTIAL MANAGEMENT METHODS ................................................................ 39
3.1
3.2
3.3
Initial Screening of Methods ........................................................................... 39
Methods Applicable to Soulajule Reservoir .................................................... 40
Reduce sediment mercury loads .............................................................. 41
Manage water chemistry to reduce methylation ..................................... 49
Decrease levels of blue-green algae ......................................................... 51
Manage fisheries to reduce methylmercury in fish tissue ....................... 56
Summary of Conceptual-level Cost Estimates for Management Methods
Applicable to Soulajule Reservoir ................................................................... 63
4 WATER LEVEL FLUCTUATION .............................................................................. 65
5 RANKING OF POTENTIAL MANAGEMENT METHODS AND RECOMMENDED
PILOT STUDIES.................................................................................................... 67
5.1
5.2
5.3
5.4
Ranking Approach ........................................................................................... 67
Ranking Results ............................................................................................... 67
Biomanipulation—sport fish stocking....................................................... 70
Hypolimnetic oxygenation system............................................................ 70
Erosion control for uplands soils in the Eastern Arm ............................... 71
VEM in the Eastern Arm............................................................................ 71
Biomanipulation—prey fish stocking ........................................................ 72
Dredging of reservoir sediments in the Eastern Arm ............................... 72
Capping of reservoir sediments in the Eastern Arm ................................. 72
Water Level Fluctuation .................................................................................. 76
Prioritization of Pilot Studies .......................................................................... 76
6 PILOT STUDIES TO FILL DATA GAPS ..................................................................... 78
6.1
Pilot Study 1—Additional Characterization of Methylmercury in Water
and Biota ......................................................................................................... 78
Objectives.................................................................................................. 78
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6.2
6.3
6.4
6.5
6.6
6.7
Hypotheses ............................................................................................... 78
Sampling design ........................................................................................ 79
Data analysis and reporting approach ...................................................... 83
Implementation schedule ......................................................................... 84
Estimated cost........................................................................................... 84
Pilot Study 2—Assessment of Littoral Zone Extent and Productivity as
Related to Increased Water Level Fluctuation ............................................... 84
Objectives.................................................................................................. 84
Hypotheses ............................................................................................... 85
Approach ................................................................................................... 85
Estimated cost........................................................................................... 86
Pilot Study 3—Assessment of Reservoir Fish Community Composition ........ 87
Objectives.................................................................................................. 87
Hypotheses ............................................................................................... 87
Sampling design ........................................................................................ 87
Estimated cost........................................................................................... 91
Pilot Study 4—Assessment of Reservoir Food Web Structure ....................... 91
Objectives.................................................................................................. 91
Hypotheses ............................................................................................... 92
Approach ................................................................................................... 92
Estimated cost........................................................................................... 94
Pilot Study 5—Evaluation of Reservoir Seasonal Oxygen Demand and
Sediment Response to Hypolimnetic Oxygenation ........................................ 94
Objectives.................................................................................................. 94
Approach ................................................................................................... 94
Data analysis and reporting ...................................................................... 96
Estimated cost........................................................................................... 97
Pilot Study 6—Evaluation of Potential for Mercury Loading from Upland
Soils in the Eastern Arm .................................................................................. 97
Objectives.................................................................................................. 97
Hypotheses ............................................................................................... 98
Sampling design ........................................................................................ 98
Data analysis and reporting approach .................................................... 100
Implementation schedule ....................................................................... 100
Estimated cost......................................................................................... 100
Summary of Pilot Study Schedule and Estimated Costs ............................... 101
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7 REFERENCES ..................................................................................................... 104
Tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
Table 17.
Table 18.
Table 19.
Table 20.
Minimum flow release requirements for Soulajule Reservoir. ....................... 3
Soulajule Reservoir and relevant case studies for primary productivity,
water level, and mercury control issues. ...................................................... 16
17 potential methods for in-lake and watershed water quality
management. ................................................................................................ 39
Applicability of selected in-lake and watershed management methods
for methylmercury control in Soulajule Reservoir. ....................................... 40
Conceptual-level cost estimates for in-lake and watershed management
methods applicable to Soulajule Reservoir. .................................................. 64
Rating criteria and weighting factors for Soulajule Reservoir in-lake and
watershed potential methylmercury control actions. .................................. 68
Ranking summary and key assumptions for Soulajule Reservoir in-lake
and watershed methylmercury control actions. ........................................... 74
Priority pilot studies for Soulajule Reservoir mercury control actions. ........ 77
Sampling sites. ............................................................................................... 79
Number of water, algae, zooplankton, and fish methylmercury samples.... 81
Estimated cost of Pilot Study 1—Additional Characterization of
Methylmercury in Water and Biota. ............................................................. 84
Estimated Cost of Pilot Study 2—Assessment of Littoral Zone Extent and
Productivity as Related to Increased Water Level Fluctuation. .................... 87
Fish sampling methods and reaches. ............................................................ 88
Estimated cost of Pilot Study 3—Assessment of Reservoir Fish
Community Composition............................................................................... 91
Estimated cost of Pilot Study 4—Assessment of Reservoir Food Web
Structure. ....................................................................................................... 94
Estimated Cost of Pilot Study 5—Evaluation of Reservoir Seasonal
Oxygen Demand and Sediment Response to Hypolimnetic Oxygenation. ... 97
Upland soil mercury characterization soil sampling locations. ..................... 99
Estimated Cost of Pilot Study 6—Evaluation of Potential for Mercury
Loading from Upland Soils in the Eastern Arm. .......................................... 101
Approximate schedule for pilot studies. ..................................................... 102
Estimated costs for pilot studies.. ............................................................... 103
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Figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Location of Soulajule Reservoir and Arroyo Sausal, Marin County,
California. ........................................................................................................ 2
Original dam and construction of larger dam at Soulajule Reservoir. ............ 6
Hypsographic curve for Soulajule Reservoir. .................................................. 7
Soulajule Reservoir high water mark along the main portion of the
reservoir east of the dam during November 2009.......................................... 8
Western Arm of Soulajule Reservoir exhibiting low water levels,
November 2009. .............................................................................................. 8
August 29–30, 2012 Soulajule Reservoir in situ water quality profiles
measured near the dam. ................................................................................. 9
Water column methylmercury concentrations in the hypolimnion in
Soulajule Reservoir during April, August, and December 2012. ................... 11
Total mercury in sediments in Soulajule Reservoir, August 29–30, 2012..... 12
Reservoir outlet concentrations of DO from Stevens Creek Reservoir
during test oxygenation in 2013 and 2014. .................................................. 20
Hypolimnion concentrations of methylmercury in Calero Reservoir
compared to volume of reservoir with DO concentration less than
1.0 mg/L ......................................................................................................... 20
Excessive algal productivity in Lake Hodges.................................................. 22
Cyanobacteria bloom in Lake Hodges on July 17, 2013, as detected by
post-processed satellite imagery. ................................................................. 23
Installation of a Speece cone in Camanche Reservoir, California, to
oxygenate hypolimnetic waters and eliminate seasonal H2S production. ... 26
Indian Creek Reservoir showing location of Speece Cone and equipment
building used as part of the hypolimnetic oxygenation system. .................. 28
Daily mean dissolved oxygen concentration measured at three locations
in Indian Creek Reservoir. ............................................................................. 29
Total phosphorus concentrations measured at Indian Creek Reservoir. ..... 29
Chlorophyll-a measured at Indian Creek Reservoir ...................................... 30
Secchi depth measured at Indian Creek Reservoir. ...................................... 30
Trailer mounted “temporary” LOX storage system at Twin Lakes, WA. ....... 33
Relation between oxygen addition and mercury in zooplankton ................. 35
(A) Relationship between mercury concentrations in young-of-the-year
yellow perch and maximum water levels relative to the previous year, for
San Point Lake, Minnesota, from 1991 to 2003. (B) Relationship between
Secchi depths and maximum water levels. (C) Mercury concentrations in
yellow perch.. ................................................................................................ 36
Area of Soulajule Reservoir considered for capping or dredging due to
elevated total mercury concentrations in sediments. .................................. 45
Cell counts and identified algal groups by site in surface water samples in
Soulajule Reservoir, at the reservoir discharge and Arroyo Sausal during
August 2012................................................................................................... 54
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Figure 24. Conceptual placement of VEM system in the shallow Eastern Arm of
Soulajule Reservoir.. ...................................................................................... 55
Figure 25. District reservoir fisheries composition at individual lakes. ......................... 58
Figure 26. Soulajule Reservoir sampling sites for additional characterization of
methylmercury in water and biota. .............................................................. 80
Figure 27. Approximate fish sampling reaches in Soulajule Reservoir. ......................... 88
Figure 28. Conceptual gill net array in the deepest area of the reservoir and along
the shoreline of Soulajule Reservoir ............................................................. 90
Appendices
Appendix A. General Applicability of 17 In-lake and 5 Watershed Management Methods
to Soulajule Reservoir and Arroyo Sausal Methylmercury Control
Appendix B. Ranking Detail for Potential Management Methods
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1
INTRODUCTION
Soulajule Reservoir is a small surface water impoundment located in the western
portion of Marin County, California. Water from Soulajule Reservoir is released
downstream to Arroyo Sausal, which then flows to the confluence of Salmon Creek and
Walker Creek, then to Tomales Bay, and eventually to the Pacific Ocean (Figure 1).
To address legacy mercury (Hg) mining issues in the surrounding watershed of Soulajule
Reservoir, the San Francisco Bay Regional Water Quality Control Board (Regional Board)
issued a 13267 letter on May 18, 2010, which requested a monitoring plan and time
schedule for assessing methylmercury (MeHg) production and bioaccumulation in
Soulajule Reservoir and its downstream waterbody Arroyo Sausal. Marin Municipal
Water District (District) subsequently undertook the development and implementation
of the Soulajule Reservoir Mercury Occurrence and Bioaccumulation Study (Brown and
Caldwell and Stillwater Sciences 2013). In their May 7, 2014 letter to the District, the
Regional Board requested a series of next actions aimed at reducing methylation and
bioaccumulation of mercury in Soulajule Reservoir. “Next Action A” was an addendum
to the 2013 Study Report, which provided clarifications and additional data analysis
requested by the Regional Board with respect to sampling locations, monthly reservoir
storage patterns, and methylmercury mass accumulation estimates (Stillwater Sciences
2014). “Next Action B” involves the development and implementation of a Study Plan to
identify and pilot test management methods for controlling methylmercury production
in Soulajule Reservoir. Accordingly, this Study Plan includes the following:
• Section 1—this introduction and statement of project goals.
• Section 2—background information relevant to methylmercury control in
•
•
•
•
Soulajule Reservoir, including an overview of factors controlling methylmercury
production and bioaccumulation in lakes and reservoirs and a summary of
relevant case studies.
Section 3—overview of 17 potential in-lake and 5 watershed management
methods and discussion of those most applicable to Soulajule Reservoir with
respect to overall reservoir management objectives as well as methylmercury
control.
Section 4—discussion of the potential for more frequent use of Soulajule Reservoir
for drinking water supply, and the potential effects of increased water level
fluctuations on mercury methylation and bioaccumulation.
Section 5—ranking of potential management methods and recommended pilot
studies to fill existing data gaps.
Section 6—pilot studies to fill data gaps identified for each of the potential
management methods, including study objectives, hypotheses, sampling design,
data analysis and reporting, anticipated implementation schedule, and estimated
costs.
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Figure 1. Location of Soulajule Reservoir and Arroyo Sausal, Marin County, California.
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1.1
Project Goals
The primary goal of Study Plan development is to identify a set of effective and fiscally
responsible management methods for controlling methylmercury in Soulajule Reservoir
that would be included and implemented in a long-term reservoir management plan.
The selected management methods must be consistent with water supply management
objectives for Soulajule Reservoir, which currently include the following:
1. Provide storage and supply for municipal drinking water;
2. Achieve flows in downstream Walker Creek (see Table 1);
3. Support appropriate water quality objectives within Soulajule Reservoir and in
downstream Arroyo Sausal.
Table 1. Minimum flow release requirements for Soulajule Reservoir.
Season
Winter
Summer
Normal Water Year
20 cfs
5 cfs
Dry Water Year
10 cfs
2 cfs
Critical Water Year
0.5 cfs
0.5 cfs
Under “Next Action B”, the Regional Board identified four mercury management
objectives for Soulajule Reservoir and the downstream Arroyo Sausal. These objectives
have been considered during development of this Study Plan:
1. Reduce loads of mercury from historical mining waste and mercury-laden
sediment;
2. Manage water chemistry, especially redox conditions, to reduce methylation;
3. Decrease levels of harmful blue-green algae and increase fish-edible green algae;
and,
4. Manage fisheries to decrease methylmercury tissue concentrations.
Combined, the aforementioned water supply management objectives and mercury
management objectives have been used to evaluate potential approaches to addressing
elevated methylmercury in fish tissue in Soulajule Reservoir. Consistent with “Next
Action B”, pilot studies of priority management methods will be undertaken by the
District during 2015−2017 to inform development of the long-term reservoir
management plan.
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2
2.1
BACKGROUND
Watershed Context
The Soulajule Reservoir watershed extends approximately 2 mi2 with grassland/
rangeland and open space as the predominant land uses. Historical land uses were
similar, with the exception of mercury mining in the Walker Creek watershed during the
1960s and early 1970s. The Gambonini Mine, the largest mercury mine in the
watershed, was active from 1964 to 1970. By 1972, all mining had ceased in the
watershed, with remediation and clean-up of the Gambonini Mine site beginning in
1999 (SFBRWQCB 2008a).
Soulajule Reservoir is situated within the Northern California Coastal region. The region
has a Mediterranean climate, characterized by rainfall in the winter and cool, dry
summers with coastal fog.
2.2
Regulatory Context
The Regional Board required the District to perform a mercury study (see also 13267
letter referenced in Section 1), based on findings and implementation actions
established in the Total Maximum Daily Load (TMDL) for Mercury in the Walker Creek
Watershed (SFBRWQCB 2008a). Actively eroding mining waste on the site of the
Gambonini mercury mine, located outside of the reservoir watershed, was the initial
concern that led first to a cleanup action on the mine site beginning in 1999 (see above),
and later to the development and adoption of the mercury TMDL (SFBRWQCB 2008a).
The TMDL established that Soulajule Reservoir was impaired because some fish in the
reservoir exceeded mercury levels for human consumption, and the narrative
bioaccumulation water quality objective was not being met.
Although there were anecdotal references to submerged mine works, a report by the
District located the historic mines along the eastern arm of Soulajule Reservoir (MMWD
2010), well above the water line. This finding suggested the possibility of direct
contamination of Soulajule Reservoir sediments as a result of runoff from the upland
mine sites, resulting in a plan to investigate the potential presence of legacy mercury
contamination in sediments related to historic mining (Brown and Caldwell and
Stillwater Sciences 2012). The Soulajule Reservoir Mercury Occurrence and
Bioaccumulation Study was completed in 2013 (Brown and Caldwell and Stillwater
Sciences 2013). Subsequently, the Regional Board requested a series of next actions
aimed at reducing methylation and bioaccumulation of mercury in Soulajule Reservoir,
including an addendum to the 2013 Study Report (“Next Action A”), which is now
complete (Stillwater Sciences 2014) and the development and implementation of a
Study Plan to identify and pilot test management methods for controlling
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methylmercury production in the reservoir (“Next Action B”), which is the focus of this
project.
The Walker Creek Mercury TMDL establishes the following numeric targets for the
Walker Creek watershed, including Soulajule Reservoir:
• To protect wildlife and rare and endangered species, the average methylmercury
concentration in fish consumed by piscivorous birds shall not exceed 0.05 mg/kg
fish (wet weight), measured in whole fish 5−15 cm in length, nor shall it exceed
0.1 mg/kg (wet weight), measured in whole fish 15−35 cm in length.
• To protect aquatic organisms, water column mercury concentrations shall not
exceed the water quality objective of 2.4 ug/L (one-hour average), measured as
total mercury.
• To protect humans who consume Soulajule Reservoir and Walker Creek fish
(assuming future conditions allow for the consumption of Walker Creek fish),
water column mercury concentrations shall not exceed the California Toxics Rule
(CTR) criterion of 0.050 ug/L (averaged over a 30-day period).
The Regional Board translated the above Walker Creek Mercury TMDL numeric targets
into the following load allocations for Soulajule Reservoir and the Arroyo Sausal
Watershed (SFBRWQCB 2008a):
• 0.04 ng/L dissolved methylmercury (annual average)
• 0.5 mg/kg total mercury in suspended sediment (riverine portions of the
watershed)
The water column load allocation (0.04 ng/L) is based on the USEPA draft national
average bioaccumulation factor (BAF) of 1,300,000 for dissolved methylmercury in lakes
and methylmercury in trophic level (TL) 1 3 fish (USEPA 2001). Dividing the Soulajule
Reservoir target tissue concentration for fish 5−15 cm in length by the average BAF
(0.05 mg/kg divided by 1,300,000) and multiplying by 106 (to convert from milligrams to
nanograms) yields 0.04 ng/L dissolved methylmercury. The suspended sediment load
allocation (0.5 mg/kg) is based on the assumption of a one-to-one relationship between
fish tissue mercury and water column particulate mercury concentrations, combined
with daily TSS measurements in Walker Creek for the period 10/01/03 to 5/31/04
(SFBRWQCB 2008a). Based on this assumption, the load allocation represents a
calculated reduction needed from pre-remediation particulate mercury levels in the
watershed to meet safe mercury levels for wildlife, expressed in terms of particulate
mercury concentration. The Regional Board anticipates revising the methylmercury
1 The common designation of trophic levels (TL) in aquatic food webs is TL1 for primary production or
algae, TL2 for primary consumers such as invertebrates, TL3 for secondary consumers such as prey fish,
and TL4 for piscivorous fish.
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allocations to reflect site-specific conditions in Soulajule Reservoir as additional data are
collected (SFBRWQCB 2008a).
2.3
Reservoir Conditions
Soulajule Reservoir was initially constructed as an earthen dam in 1969 on privately
owned lands to serve a recreational second-home development, Soulajule Ranch. In
1979, after the District acquired the reservoir area, a larger impoundment was created
by the construction of a larger dam (Figure 2).
Original, small
earthen dam
constructed in
1969
Larger dam
constructed in
1979, downstream
of original dam
Figure 2. Original dam and construction of larger dam at Soulajule Reservoir. Source: MMWD.
Currently, Soulajule Reservoir has a (maximum) surface area of 310 ac, a storage
capacity of 10,560 ac-ft, and a maximum depth of approximately 80 ft. The reservoir
serves as an infrequent water supply, providing approximately 13 percent of the
District’s total storage capacity. Regulated flow releases in the summer and winter
(Table 1) result in typical seasonal volume fluctuations between approximately 8,000
and 10,500 ac-ft (roughly 25%), and water level fluctuations of approximately 9 ft
(elevation between 332 ft and 323 ft) (Figure 3). The District is currently considering
more frequent use of Soulajule Reservoir for drinking water supply, which could alter
the future magnitude, frequency, and duration of volume and water level fluctuations
(Section 4).
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0
2,000
4,000
Capacity (ac-ft)
6,000
8,000
10,000
12,000
0
10
Typical seasonal volume fluctuations (~9 ft)
Depth from top (ft)
Spillway Elevation = 332.0'
20
30
40
50
60
70
80
90
Figure 3. Hypsographic curve for Soulajule Reservoir. Data source: MMWD (2013).
Under normal operating conditions, the theoretical hydraulic residence time
(storage/flow) for Soulajule Reservoir is approximately two years; however, the
significant additional flow that occurs over the spillway during most wintertime rain
events results in a substantially reduced actual hydraulic residence time. For example,
during December 2012, when the reservoir was spilling, the estimated hydraulic
residence time was approximately 2 months (MMWD unpub. data).
Soulajule Reservoir shorelines tend to be steep and not well vegetated (Figure 4). The
eastern and western arms are shallow and exhibit relatively broad shorelines; however,
these shorelines tend not to be vegetated other than seasonal grasses which are grazed
by cattle when reservoir levels are low (Figure 5).
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Figure 4. Soulajule Reservoir high water mark along the main portion of the reservoir east of
the dam during November 2009.
Figure 5. Western Arm of Soulajule Reservoir exhibiting low water levels, November 2009.
Soulajule Reservoir is monomictic, typically exhibiting complete water column mixing
throughout the winter months. Although thermal stratification tends to develop during
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late spring/early summer (May to June), the onset of stratification has occurred as early
as the end of February. The shallow side arms of the reservoir become fully mixed
earlier in the season than the deeper portions of the reservoir, which remain fully
stratified July through October (Figure 6). Full turnover usually occurs by mid-November
(Brown & Caldwell and Stillwater Sciences 2010).
0
Water Temperature (°C), Dissolved Oxygen (mg/L), & pH (s.u.)
5
10
15
20
25
0
10
20
Temp. (°C)
Depth (ft)
30
DO (mg/L)
40
pH
50
60
70
80
Figure 6. August 29–30, 2012 Soulajule Reservoir in situ water quality profiles measured near
the dam (Site S-WQ4).
Soulajule Reservoir is also eutrophic, supporting large seasonal algal blooms that
typically occur between March and May, but have also been observed in the fall during
some years. Water clarity during bloom periods is low, with mean Secchi depth
measured at 0.7 ft in April 2012 and 2.2 ft in August 2012 (Brown and Caldwell and
Stillwater Sciences 2013). The seasonal blooms provide readily degradable organic
carbon from decomposing algal cells, which depress DO levels in the hypolimnion and
can result in low oxygen to anoxic conditions near the dam during late summer/early
fall. Conversely, reservoir surface waters can become supersaturated with DO during
bloom periods, particularly in the shallow eastern arm (Brown and Caldwell and
Stillwater Sciences 2013). Seasonal pH profiles during stratification periods are
consistent with high levels of photosynthesis in the surface waters and high levels of
algal and bacterial respiration in the bottom waters (Brown and Caldwell and Stillwater
Sciences 2013). Abundant phosphorus and limited nitrogen in Soulajule Reservoir create
conditions where nitrogen fixing blue-green algae thrive. Despite the abundance of
blue-green algae, levels of cyanotoxins measured in the reservoir have not historically
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been of concern with respect to drinking water standards (Brown and Caldwell and
Stillwater Sciences 2013).
The seasonally low DO and algal source of organic carbon in Soulajule Reservoir provide
ideal conditions for methylating bacteria, including sulfate-reducing and iron-reducing
bacteria associated with mercury methylation (see also Section 3.1). Results from the
recent mercury occurrence and bioaccumulation study undertaken in Soulajule
Reservoir (Brown and Caldwell and Stillwater Sciences 2013) indicated that the highest
methylmercury concentrations (2−3 ng/L) occurred in the hypolimnion, at the reservoir
outlet, and in Arroyo Sausal during August, when the reservoir was stratified and
bottom waters were anoxic (Figure 7). The opposite pattern was observed in the
shallow eastern arm of the reservoir during spring and summer months, when surface
water methylmercury concentrations were relatively higher (0.4−1.2 ng/L) and were
associated with supersaturated DO and high algal cell counts (Brown and Caldwell and
Stillwater Sciences 2013).
Sediment sampling results indicated relatively higher concentrations of total mercury
(390−5,940 ng/g dry weight) proximal to the historical Cycle and Franciscan mine sites
on the eastern arm of the reservoir, with concentrations at or near the reported
watershed background concentration of 200 ng/g (SFBRWQCB 2008a) moving towards
the dam and in the western arm of the reservoir (Figure 8). Sediment results also
indicated generally low methylation efficiency, as evidenced by MeHg:TotHg ratios of
much less than 0.01, both in the shallow eastern arm near the mine sites and in deeper
waters near the dam. The low methylation efficiency of the sediments may be due to a
predominance of insoluble mercury sulfides, low organic carbon availability, low sulfate
availability, and high pH, or a combination of these factors (Brown and Caldwell and
Stillwater Sciences 2013). All fish tissue methylmercury results from 2012 exceeded the
Walker Creek TMDL numeric targets of 0.05 mg/kg (wet weight) for prey fish (5–15 cm
FL) and 0.1 mg/kg (wet weight) for piscivorous fish 15–35 cm FL. As compared with
other mercury-mining impacted reservoirs in the San Francisco Bay Area, Soulajule
Reservoir generally exhibits lower overall methylmercury bioaccumulation factors (e.g.,
106.0-106.3 L/kg for piscivorous fish) and lower fish tissue total mercury concentrations
(e.g., 546-1,080 ng/g wet weight for piscivorous fish) (Brown and Caldwell and Stillwater
Sciences 2013).
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Figure 7. Water column methylmercury concentrations in the hypolimnion in Soulajule
Reservoir during April, August, and December 2012. Note that the water column was
not stratified in December, so the data for this month represent bottom water
concentrations.
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Figure 8. Total mercury (TotHg) in sediments in Soulajule Reservoir, August 29–30, 2012.
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2.4
Methylmercury in Reservoirs
Overview of factors controlling methylmercury production and
bioaccumulation in lakes and reservoirs
Hg biogeochemical cycling is complex, as mercury can exist in many forms in the
environment, including elemental mercury (Hg0), other inorganic mercury compounds,
and organic mercury. Inorganic mercury compounds include a variety of relatively
soluble mercury oxide, sulfate, and chloride complexes, as well as the less soluble
mercuric sulfide, or cinnabar (HgS), the form of mercury that has been historically mined
for use in gold recovery. Organic mercury compounds include mercury bound to carbon.
The organic mercury compound (mono)methylmercury (MeHg), is of particular interest
as it is bioavailable and toxic at elevated concentrations. The following is a brief
overview of factors controlling methylmercury production and bioaccumulation in lakes
and reservoirs.
2.4.1.1
Methylation and demethylation
Many factors affect the formation and degradation of methylmercury in lakes and
reservoirs, such as overall mercury concentrations, redox conditions (i.e., availability of
oxygen, nitrate), labile or readily bioavailable organic carbon, sulfate (SO42-), sulfide (S2-),
temperature, solar radiation, and salinity (Ulrich et al. 2001). Mercury methylation is
primarily mediated by heterotrophic sulfate-reducing bacteria, which are broadly
present in lake and reservoir sediments and convert inorganic mercury to
methylmercury during cellular respiration (Compeau and Bartha 1985, Gilmour et al.
1992, Fleming et al. 2006). Heterotrophic bacteria require an organic carbon source,
such as dissolved organic carbon (DOC), to fuel respiration. Numerous other types of
heterotrophic bacteria are commonly found in lake and reservoir sediments and the
overlying water column, such as aerobic bacteria, denitrifiers, and iron-reducing
bacteria; these microorganisms also require an organic carbon source for respiration,
but they use oxygen (O2), nitrate (NO3-), or iron (Fe3+), respectively, as electron
acceptors during cellular respiration. Sulfate-reducing bacteria are generally suppressed
by the presence of these other electron acceptors.
Anaerobic (low oxygen) sediments of mercury-contaminated lakes have been found to
be particularly active zones of methylmercury production and an important internal
source of methylmercury, especially in lakes experiencing summer anoxia (Regnell et al.
1996). Methylmercury can also be produced directly in the hypolimnion of the water
column if other electron acceptors such as oxygen and nitrate are in short supply (Eckley
and Hintelman 2005, Watras et al. 1995). Although readily available sulfate and labile
organic carbon can result in an increase in mercury methylation, sulfide produced
during cellular respiration can, at high concentrations, form Hg-S complexes that are not
bioavailable, and in the case of mercury sulfide (HgS) are insoluble (Benoit et al. 1999).
Similarly, while DOC can stimulate sulfate-reducing bacteria, at high concentrations it
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can also complex mercury such that it is not as readily available to methylating bacteria
(Gorski et al. 2008).
Demethylation is the degradation of methylmercury, which can occur under microbially
mediated or abiotic conditions. The difference between the rate of mercury methylation
and that of methylmercury degradation represents the rate of net methylmercury
production. Photodemethylation is the abiotic degradation of methylmercury in the
presence of solar radiation (Sellers et al. 1996, Gardfeldt et al. 2001). While the absolute
rates of methylmercury photo-demethylation are uncertain, the process has been
identified as a significant component of the mass balance of methylmercury in the Delta
(Stephenson et al. 2008). Given sufficient water column clarity, photodemethylation
may be an important loss pathway for methylmercury in lakes and reservoirs as well.
2.4.1.2
Bioaccumulation
Methylmercury bioaccumulates, or biomagnifies, in the food web, which can result in
biota concentrations that are orders of magnitude greater than ambient water
concentrations (Weiner et al. 2003). Developing fetuses and young in humans and
wildlife are primarily at risk from bioaccumulation, where the main route of exposure is
consumption of mercury-contaminated fish (Fitzgerald et al. 1998). Many environmental
factors affect mercury bioaccumulation in the food web of lakes and reservoirs,
including physical-chemical properties of habitat that affect the formation of
methylmercury, exposure time to methylmercury (via diet for higher tropic levels),
potential for biodilution, and the growth rate of food web organisms. Phytoplankton
(algae) is the primary entry point for methylmercury into the food web. Methylmercury
concentrations in phytoplankton are typically 100-fold greater than concentrations in
water (Mason et al. 1996; Pickhardt and Fisher 2007; Gorski et al. 2008), while
methylmercury increases at each subsequent trophic level are typically only a factor of
two to five (Wood et al. 2010; Stewart et al. 2008; Cabana et al. 1994; Peterson et al.
2007). As methylmercury exposure and subsequent bioaccumulation is directly related
to food consumption, habitat and feeding niches for aquatic organisms at various lifestages are likely to be important determinants of ultimate methylmercury tissue
concentrations at the higher trophic levels.
Because phytoplankton serve such an important role in concentrating methylmercury in
the aquatic food web, the degree of primary productivity or trophic status of a lake or
reservoir may affect the degree of methylmercury bioaccumulation in higher trophic
levels. Algal bloom dilution is a biological mechanism whereby a constant mass of
methylmercury is distributed throughout a relatively large biomass of algae, resulting in
a generally low concentration of methylmercury per cell and accordingly, low exposure
for consumers. As more productive waterbodies tend to be dominated by larger-sized
algal species, such as diatoms and green algae, the phenomenon of algal bloom dilution
may be amplified by the tendency of more productive water bodies to contain larger
bodied phytoplankton with a lower methylmercury bioaccumulation potential (Foe and
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Louie 2014). Overall, the mechanism of bloom dilution suggests that methylmercury
bioaccumulation in eutrophic lakes and reservoirs would be relatively low. While there
is some evidence to support this hypothesis in California lakes and reservoirs (Foe and
Louie 2014), the applicability of the biodilution hypothesis in eutrophic waterbodies
such as Soulajule Reservoir, which have elevated fish tissue concentrations of
methylmercury, is not well understood.
Another form of biodilution, called somatic growth dilution, occurs when the growth
rate of an organism is higher than the rate at which methylmercury is incorporated into
tissue. At basal metabolism, the amount of food taken in by the organism balances
metabolic requirements and no tissue growth occurs. Any methylmercury assimilated in
food would gradually increase in concentration over time given the constant tissue
mass. Alternatively, as food resources increase above basal metabolic needs, tissue
growth occurs. In this case, any methylmercury assimilated in food would gradually
decrease in concentration over time given the comparatively rapid increase in tissue
mass. Somatic growth dilution has been observed in several lakes in Canada, where fish
growth rates were significantly correlated to mercury concentrations and faster-growing
fish exhibited lower mercury concentrations than slower-growing fish at a given length
(Verta 1990, Gothberg 1983, Simoneau et al. 2005).
Relevant case studies
Case studies pertinent to Soulajule Reservoir, and for which there are existing available
data, include the following (see also Table 2):
• regionally proximal reservoirs with mercury contamination issues and high algal
productivity (Almaden, Calero, and Guadalupe Reservoirs, Lake Almaden, Lake
Hodges);
• productive reservoirs in California being managed for algal impacts, but not
necessarily mercury (Camanche Reservoir, Indian Creek Reservoir);
• lakes in other regions where methylmercury control studies are currently
underway (Onondaga Lake, New York); and
• lakes in other regions undergoing management actions (i.e., oxygen addition,
water level fluctuation) that may affect methylmercury production and
bioaccumulation (Twin Lakes, Washington; various northern hemisphere lakes).
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Table 2. Soulajule Reservoir and relevant case studies for primary productivity, water level, and mercury control issues.
Waterbody Name
Soulajule Reservoir
Region and/or
State
Northern
California
Coast
Almaden, Calero,
Guadalupe Reservoirs,
Lake Almaden
Northern
California
Coast
Lake Hodges
Southern
California
Coast
Camanche Reservoir
Northern
California,
Sierra Nevada
Watershed Area (WA), Surface
Area (SA), Volume (V),
Max Depth 1, Mean Water Level
Fluctuation (WL)
WA = 2 mi2
SA = 125 ha
V =10,560 ac-ft
Max depth = 80 ft
WL= 9 ft
Almaden Reservoir
WA = 12.5 mi2
SA = 25 ha
V = 1,780 ac-ft
Calero Reservoir
WA = 7.1 mi2
SA = 136 ha
V = 10,050 ac-ft
Max depth = approx. 100 ft
Guadalupe Reservoir
WA = 6 mi2
SA = 30 ha
V = 3,723 ac-ft
Lake Almaden
SA = 12.9 ha
Max depth = 42 ft
WA = 303 mi2
SA = 1,234 ac
V = 30,251 ac-ft
Max depth = 117 ft
WA = 619 mi2
SA = 3,100 ha
V = 423,000 ac-ft
Max depth = 100 ft
Mixing
State
Seasonal Primary
Productivity/Trop
hic Status
Mercury Control Strategy?
Monomictic
High/Eutrophic
To be determined
Water column mixing (solar
bees) (2007)
Monomictic
High/Eutrophic
Monomictic
High/Eutrophic
Hypolimnetic oxygenation
system (HOS)—Speece cone
Monomictic
Low/Oligotrophic
system (HOS)—Speece cone
system (HOS)—line diffuser
(2013—current)
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Waterbody Name
Region and/or
State
Indian Creek Reservoir
Northern
California,
Sierra Nevada
Twin Lakes
Onondaga Lake
Finnish Reservoirs (40
reservoirs)
Eastern South Dakota
Lakes (18 natural lakes)
Northern
Washington
New York
Finland
South Dakota
Fluctuation (WL)
WA = 122 mi2
SA = 55 ha
V = 3,160 ac-ft
Max depth = 37 ft
North Twin Lake
WA = 28.2 mi2
SA = 370 ha
V = 29,600 ac-ft
Max depth = 49.9 ft
South Twin Lake
WA = 5.4 mi2
SA = 413 ha
V = 34,200 ac-ft
Max depth = 57.1 ft
WA = 285 mi2
SA = 1,191 ha
V = 106,200 ac-ft
Max depth = 63 ft
SA = 100–41,700 ha
V = 3,080–1,672,500 ac-ft
Max depth = 4.9–22 ft
WL= 1.3–8 ft
SA = 157–7,331 ha
WL= 0–3,580 ha 2
Mixing
State
Seasonal Primary
Productivity/Trop
hic Status
Dimictic
High/Eutrophic
Not applicable
Low/Oligotrophic
Not required, although
hypolimnetic oxygenation
system (HOS)—line diffuser is
being used to control seasonal
anoxia
Dimictic
Moderate/
Mesotrophic
Nitrate addition
−
−
−
Polymictic
Eutrophic—
hypereutrophic
Dimictic
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Waterbody Name
Voyageurs National Park
Lakes (6 reservoirs and 8
lakes)
Sand Point Lake
Region and/or
State
Northeastern
Minnesota
Northeastern
Minnesota
Fluctuation (WL)
Reservoirs:
SA=540–894,00 ha
Depth =25–45 ft
WL= 1.6–7 ft
Lakes:
SA=670–11,600 ha
Depth = 7–20 ft
WL= 0.7–13 ft
SA = 3,580 ha
Max depth = 184 ft
WL= 1.9 ft
Mixing
State
Seasonal Primary
Productivity/Trop
hic Status
−
−
−
−
Oligotrophic
−
Values provided where data are readily available.
Water level fluctuations were presented in change of surface area (Selch et. al 2007)
“−” indicates no information available
1
2
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2.4.2.1
Almaden, Calero, and Guadalupe reservoirs and Lake Almaden
The Santa Clara Valley Water District (SCVWD), located in the South San Francisco Bay
Area, California, manages a number of reservoirs within the historical New Almaden
Mining District for flood control and water capture/groundwater recharge. Some of the
reservoirs have been impacted by past mercury mining activities and currently exhibit
high levels of methylmercury accumulation (sometimes > 30 ng/L) in anoxic bottom
waters during the summer stratification period. Lake Almaden, and Guadalupe,
Almaden and Calero reservoirs are part of the Guadalupe River Watershed Mercury
TMDL includes the following specific mercury criteria for a range of ecosystem
compartments (SFBRWQCB 2008b):
• Total mercury (TotHg) in creek waters—0.2 mg Hg per kg dry suspended sediment,
annual median;
• Methylmercury in reservoir bottom waters—1.5 ng/L seasonal maximum; and
• Methylmercury in fish tissue—< 50 ug/kg wet weight for 5-15 cm fork length and
< 100 ug/kg wet weight for 15-35 cm fork length.
During 2005−2007, SCVWD installed SolarBee® solar-powered vertical mixing devices in
Lake Almaden and in Almaden and Guadalupe reservoirs. While some benefit in water
quality was achieved in Lake Almaden, including an apparent improvement in the
oxidation-reduction potential (ORP) in lake bottom waters, there was little actual
increase in DO levels or substantial drops in water column methylmercury
concentrations. SCVWD then embarked on an ambitious program to design, construct,
and install a hypolimnetic oxygenation system (HOS) at four reservoirs including Stevens
Creek (outside of the Guadalupe River Watershed and not impacted by mining), Calero
(in the Guadalupe River Watershed but not impacted by mining), Almaden, and
Guadalupe reservoirs. The HOS line-diffuser systems each deliver less than 1 ton/day of
oxygen to the sediment-water interface of the two reservoirs. Preliminary data from
2013 and 2014 indicate that oxygen addition is increasing DO levels in bottom waters of
both reservoirs (D. Drury, SCVWD, pers. comm.). Results of test oxygenation in Stevens
Creek Reservoir indicated a rapid decline in DO in early spring during both 2013 and
2014, at a rate typical of productive reservoirs (approximately 0.1 mg/L/d) (Figure 9).
During test oxygenation periods, DO levels in Stevens Creek Reservoir outlet waters
increased, ranging 4−6 mg/L. In Calero Reservoir, test oxygenation decreased peak
hypolimnetic methylmercury levels from 2−4 ng/L to < 0.7 ng/L (Figure 10) (D. Drury,
SCVWD, pers. comm.).
Future testing of the systems in the SCVWD reservoirs will likely involve oxygenation
prior to the onset of low DO (i.e., < 5 mg/L) each spring, as preliminary data from 2013
and 2014 indicate that it is considerably more difficult to increase DO concentrations in
bottom waters once anoxia has become established. Small fish (50−150 mm fork length,
multiple species) sampled in fall 2014 at Calero Reservoir were not lower in
methylmercury than in the three years prior to pilot HOS operation. Monitoring of water
and small fish continues as additional HOS installations become operational.
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Figure 9. Reservoir outlet concentrations of DO from Stevens Creek Reservoir during test
oxygenation in 2013 and 2014. HOS was cycled on and off in May−June 2013 and
June−August 2014. Preliminary data provided by David Drury (SCVWD) and Stephen
McCord (McCord Environmental).
Figure 10. Hypolimnion concentrations of methylmercury in Calero Reservoir (right axis)
compared to volume of reservoir with DO concentration less than 1.0 mg/L (left
axis). The reservoir was oxygenated in 2014. Seasonal maximum criterion for
methylmercury in the hypolimnion (1.5 ng/L) is shown in red. Preliminary data
provided by David Drury (SCVWD).
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2.4.2.2
Lake Hodges
The City of San Diego (City) Public Utilities Department owns and operates Lake Hodges
Reservoir, located about 30 miles north of San Diego, California. The reservoir has a
maximum capacity of 30,251 ac-ft, maximum depth of 117 ft, and encompasses an area
of 499 ha, with 303 mi2 of upstream catchment area (see also Table 2). It is an important
part of the San Diego County Water Authority (SDCWA) Emergency Storage Projects and
is needed to increase the ability to deliver water within San Diego County during
significant water supply shortage (Brown and Caldwell 2014).
The Regional Water Quality Control Board, San Diego Region (RWQCB) 2008 Clean
Water Act Sections 305(b) and 303(d) Integrated Report states that Lake Hodges
Reservoir currently does not meet water quality objectives for the following five
parameters: pH, manganese, turbidity, nitrogen and phosphorous. This assessment
means that current Lake Hodges conditions no longer fully support one or more of its
beneficial uses. In addition, in the Statewide 2010 Integrated Report (Clean Water Act
Section 303(d) List), the State Water Resources Control Board (SWRCB) included
mercury on the list of pollutants causing impairment in Lake Hodges. The mercury listing
is based on findings in the 2009 Surface Water Ambient Monitoring Program report
entitled Contaminants in Fish from the California Lakes and Reservoirs.
Excessive algal productivity in Lake Hodges impairs the reservoir’s usability as a drinking
water source because of taste and odor events, high levels of disinfection by-product
precursors, filter clogging, high turbidity, and contribution to anoxic conditions in the
reservoir’s deeper water. Senescent algae settle out of the water column, providing
labile organic carbon that fuels sulfate reduction in deeper, anoxic waters and supports
methylmercury production (see also Section 2.4.1.1). Thus, managing odor and taste
producing algae is key to restoring and sustaining Lake Hodges’ dominant and
overarching beneficial use as a source of drinking water supply. It also concurrently
reduces the potential for methylmercury formation since it removes a large source of
organic carbon to bottom sediments. Excessive loading of nutrients (nitrogen and
phosphorous) and organic carbon, both from both external catchment sources and
internal nutrient cycling, currently fuel the lake’s high algae productivity (Figure 11 and
Figure 12).
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Figure 11. Excessive algal productivity in Lake Hodges
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Figure 12. Cyanobacteria bloom in Lake Hodges on July 17, 2013, as detected by postprocessed satellite imagery.
Currently, multiple in-lake management methods to manage and control excessive algal
productivity and address the 303(d) listings of water quality impairments have been
identified for Lake Hodges. The alternatives have been combined into an overall plan
with several key components, presented in order of implementation priority:
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• Speece Cone HOS is proposed to add DO to the reservoir’s bottom water, to
•
•
•
•
prevent anaerobic conditions from occurring.
Mid-lake vigorous epilimnetic mixing (VEM) is proposed at the middle section of
the reservoir to mix shallow areas to discourage the growth of potentially toxic
blue green algae.
Upper wetland filtering through constructed wetlands is proposed at the upper
section of the reservoir, which would skim and pump floating algae from the lake
and remove it through the wetland system.
Biomanipulation is being considered to focus on harvesting carp that stir up the
bottom sediments and hence recycle nutrients, as well as netting out small fish
that feed on zooplankton (good organisms that feed on algae).
Algaecides/molluscicides are being considered if Quagga mussels (or other
deleterious organisms) establish themselves in Lake Hodges.
In addition, a comprehensive sediment testing effort and mercury monitoring program
are underway at Lake Hodges. The sediment testing effort will assess sediment oxygen
demand (SOD) and sediment release rates of redox sensitive compounds (i.e., ammonia,
phosphate, iron, manganese, and methylmercury) under oxic versus anoxic conditions
using laboratory bench-scale chambers. Results will help to determine the design
oxygen delivery rate for an HOS and confirm that maintenance of an oxygenated
sediment-water interface will improve water quality by suppressing the sediment
release of redox sensitive compounds. The mercury monitoring program also will
evaluate monthly total mercury and methylmercury at several locations within the
reservoir’s water column at multiple sites. Collection of profile data, rather than discreet
surface, middle and bottom samples, will support the development of a more precise
mercury mass balance for the reservoir. Lastly, the monitoring program will evaluate
monthly total mercury and methylmercury levels in zooplankton, a key link between
water column mercury and fish tissue concentrations.
2.4.2.3
Camanche Reservoir
Camanche Reservoir is a large East Bay Municipal Water District (EBMUD) reservoir
(423,000 ac-ft, max depth approx. 100 ft) located on the east side of the California
Central Valley, approximately 15 mi northeast of Lodi. It is fed by the Mokelumne River,
which originates in the Sierra Mountains. Camanche Reservoir was constructed in 1964,
primarily as an agricultural water supply. It is situated just downstream of Pardee
Reservoir, which supplies drinking water to 1.3 million EBMUD customers. The
combination of the two reservoirs allows EBMUD to better manage water supply. As the
construction of Camanche Dam blocked access to upstream spawning habitat for
Chinook salmon and steelhead, the Mokelumne River Fish Facility was constructed just
below the dam and supplied with cool water from the hypolimnion of Camanche
Reservoir. In the mid-1980s, two large fish kills occurred in the hatchery and were
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ascribed to hydrogen sulfide (H2S) production in the anoxic hypolimnion waters and
sediments of the reservoir.
In 1992, an HOS consisting of a Speece Cone pure oxygen, bubble-free plume system
was installed near Camanche dam to add oxygen directly to the bottom waters and
sediments and eliminate H2S production (Figure 13). Because it was determined that
one of the fish kill incidents was induced by reservoir draw-down in the summer to meet
downstream flow requirements, thereby exacerbating fish exposure to H2S, the HOS
implementation was combined with minimum hypolimnion pool volume and maximum
temperature requirements. During the last 20 years, there have been no reported fish
kills due to poor water quality at the Mokelumne River Fish Facility. Further, in 2014,
during a record drought in California, small hatchery fish from other state facilities were
relocated to the Mokelumne River Fish Facility because of its preferred water quality.
Other reservoir water quality parameters, such as chlorophyll-a and water clarity,
improved following Speece Cone implementation, and seasonal blue-green algae
nuisance blooms were eliminated because nutrients fueling the blooms were no longer
recycled from anoxic reservoir sediments. Although H2S was the prime cause of fish kills,
concentrations of ammonia, trace metals (e.g., copper), and turbidity were high enough
in reservoir bottom waters to cause concern for hatchery and river fish, especially at the
egg stages. These contaminants were also reduced by HOS, including copper, which can
be elevated in the reservoir due to an upstream abandoned copper mine. Hydropower
operations at Camanche Dam, which had been reduced due to the need to first remove
H2S through a spray release valve, were restored following HOS implementation. In
addition, the “temperature-oxygen squeeze” that previously limited reservoir habitat
for trout has expanded to encompass much of the hypolimnion, to the benefit of local
anglers and EBMUD licensees. The added flexibility provided by a minimum oxygenated
and cool hypolimnion allowed reservoir operation at lower water elevations during
drought periods, such that emergency water did not have to be purchased at relatively
high prices.
Downstream water quality in the Mokelumne River has also improved and, over the last
20 years, numbers of spawning wild Chinook salmon have increased almost three fold
and spawning Steelhead have increased from zero to 278/yr. Although some of the
fisheries benefits were due to in-river improvements such as additional spawning gravel,
better hatchery operation, and targeted Delta releases, none would have been possible
without the improved water supply downstream of Camanche Dam provided by HOS.
Unlike other case studies presented herein, no known mercury contamination exists at
Camanche Reservoir; however, this case study is included because it presents an
example of an engineered system designed to reduce hypolimnion anoxia.
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Figure 13. Installation of a Speece cone in Camanche Reservoir, California, to oxygenate
hypolimnetic waters and eliminate seasonal H2S production. Photo Source: East Bay
Municipal Utility District.
2.4.2.4
Indian Creek Reservoir
Owned and operated by South Tahoe Public Utility District (STPUD), Indian Creek
Reservoir (ICR) is a small, relatively shallow high-elevation reservoir located in Alpine
County, California, about 26 miles southeast of Lake Tahoe. The reservoir comprises
approximately 55 ha, with a total volume of approximately 3,160 ac-ft, a mean depth of
14–20 ft and typical maximum depth of about 37 ft. At times the maximum depth has
increased to about 45 ft (see also Table 2). It was originally constructed in 1967 to hold
tertiary treated, high-nutrient wastewater from the STPUD wastewater treatment plant
prior to re-use for irrigation purposes in Alpine County. No longer used for this purpose,
STPUD continues to operate the reservoir as a recreational facility, primarily for trout
fishing. Located in a dry valley on the eastern slopes of the Sierra Nevada Mountains,
the reservoir has almost no natural inflows and would have dried up in a few years, but
since 1989 it has been solely supplied with water diverted through several miles of
channels from Indian Creek or the Carson River.
Despite inflow water quality improvement, ICR is included on the Clean Water Act
Section 303(d) list for eutrophication. Since the 1970s, it has exhibited symptoms of
eutrophication including impairment of aquatic life and recreational uses and violation
of narrative and numerical water quality objectives. Extensive sampling has identified
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phosphorus as the primary nutrient contributing to eutrophication, with internal loading
of phosphorus from bottom sediments as the main source. Unlike most other case
studies presented herein, no known mercury contamination exists at ICR; however this
case study is also included because it presents an example of an engineered system
designed to meet TMDL requirements, especially hypolimnion anoxia.
In July 2002, the Lahontan Regional Water Quality Control Board (LRWQCB) established
numeric targets for five (5) indicator parameters for ICR: total phosphorus, DO Secchi
Depth, chlorophyll-a, and Carlson Trophic State Index (TSI). STPUD studied a variety of
alternatives and determined that HOS was the optimal method to address the beneficial
use impairment by reversing eutrophication in the reservoir and meeting TMDL
objectives (Kennedy Jenks Consultants 2004). Specifically, investigations identified a
submerged down-flow contact oxygenation system (SDCO or Speece Cone) as the best
device for use at ICR (A. Horne Associates 2005). In December 2006, the District was
awarded a Clean Water Act Section 319 (h) Nonpoint Source Implementation Grant for
the purpose of reducing internal loading of phosphorus to ICR and optimizing reservoir
management for protection and enhancement of aquatic life and recreational uses. The
recommended ICR HOS consisted of a Speece Cone located in the deepest portion of the
reservoir, an on-site oxygen generation system, and underground and submerged
utilities connecting the oxygen generator to the Speece Cone (Figure 14) (Brown and
Caldwell 2007). The ICR HOS became operational in 2009. STPUD now operates the HOS
during the late spring and summer to deliver oxygen to ICR for water quality and aquatic
habitat improvement.
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Figure 14. Indian Creek Reservoir showing location of Speece Cone and equipment building
used as part of the hypolimnetic oxygenation system. (STPUD 2014)
The ICR TMDL tracking plan requires collection and evaluation of indicator parameters
to ascertain progress toward achieving ICR numeric targets. The following figures from
the 2013 TMDL Progress Report (STPUD 2014) show the marked improvement in water
quality. With an exception in 2011, daily mean dissolved oxygen concentrations have
remained above the 4.0 mg/L interim target (Figure 15). This change has led to lower
total phosphorus in the reservoir (Figure 16), lower algae concentrations (as measured
by chlorophyll-a) (Figure 17), and greater Secchi depths (Figure 18). Operating the HOS
routinely has allowed STPUD to comply fully with interim ICR TMDL limits.
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Figure 15. Daily mean dissolved oxygen concentration (averaged in water column) measured at
three locations in Indian Creek Reservoir. ICR HOS began operating in 2009. (STPUD
2014)
Figure 16. Total phosphorus concentrations measured at Indian Creek Reservoir. ICR HOS began
operating in 2009. (STPUD 2014)
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Figure 17. Chlorophyll-a measured at Indian Creek Reservoir. ICR HOS began operating in 2009.
(STPUD 2014)
Figure 18. Secchi depth measured at Indian Creek Reservoir. ICR HOS began operating in 2009.
(STPUD 2014)
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2.4.2.5
Onondaga Lake
Onondaga Lake is a large, temperate urban lake located north of Syracuse, New York.
The lake has a surface area of 1,191 ha, a volume of 106,200 ac-ft, and a maximum
depth of 63 ft (see also Table 2). Its surrounding watershed extends 285 mi2 with
predominant land use activities including agriculture, forestry, and urbanization.
Onondaga Lake is dimictic, mixing once from top to bottom in the spring following ice
melt, and again at fall turnover. The lake flushes approximately 4 times per year and as a
result responds relatively rapidly to changes in external loading (Doerr et al. 1994). The
lake is currently mesotrophic, with summer mean chlorophyll-a concentrations less than
10 ug/L and no major summer algal blooms observed since 2007 (Onondaga County
2011, Suryadevara 2014). Despite this, anoxic conditions in the hypolimnion typically
prevail from July through October, during the summertime stratification period
(Matthews et al. 2008). Due to the abundance of gypsum (CaSO4) in the surrounding
watershed, sulfate water column concentrations are relatively high (approximately 96
mg/L) and can account for a high percentage of total annual organic matter
mineralization in the lake (Matthews et al. 2008).
Over the past century, development of the watershed surrounding Onondaga Lake
resulted in contamination from both domestic and industrial sources. The lake’s bottom
sediments and numerous subsites around the lake and its tributaries were added to the
USEPA Superfund National Priorities List in 1994 due to industrial inputs of mercury,
aromatic volatiles, chlorinated benzenes, polycyclic aromatic hydrocarbons, and
polychlorinated biphenyls (Parsons and UFI 2011). An estimated 75,000 kg of mercury
were discharged to the lake during historical shoreline manufacturing operations,
resulting in subsequent contamination of lake water column, sediments, and biota
(Matthews et al. 2013). Effluent from the Metropolitan Syracuse Wastewater Treatment
Plant has accounted for a significant fraction of annual Onondaga Lake inflow over time
(currently at 20% [Matthews et al. 2013]), adding high levels of phosphorus and
nitrogen to the lake. During the mid-2000s, elevated total phosphorus concentrations in
treated wastewater effluent resulted in routine violations of receiving water quality
standards (Effler et al. 2010). Further, internal loading of methylmercury from bottom
sediments has resulted in hypolimnetic concentrations >15 ng/L during periods of
summer anoxia (Bloom and Effler 1990; PTI Environmental Services 1994; Sharpe 2004).
In recent decades, major rehabilitation programs have been underway to address the
impacts of both domestic and legacy industrial wastes in Lake Onondaga, including
source control, dredging, in situ sediment capping, habitat restoration, and monitored
natural recovery (Parsons and UFI 2011). In 2004, year-round nitrification of wastewater
was initiated at the Metropolitan Syracuse Wastewater Treatment Plant, resulting in
decreased ammonia levels and a doubling of water column nitrate concentrations in the
lake at spring turnover (Effler et al. 2010). Subsequent treatment upgrades in 2005
caused major decreases in phosphorus concentrations and transformed the lake from
eutrophic to mesotrophic (Effler et al. 2008). High nitrate concentrations (approximately
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2 mg N/L) and elevated N:P ratios were maintained to discourage blooms of nuisance Nfixing cyanobacteria in the epilimnion. As a consequence, the duration of nitrate
presence in the hypolimnion was increased by nearly two months, and the onset of
sulfate reduction shifted to later in the summer or was completely eliminated. The
enhanced supply of nitrate alleviated anaerobic conditions and exerted strong
regulation on methylmercury concentrations, resulting in > 50% decreases in
hypolimnetic accumulations of methylmercury (Todorova et al. 2009). These findings
prompted investigations into the feasibility of adding nitrate to the hypolimnion to
further reduce transport of methylmercury from profundal sediments.
During 2011–2013, a pilot study was conducted in Onondaga Lake to evaluate the
effectiveness of nitrate application in controlling hypolimnetic methylmercury
concentrations during summer anoxia. A neutrally-buoyant plume of liquid calcium
nitrate solution was added to the hypolimnion during summer stratification from a
motorized barge at three locations. Each location received 18,000 L of the solution
approximately once per week, such that nitrate concentrations were maintained at
greater than 1 mg N/L at the sediment-water interface (Matthews et al. 2013). During
the first year of the pilot study, maximum hypolimnetic concentrations of
methylmercury were reduced by 94% and maximum concentrations of soluble reactive
phosphorus were decreased by 95%. Hypolimnetic concentrations of both
methylmercury and phosphorus were maintained at background levels during the final
two years of the nitrate addition pilot study. The results were attributed to increased
redox potentials and enhanced sorption to iron and manganese oxyhydroxides in
surficial sediment layers. Methylmercury concentrations in the upper waters during fall
turnover were also substantially reduced, resulting in decreased exposure of aquatic
organisms to methylmercury (Matthews et al. 2013). Recent reductions in fish mercury
levels have been measured in Onondaga Lake; however, in general the majority of fish
tissue samples still exceed levels safe for human consumption (Honeywell International,
Inc. 2013).
Based on the pilot study, the New York State Department of Environmental
Conservation (NYSDEC) and USEPA are modifying the proposed remedy for clean-up of
the Onondaga Lake Superfund Site to require full-scale implementation of nitrate
addition in lieu of the original plan to evaluate the effectiveness of oxygenation (NYSDEC
and USEPA 2014). This decision was based upon an assessment of relatively low
recreational and environmental impacts of nitrate addition, as well as low costs.
Dredging activities included in the clean-up plan were completed in November 2014,
removing approximately 2.2 million cubic yards of sediment from Onondaga Lake
(Honeywell International, Inc. 2015). In situ capping of an additional 450 ac of lake
bottom sediments and 50 ac of wetland restoration are scheduled for completion in
2016 (Honeywell International, Inc. 2015).
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2.4.2.6
Twin Lakes
North Twin (SA = 370 ha); maximum depth = 49.9 ft) and South Twin (SA = 413 ha);
maximum depth = 57.1 ft) (see also Table 2) lakes are mesotrophic, dimictic lakes
located on the Confederated Tribes of the Colville Indian Reservation, a 1.4 million-ac
reservation in eastern Washington State that is rich in timber, wildlife, fish, and water
resources. While the lakes have fairly similar bathymetry, North Twin Lake has a larger
watershed (28.2 mi2 versus 5.4 mi2) and a shorter mean hydraulic residence time (2.7 yr
versus 9.4 yr) relative to South Twin Lake. The lakes are especially prized for warm- and
cold-water recreational fishing opportunities. Typical Secchi depths in the lakes are
13.1−19.7 ft. Both North Twin and South Twin lakes exhibit symptoms of summertime
hypolimnetic anoxia including bottom water buildup of redox-sensitive compounds such
as manganese, iron, and sulfide. To improve summertime cold-water habitat for trout,
hypolimnetic oxygen addition to North Twin Lake was initiated in 2009. The oxygenation
system consisted of a 6,000 gallon trailer-based on-shore liquid oxygen storage tank and
evaporator connected to a 2,461 ft fine-bubble line diffuser located near the lake
bottom (Figure 19). The system was typically operated between May and October and
delivered 25−40 cfm of pure oxygen gas to the lake, of which an estimated 80 percent
dissolved into bottom waters.
Figure 19. Trailer mounted “temporary” LOX storage system at Twin Lakes, WA. Black pipe on
bottom left delivers oxygen gas to lake through a line diffuser near the bottom of
the lake.
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Hg in the water column, zooplankton and fish, and related water quality parameters
were assessed in Twin Lakes from 2009 to 2012. Since methylmercury buildup in bottom
waters is commonly associated with hypolimnetic anoxia (Section 2.4.1), hypolimnetic
oxygenation was hypothesized to reduce mercury in bottom waters and biota in North
Twin Lake relative to South Twin Lake. Part of this hypothesis was confirmed, but
unexpectedly another was refuted. Oxygen addition led to significantly higher DO (mean
hypolimnetic DO: 2−8 mg/L versus < 1 mg/L) and lower methylmercury (peak mean
hypolimnetic MeHg: 0.05−0.2 ng/L versus 0.15−0.4 ng/L) in North Twin Lake compared
to South Twin Lake. But mercury levels were significantly higher in zooplankton (TotHg
range: 100−200 versus 50−100 ug/kg dry weight) and trout (spring 2010 stocking cohort
of eastern brook trout mean total Hg: 74.9 versus 49.9 ug/kg wet weight) in North Twin
Lake when compared to South Twin Lake, where the latter appeared to be a naturally
low mercury bioaccumulating system. Very high chlorophyll levels (20-80 ug/L) were
observed in the bottom water of South Twin Lake, so methylmercury in bottom waters
may have been incorporated into a large pool of algae, thereby resulting in low
methylmercury uptake per unit of algae consumed by zooplankton, a phenomenon
known as algal bloom dilution (Section 2.4.1). In addition, in South Twin Lake the
bottom waters were so anoxic (DO < 0.1 mg/L and sulfide odor in very deep waters) that
zooplankton did not migrate into methylmercury-rich bottom waters. The vertical
disconnect between the location of zooplankton and methylmercury in the water
column could have accounted for low mercury in South Twin Lake zooplankton. Finally,
bottom waters in South Twin Lake were high in sulfide approximately 0.2 mg/L), which
has a high binding affinity with methylmercury (Section 2.4.1). This may have repressed
the bioavailability of methylmercury for uptake into algae at the base of the food web.
These results do not necessarily indicate that lake oxygenation enhances methylmercury
bioaccumulation. In North Twin Lake, high DO in 2009 and 2011 co-occurred with lower
levels of methylmercury in bottom waters and less total mercury in zooplankton,
confirming a positive linkage between oxygen addition and lower bioaccumulation
(Figure 20). Another critical facet of this study was the measurement of iron and
manganese in North Twin Lake bottom waters even during oxygenation. Typically,
oxygen addition suppresses iron and manganese accumulation in bottom waters by
locking these metals into sediments in the form of particulate metal oxides. The
presence of iron and manganese in bottom water indicated that the line diffuser
oxygenation system, while maintaining elevated DO in the upper hypolimnion and
improving cold-water trout habitat, was not oxygenating the sediments. If the
oxygenation system had been successful in oxygenating sediments, it is possible that
methylmercury levels would have been lower in North Twin Lake bottom water and
zooplankton than were observed in the study.
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Figure 20. Relation between oxygen addition (x-axis) and mercury in zooplankton (y-axis).
Years with high oxygen addition (2009 and 2011) had lower mercury levels in
zooplankton.
2.4.2.7
Northern hemisphere lakes with fluctuating water levels
Fluctuating water levels have been correlated with increased fish tissue mercury
concentrations in several reservoirs and natural lakes in the northern hemisphere (Selch
et al. 2007, Sorensen et al. 2005, Evers et al. 2007, Verta et al. 1986). For example, Verta
et al. (1986) indicated that water-level fluctuations were a key explanatory variable
related to elevated mercury fish tissue concentrations in a study of 20 dimictic
hydroelectric and flood control reservoirs in Finland. The authors suggested that
fluctuations can increase shoreline erosion, delivering organic carbon and mercury into
the reservoir; this process was particularly important for reservoirs that experienced ice
cover during reservoir drawdown, which appeared to increase erosive forces in steepsided reservoirs.
Increased mercury concentrations in fish tissue were observed in 18 natural lakes in
eastern South Dakota following an extended wet period that significantly increased lake
surface areas. Mercury concentrations in walleye were positively correlated with the
magnitude of surface area expansion, and higher growth rates of walleye were
positively correlated with mercury concentrations in fish tissue (Selch et. al. 2007).
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In a three-year study of 14 northeastern Minnesota lakes, there was a strong positive
correlation between mercury concentrations in yellow perch young-of-the-year (YOY)
and changing water levels (Sorensen et al. 2005). For all 14 lakes, the authors observed
significant positive correlations between mercury in fish and multiple measures of water
level, with the strongest apparent correlation for mean water levels occurring during
summer/early fall. Water color, TOC, and conductivity also correlated positively with
mercury fish tissue concentrations in these lakes. In one lake in particular (San Point
Lake), where 12 years of additional data are available, YOY yellow perch mercury levels
increased as average Secchi depth decreased (r2 = 0.62), and Secchi depth decreased as
maximum water increased (r2 = 0.77) (Figure 21), suggesting a positive link between
reservoir level, algal productivity, and fish tissue mercury concentrations in this lake
(Sorensen et al. 2005).
Figure 21. (A) Relationship between mercury concentrations in young-of-the-year yellow perch
and maximum water levels (March–July) relative to the previous year, for San Point
Lake, Minnesota, from 1991 to 2003. (B) Relationship between Secchi depths and
maximum water levels. (C) Mercury concentrations in yellow perch. Bars represent
standard error ranges. Source: Sorensen et al. (2005).
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Mechanistic explanations for the observed effects of water level fluctuations on fish
tissue mercury concentrations vary depending on study conditions. In general, microbial
and chemical processes supporting methylation appear to be relatively greater in
organic-rich littoral zones as compared with pelagic (open water) zones, and
bioaccumulation from detrital/ bacterial matter consumed by littoral zooplankton is
greater than bioaccumulation from algae consumed by pelagic zooplankton (Tremblay
et al. 1998). Alternating drying and flooding of soils within the littoral zone may further
enhance littoral zone methylation and bioaccumulation, whereby flooding increases
rates of mercury methylation due to decomposition of organic matter and subsequent
stimulation of sulfate reducing bacteria (Munthe et al. 2007).
Accordingly, the ratio of littoral zone area to total reservoir volume may inform the
potential for methylmercury bioaccumulation in lakes/reservoirs experiencing a high
degree of water level fluctuation. Steep-sided reservoirs with organic-poor substrates
may exhibit less efficient methylmercury production, lower ambient methylmercury
concentrations, and less bioaccumulation compared to reservoirs possessing relatively
large, shallow littoral zones with organic rich soils (Evers et al. 2007). Further, less
frequent water level fluctuations within the littoral zone may reduce average mercury
concentrations in fish tissue.
2.5
Climate Change
Climate change could indirectly affect methylmercury cycling in Soulajule Reservoir and
Arroyo Sausal through associated changes to physical, chemical, and biological
processes in the reservoir and creek. Future projections of climate change under the
Fourth Assessment of the International Panel on Climate Change indicate that by late in
this century (2080s) average annual surface temperatures in northern coastal California
will rise from current levels by 2–8oF (1–4.5oC) assuming a range of potential
greenhouse gas emissions scenarios.
The long-term effects of global climate change on the North Coast region are uncertain.
However, the following effects are likely, based on the current state of scientific
knowledge and associated climate change projections:
• Increased average air temperatures. These increases may be somewhat mitigated
by potential increases in coastal fog formation.
• Increased water temperatures. Again, these may be somewhat mitigated by
potential increases in coastal fog formation.
• Increased intensity and frequency of storm events, shifts in inter-annual timing
and magnitude of peak floods, and associated increases in erosion and
sedimentation.
• Increased intensity and frequency of droughts and, potentially, wildfires.
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• Increased frequency and magnitude of coastal storm surges/flooding events due
to sea level rise.
Increases in air and water temperatures may result in earlier seasonal stratification and
a longer overall period of stratification for Soulajule Reservoir. This could extend the
growth season for algae in reservoir surface waters and the period of methylmercury
production in anaerobic/anoxic bottom waters, affecting concentrations that are
released to Arroyo Sausal. Increased intensity and frequency of storm events and
associated increases in erosion could affect the rate of sedimentation in Soulajule
Reservoir from eroding upland soils. If the latter are a source of total mercury to the
reservoir, this could affect overall mercury loading rates. Increased intensity and
frequency of droughts could change the way in which Soulajule Reservoir is managed for
water supply. The District is currently considering more frequent use of Soulajule
Reservoir for drinking water supply. Although speculative at this time, this Study Plan
includes consideration of the potential effects of climate change on mercury
methylation in Soulajule Reservoir as part of developing successful long-term strategies
for methylmercury control and overall reservoir management.
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3
3.1
POTENTIAL MANAGEMENT METHODS
Initial Screening of Methods
While the Regional Board had already identified a number of potential management
approaches in their May 7, 2014 letter (e.g., erosion control plan for targeted areas in
the Eastern Arm, capping of reservoir sediments in the Eastern Arm, hypolimnetic
oxygenation/aeration, fisheries management), development of the Study Plan
commenced with a screening workshop to evaluate 17 potential in-lake management
methods, as well as 5 broader watershed management methods (Table 3), for their
applicability in addressing overall reservoir management objectives as well as
methylmercury control. Initial consideration of a broader list of potential management
methods, including those suggested by the Regional Board, was undertaken to ensure
that the project did not inadvertently overlook possible viable approaches. Workshop
participants included District staff involved in managing and monitoring Soulajule
Reservoir and Arroyo Sausal, the San Francisco Bay Regional Water Quality Control
Board, as well as project team members with expertise in water quality, fisheries,
watershed management, methylmercury biogeochemical cycling, and engineering.
Table 3. 17 potential methods for in-lake and watershed water quality management.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
2
3
4
5
In-lake/Reservoir Method
Dredging
Water level fluctuation
Mixing and/or destratification
Macrophyte harvesting
Wetland filters (fringe)
Algae harvesting
Selective withdrawal of hypolimnion
Dilution/flushing
Sediment sealing/capping (fabrics)
Algaecides/herbicides or molluscicides
Oxygenation/aeration/nitrate addition
Shading/dyes
Sediment sealing (chemical, alum, etc.)
Pathogens/diseases of algae
Grazers (on algae or macrophytes)
Nutrient harvesting from fish/weeds
Biomanipulation
Watershed Method
Land re-sculpture & stream re-routing
Standard erosion control measures (BMPs)
Unit process natural treatment (UPNT)
wetlands
Volatilization in “dry” cultivated wetlands
Volatilization in crops
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Assignment of general applicability and the associated rationale for each of the 17
potential in-lake and 5 watershed management methods discussed at the workshop is
presented in Appendix A. Of the 22 total management methods considered, 7 were
judged to have general applicability or potential applicability for methylmercury control
in Soulajule Reservoir, given its broader water quality challenges, physical
characteristics, and consideration of the four mercury management objectives identified
by the Regional Board (Table 4).
Table 4. Applicability of selected in-lake and watershed management methods for
methylmercury control in Soulajule Reservoir.
Method
Applicability
Mercury management objective 1: Reduce sediment mercury loads
Erosion control
Potentially applicable
Sediment sealing/capping (fabrics)
Dredging
Mercury management objective 2: Manage water chemistry to reduce methylation
Oxygenation/aeration (not including nitrate addition)
Applicable
Mercury management objective 3: Decrease levels of blue-green algae
Mixing and/or destratification—vigorous epilimnetic mixing
Applicable
Mercury management objective 4: Manage fisheries to reduce methylmercury in fish tissue
Biomanipulation—stocking of predator fish un-impacted by mercury
Biomanipulation—stocking prey fish un-impacted by mercury
Water level fluctuation, while occasionally used to control fisheries, aquatic vegetation,
and algal growth, is not currently understood to be a method for controlling mercury
methylation and bioaccumulation in lakes and reservoirs. The District is considering
more frequent use of Soulajule Reservoir for drinking water supply, which could alter
the future magnitude, frequency, and duration of volume and water level fluctuations.
Accordingly, workshop participants recommended that the potential effects of
increased water level fluctuation on methylmercury production and bioaccumulation be
considered as part of the Study Plan. Because it represents a potential change in
reservoir conditions that may affect mercury methylation and bioaccumulation, rather
than a true mercury management approach, water level fluctuation is addressed
separately in Section 4.
Each of the seven applicable (or potentially applicable) management methods is
discussed further below, organized by mercury management objective (i.e., reduce
sediment mercury loads, manage water chemistry, decrease levels of blue-green algae,
manage fisheries).
3.2
Methods Applicable to Soulajule Reservoir
The intent of the below methods is to reduce mercury loads from external (watershed)
and internal (in-lake) sources into Soulajule Reservoir.
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Reduce sediment mercury loads
3.2.1.1
Erosion control for upland soils in the Eastern Arm
Eroding sediments from upland soils may significantly contribute to total mercury
loading to Soulajule Reservoir, particularly in the Eastern Arm, near the historical
Franciscan and Cycle Mines (Figure 8). If erosion of soils containing elevated mercury is
occurring, a soil erosion control plan could be developed to reduce sediment loads to
the reservoir. Specific recommendations arising from an erosion control plan would be
based on data gathered as part of a site reconnaissance and soil testing to assess the
erosion contribution of mercury from anthropogenic (mining/mine tailings) and natural
background sources under current conditions (see Pilot Study 6, Section 6.6).
Implementation of the upland source evaluation could be initiated in 3 to 6 months.
Development of a soil erosion control plan, should it be required, could be accomplished
within 6 to 12 months, with the caveat that implementation of erosion control BMPs
would require landowner agreements; the District does not own the upland areas
surrounding Soulajule Reservoir. Reduction of mercury concentrations in the reservoir
water column and/or fish tissue may not be easily observable in the short term, but the
decreased mercury loading would be effective in the long term.
Goals and capabilities
The goal of sediment erosion control is to reduce mercury source loading from upland
soils and sediments adjacent to Soulajule Reservoir. At the nearby Gambonini Mine,
slope stabilization and native-plant revegetation were successfully employed as part of a
1999 Superfund (CERCLA) cleanup to control surface erosion from a 12-ac mercury
mining waste pile, avoiding the following: offsite disposal, importing of clean soils, and
placement of an impervious cap. Intensive stormwater discharge monitoring conducted
in 2005 and 2010 indicated that site cleanup had reduced mercury loads by an
estimated 92–93% (Kirchner 2011).
Example applications
The focus of BMPs is to prevent sediment containing elevated levels of mercury from
contact with streams, ponds or the reservoirs, where methylmercury can be formed. For
example at the New Almaden Mine in Santa Clara County (the 5th largest mercury mine
in the world), soil erosion control BMPs have been used to stabilize slopes and protect
waterbodies within the Guadalupe River watershed and reduce TMDLs of mercury that
contribute to mercury levels in San Francisco Bay (Stantec 2010). Phase I of the TMDL
corrective action focused on “erosion control at former mine sites, methylmercury
controls at reservoirs, and an assessment of Alamitos Creek” (URS 2011). The Phase I
work included reducing erosion from upland sources of fresh and spent ore, kiln dust
and any soil exceeding 0.2 mg/kg mercury in sediments (URS 2011). Phase I of this effort
also included mapping mercury concentration in sediments, identifying seeps, mapping
slopes, vegetative cover, gullies and other erodible areas. Upland soil erosion control
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focused on stabilizing mercury impacted sediments within 100 feet of a stream, river or
surface water body.
The Phase II long term remedy to stabilizing these areas within the riparian corridor
began with recommended earthwork needed to remove high concentration sources of
mercury (tailings), reducing slope length, enhancing natural terraces, catchments or
other topographic features to reduce stormwater runoff velocity, gully and rill erosion
(URS 2011).
Relevant existing information
The highest concentrations of total mercury in Soulajule Reservoir sediments are
adjacent to the Cycle Mine, in the Eastern Arm of the reservoir (Figure 8). In addition,
the Environmental Impact Report (EIR) for construction of Soulajule Reservoir suggests
that cinnabar mine tailings were present in upland portions of the reservoir watershed
(Madrone Associates 1976). The EIR concluded that the potential environmental impact
on water quality from the mines was insignificant, because they were small, operated
only for a year or two each, and because cinnabar is sparingly soluble unless exposed to
concentrated acidity (less than pH 4). Despite the perceived lack of significance, the EIR
suggested placing any associated tailings into the mines prior to construction of the
reservoir. Inundation or flooding of the mines was not considered as a potential
environmental impact; however, the EIR suggested that mine tailings be placed above
the high water level of Soulajule Reservoir. Although the formation of methylmercury
was generally understood in the 1970s, conditions that could contribute to methylation
of mercury (e.g., mine tailings eroding into the reservoir, stratification of the water
column, elevated organic carbon and anoxia in the upper sediment layer) were not
specifically analyzed in the EIR (Madrone Associates 1976).
Lastly, the Franciscan and Cycle Mines are small mercury mines with a short (1–2 year)
history of operation and no documented use of processing equipment. The Franciscan
Mine is located several hundred feet upland of Soulajule Reservoir, whereas the Cycle
Mine is closer to the reservoir water level and thus may be a potential source of soil
erosion to the reservoir.
Additional information needs
The following additional information is needed to assess the need for an erosion control
plan in the Eastern Arm of Soulajule Reservoir compared with other applicable reservoir
management approaches:
• Evidence of erodible, unvegetated, or disturbed soils with elevated total mercury
concentrations in the vicinity of the Cycle and Franciscan mine sites.
• Surface soil analytical data for total mercury and grain size at and downslope of
the Cycle and Franciscan mine sites and at undisturbed background sites.
Analyzing grain size along with the mercury content of soil samples will provide
information to support the appropriate selection of sediment control measures. For
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example, if mercury is associated with a larger grain size such as gravel, sediment
transport and soil erosion control BMPs will likely be an effective means of reducing
mercury transport from upland sediments. Conversely, if mercury is primarily associated
with smaller grain sizes (e.g., silts, clayey soils), sediment transport BMPs (e.g., fiber
rolls, silt fences) will be less effective and the mercury-laden soils may need to be
capped and stabilized with clean soils and /or vegetation to reduce mercury transport.
Compatibility with reservoir management objectives and downstream beneficial uses
Development (and eventual implementation) of an erosion control plan in the Eastern
Arm of Soulajule Reservoir would not affect reservoir storage, downstream flows, water
temperatures, or dissolved oxygen in the reservoir or Arroyo Sausal and thus would be
compatible with all current water supply objectives.
Permit requirements
No permits would be required for surface soil sample collection at the upland mine site
or the eventual implementation of BMPs. Landowner permission is needed to allow for
reconnaissance and soil sampling at the mine sites and eventually for BMP installation.
Conceptual engineering, infrastructure, and cost considerations
There are no engineering and/or infrastructure requirements identified for site
reconnaissance, surface soil sampling, or development of a soil erosion control plan.
Requirements specific to BMP implementation will be determined as part of plan
development.
The cost of this effort would include an initial evaluation of potential upland mercury
sources (see Pilot Study 6, Section 6.6) and subsequent development of a soil erosion
control plan, if needed, based on the upland investigation. The anticipated cost for this
effort is $32,000 for the initial evaluation. Development of a soil erosion control plan
may cost an additional $50,000–150,000, depending on the area included in the plan
and the complexity of the likely erosion control measures. The cost of developing an
erosion control plan (if necessary) does not include implementation of the erosion
control measures recommended in the plan.
3.2.1.2
Cap reservoir sediments in the Eastern Arm
In-situ capping of mercury impacted sediments involves the placement of an
impermeable liner between the sediments and the overlying water column of a lake or
reservoir. With respect to mercury, this management approach has the potential to
reduce the solubility, transport, methylation and bioaccumulation of mercury in the
aquatic food web of Soulajule Reservoir. Capping is typically accomplished in 6–12
months. While reduction of mercury concentrations in the water column may not be
easily observable in the short term, overall reduced mercury loading to the waterbody
would be effective in the long term.
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The goal of sediment capping is to reduce the methylation of mercury and reduce the
transport of both total mercury and methylmercury into the water column. Limitations
of this technology are tied to accurately delineating the extent of mercury impacted
sediments. Other technical and logistical constraints include installing the cap without
disturbing mercury impacted sediments, maintaining a durable, impermeable cap,
avoiding or managing obstructions (e.g., stumps, etc.) and ability to limit disturbance by
human activity (e.g., boating) or by natural processes (e.g. current, sedimentation)
especially if the reservoir level fluctuates.
In-situ sediment capping is a remediation technique that has been implemented
effectively at approximately 15 EPA Superfund (CERCLA) sites throughout the United
States to control the release of contaminants to the aquatic food web. Successful
sediment capping also has been used, for example, in Emerald Bay and other areas
around Lake Tahoe, California for aquatic weed and invasive species control (about 10
acres total).
Current data suggests that the highest concentrations of total mercury in Soulajule
Reservoir sediments are adjacent to the Cycle Mine, and to a lesser degree, the
Franciscan Mine (Figure 5). The rough extent of elevated concentrations is
approximately 8 ac. These sediments appear to be largely non-mobile (Brown and
Caldwell and Stillwater Sciences 2013).
The reservoir bathymetry in the Eastern Arm is generally low gradient, with typical
water depths ranging 10−50 ft depending on distance from the dam. Reservoir water
levels typically fluctuate by approximately 9 ft on an annual basis (Figure 3), indicating
that sediments containing elevated concentrations of mercury in the Eastern Arm
remain submerged under normal reservoir conditions. Presumably, reservoir storage
would need to drop to roughly 50% capacity or less before these sediments would be
exposed (Figure 3).
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Figure 22. Area of Soulajule Reservoir considered for capping or dredging due to elevated total
mercury concentrations in sediments. Based on existing data, the current extent of
sediments exhibiting 600 to >1,000 ng/g total mercury (orange polygon) is
approximately 8 ac and 300-600 ng/g total mercury (red polygon) is 27 ac, for a
total of approximately 35 ac.
Given the potentially high cost of capping, and the relatively large area of elevated
sediments (Figure 22), additional characterization of the surficial extent of elevated
mercury in the sediments of the Eastern Arm is needed. Mercury speciation/reactivity
may provide additional useful information for better understanding the efficacy of
capping, but total mercury is likely to be the most important factor. Supplemental
sediment sampling is a key component of this management approach to more precisely
define the extent of any proposed cap.
Detailed bathymetry of the Eastern Arm also would be needed for sediment capping,
including information regarding potential obstructions from stumps, abandoned mining
equipment, or other underwater conditions that would affect capping. Based on the
aforementioned information, appropriate capping materials and costs would be
determined.
Lastly, additional information regarding the potential for continuing upland erosion to
deliver elevated mercury to Soulajule Reservoir is needed (see Study 6—Section 6.6). As
a method to address internal loading of mercury to the reservoir, sediment capping
would be largely ineffective if the historical mines and/or other sources are a continuing
source (i.e., external loading source) of elevated mercury to the Eastern Arm.
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Compatibility with reservoir management objectives and downstream aquatic beneficial
uses
In-situ capping of sediments in the Eastern Arm of Soulajule Reservoir would not affect
reservoir storage, downstream flows, water temperatures, or dissolved oxygen in the
reservoir or Arroyo Sausal and thus would be compatible with all current water supply
objectives. In-situ capping may impact the Eastern Arm of the Reservoir temporarily due
to higher turbidity during capping activities.
Permit requirements
We anticipate that in-situ capping would require a California Department of Fish and
Wildlife (CDFW) Lake or Streambed Alteration permit (Fish and Game Code Section
1602) and may also require consultation with the U.S. Army Corps of Engineers (USACE)
under Clean Water Act (CWA) Section 404 and with the State Water Resources Control
Board under CWA Section 401. USACE may consult with the U.S. Fish and Wildlife
Service (USFWS) and National Marine Fisheries Service (NMFS) on permit applications
depending on species that may be impacted; permits may include conditions to avoid,
minimize and/or mitigate impacts to these species. The RWQCB and the Department of
Toxic Substances Control may also require permits for dredging, as related to temporary
water quality impacts and the dewatering and disposal components of the project.
Conceptual engineering of the sediment cap, extent required, and installation methods
are required. This process would including better determining the areas requiring
capping, investigating existing bottom conditions, and coordinating with diving
contractors regarding access requirements and likely methods and cost.
Based on initial conversations with an experienced installation contractor (Underwater
Resources, San Francisco, CA), we estimate that the cost of materials and installation to
be approximately $300,000 to $600,000 per acre required, not including contingency or
engineering/permitting. This cost is based on recent work conducted by the contractor
in Lake Tahoe. Assuming a 40% contingency allowance and 25% for engineering design
and permitting, the total capital cost could be $0.55M–1.1M per acre, which would
result in a total project cost of approximately $4.4M–8.8M for 8 acres of capping.
3.2.1.3
Dredge and dispose of sediments with elevated mercury from the Eastern
Arm
Dredging is the removal of accumulated lake sediments from lake bottoms with the goal
of improving water quality, recreation, navigation or other uses (IEPA and NIPC 1998).
With respect to Soulajule Reservoir, dredging would remove mercury impacted
sediments from the Eastern Arm and transfer them to an above-ground processing
facility for dewatering, testing, transport and off-site disposal. Implementation would
require at least 12 months. Reduction of mercury concentrations in the reservoir would
be easily observable in the short term and permanent as long as other mercury sources
are eliminated.
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Assuming that all other mercury source areas are controlled, the goal of sediment
dredging and disposal is to eliminate mercury methylation by removing the primary
source of mercury to the waterbody. In some cases, dredging can improve water quality
by deepening a lake or other water body enough to create thermal stratification and
limit nutrient movement from the deeper areas to the upper waters. The entire lake
bottom can be dredged or dredging can be conducted in certain zones where it may be
most beneficial (e.g., areas with the thickest layer of contaminated sediments).
There are two primary methods used for lake dredging: mechanical dredging and
hydraulic dredging. Hydraulic dredging is the preferred method for dredging lake
sediments because it does not require drawing down lake levels, is faster than
mechanical dredging, creates less turbidity and can effectively remove loose, watery
sediments. Thus, dredging of sediments in the Eastern Arm of Soulajule Reservoir would
most likely be accomplished by a hydraulic dredge. Hydraulic dredging also requires the
design and construction of a settling basin to which the slurry (sediment/water mix) is
piped and sediments settle out from the water column. In some cases the sediment
slurry can be decanted prior to being transported to the settling basin, which
significantly decreases the amount of land required for the basin (CDM 2011). After
settling (and treatment, in some cases), the water is pumped back into the lake and the
sediments are left in the basin to dry. Ultimately the sediments are utilized or disposed
of in a variety of ways, including agricultural soil augmentation, fill for planned projects
or, if contaminated, hauled to a landfill.
Sediment removal and disposal by dredging is a common remediation technique that
has been implemented effectively at more than 100 EPA Superfund (CERCLA) sites and
other sites. For example, Caltrans removed, dewatered and disposed of sediments in
support of constructing the eastern span of the Bay Bridge.
Current data suggests that the highest concentrations of total mercury in Soulajule
Reservoir sediments are adjacent to the Cycle Mine, and to a lesser degree, the
Franciscan Mine (Figure 8). The rough extent of elevated concentrations is
approximately 8 acres (Figure 22). The reservoir bathymetry in the Eastern Arm is
generally low gradient, with typical water depths ranging 10−50 ft depending on
distance from the dam. Reservoir water levels typically fluctuate by approximately 9 ft
on an annual basis (Figure 3), indicating that sediments containing elevated
concentrations of mercury in the Eastern Arm remain submerged under normal
reservoir conditions. Presumably, reservoir storage would need to drop to roughly 50%
capacity or less before these sediments would be exposed (Figure 3). Additionally,
upland side slopes may be locally steep, which can be an additional challenge for
dredging and for installing silt curtains required to control water column turbidity.
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Similar to in situ capping, dredging design would rely heavily on characterization of total
mercury in impacted sediments. However, while additional information needs for
capping are limited to the horizontal extent of elevated mercury in sediment in the
Eastern Arm, dredging also requires information regarding the vertical extent of
mercury in sediment. Supplemental sediment sampling is a key component of this
alternative to define precisely the extent of the proposed dredging and removal.
Mercury speciation may have some benefit in defining the limits of excavation and
removal, but total mercury is likely to be the most important factor. Ultimately,
dredging would require more complete testing of the sediments (e.g., all metals, organic
contaminants) to determine proper re-use or disposal options.
Lastly, additional information regarding the potential for continuing upland erosion to
deliver elevated mercury to Soulajule Reservoir is needed (see Study 6—Section 6.6).
Dredging would be largely ineffective if the historical mines are a continuing source of
elevated mercury to the Eastern Arm.
uses
Properly executed, dredging, transporting and removing mercury impacted sediments in
the Eastern Arm of Soulajule Reservoir would not adversely affect reservoir storage,
downstream flows, or water temperatures or dissolved oxygen in the reservoir or
Arroyo Sausal and thus would be compatible with all current water supply objectives.
Dredging may impact the Reservoir’s Eastern Arm temporarily with higher turbidity
during dredging activities. Depending on the depth of sediments to be removed from
the Eastern Arm, reservoir storage may be slightly increased as a result of dredging.
Permit requirements
Typically, dredging requires a California Department of Fish and Wildlife (CDFW) Lake or
Streambed Alteration permit (Fish and Game Code Section 1602) and may also require
consultation with the U.S. Army Corps of Engineers (USACE) under Clean Water Act
(CWA) Section 404 and with the State Water Resources Control Board under CWA
Section 401. USACE may consult with the U.S. Fish and Wildlife Service (USFWS) and
National Marine Fisheries Service (NMFS) on permit applications depending on species
that may be impacted; permits may include conditions to avoid, minimize and/or
mitigate impacts to these species. The RWQCB and the Department of Toxic Substances
Control may also require permits for dredging, as related to temporary water quality
impacts and the dewatering and disposal components of the project.
Dredging implementation requires a suite of complex technical and logistical
considerations. The dredge must be safely transported to and deployed within the
Eastern Arm. Turbidity and other required water quality protection measures must be
implemented before, during and after dredging. After dredging the sediment slurry
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must be transported from the dredge to the shore (a distance that may exceed 0.5
miles). A dewatering facility, most likely located near the dam, must be constructed to
contain and separate water from sediments prior to transport. All water must be
contained until tested. Sediments must be dewatered for transport and disposal. Water
extracted during dewatering also likely would require special treatment before
returning it to the reservoir. Impacted sediments must be loaded onto trucks and
transported off-site at an approved landfill.
Cost considerations include the following:
• Site selection for disposal facility or beneficial re-use
• Final design
• Permitting
• Sediment quality characterization (i.e., beyond total mercury)
• Dredging (including mobilization/demobilization, turbidity control)
• Operation and maintenance
• Monitoring at disposal facility or re-use site
Based on existing information, we estimate that the cost of this work to be in the range
of $375,000 per acre for dredging and disposal. Including a 40% contingency and 25%
for design engineering and permitting, the total cost could be approximately $660,000
per acre, or roughly $5.3M assuming 8 acres of dredging required.
Manage water chemistry to reduce methylation
The intent of the below method is to manage water chemistry in Soulajule Reservoir to
prevent anaerobic/anoxic conditions that lead to mercury methylation.
3.2.2.1
Hypolimnetic oxygenation/aeration
A hypolimnetic oxygenation system (HOS) could be used to prevent the occurrence of
anaerobic conditions in Soulajule Reservoir and make the environment unsuitable for
sulfate reducing bacteria that also produce methylmercury. As observed for Lake
Hodges (Section 2.4.2.2) and Indian Creek Reservoir (Section 2.4.2.4), chlorophyll-a,
water clarity, and large seasonal blue-green algae blooms would be expected to
improve following HOS implementation because nutrients fueling the blooms would no
longer be recycled from anoxic reservoir sediments. Water enriched with dissolved
oxygen using pure oxygen gas would be injected into the deepest area of the reservoir
near the dam and Eastern Arm, and possibly the Western Arm. Design and construction
of the HOS and supporting equipment could take approximately 1–2 years. Oxygen
addition would take place yearly from late spring shortly after stratification until after
overturn in late fall/early winter.
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The main goal of a HOS is to oxygenate the hypolimnion and adjacent sediments during
summer months to prevent anaerobic conditions that result in mercury methylation.
The systems are designed and operated to maintain positive dissolved oxygen in the
water column and at sediment/water interface, which will reduce methylation and is
also a benefit to general water quality and fish populations. The HOS would be sized
based on oxygen demands calculated from historical and new dissolved oxygen profiles
and from sediment oxygen demands (SODs) measured through chamber incubation of
reservoir sediment samples.
Although we do not know of any HOS that has been implemented specifically for
mercury methylation control, numerous successful applications of hypolimnetic
oxygenation systems are available for preventing anoxia and improving bottom water
quality (Section 2.4.1).
The District’s ongoing water quality monitoring and sampling and field measurements
carried out in 2012 demonstrated the occurrence of an anaerobic water column and
sediments (Brown and Caldwell and Stillwater Sciences 2013). For example, in August
2012, the hypolimnion was anaerobic and sulfide levels exceeded 0.3 mg/L. In
combination with reservoir-capacity (hypsographic) curves, these data can be used to
estimate the oxygen mass required for HOS design.
Ultimately, a preliminary engineering design effort is required to better define the
project and develop estimated capital and operating costs for an HOS. As a first step,
Pilot Study 5 will inform certain aspects of HOS design, namely those that involve
system size, extent, and in-reservoir location (Section 6.5).
uses
A properly designed HOS could minimize mercury methylation in sediments by
maintaining a positive dissolved oxygen concentration in the hypolimnion. The
installation of an HOS is not expected to significantly impact reservoir storage, although
a minimum storage may be necessary during the summer to properly operate the
system. The HOS would improve dissolved oxygen concentrations in the reservoir and
Arroyo Sausal, but would not affect downstream flows or water temperatures.
Permit requirements
The installation of an HOS would likely require CEQA compliance and potential California
Department of Water Resources Division of Safety of Dams clearance, depending upon
final facilities configuration.
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Conceptual engineering, infrastructure requirements
An HOS is an engineered system, and numerous engineering investigations and cost
estimates will be required as part of preliminary and final design. As a first step, Pilot
Study 5 will inform certain aspects of HOS design, namely those that involve system size,
extent, and in-reservoir location (Section 6.5).
Decrease levels of blue-green algae
Because algae provide organic carbon to fuel methylation and serve to concentrate
methylmercury in the aquatic food web, the degree of primary productivity or trophic
status of a lake or reservoir may affect the degree of methylmercury bioaccumulation in
higher trophic levels. Blue-green algae, along with other bacteria (e.g., actinomycetes)
and several types of green and brown algae, also produce intracellular by-products that
are associated with earthy and musty tastes and odors, including geosmin (trans-1, 10dimethyl-trans-9-decalol) and MIB (2-methylisoborneol). The presence of these
compounds in drinking water is associated with taste and odor complaints at
concentrations as low as 5 to 10 parts per trillion. During intense algae blooms,
relatively high concentrations of these compounds are released into the water.
Minimizing the production of taste and odor compounds often requires control of algae
blooms in the affected waterbody since removal alternatives at the water treatment
plant can be very costly. Further, some blue-green algae, such as Anabaena flos-aquae
and Microcystis aeruginosa can produce toxins that are harmful to fish, mammals and
humans.
In contrast, diatoms and green algae are typically the more desirable species for lake
and reservoir primary productivity because they are a preferred food source for
zooplankton and fish. With respect to mercury control, the effects of reducing bluegreen algae (and, concomitantly, increasing diatoms and green algae) on
bioaccumulation in the food web of lakes and reservoirs are currently untested,
particularly with respect to algal bloom dilution (Section 1.1.1.1).
3.2.3.1
Vigorous epilimnetic mixing
Vigorous epilimnetic mixing (VEM) is a physical management method that focuses on
the surface waters (epilimnion) of a lake or reservoir. VEM is intended to reduce algal
scums and it is typically situated in relatively shallow areas of the reservoir where the
photic zone makes up a significant fraction of the water column and seasonal algal
blooms tend to occur. VEM installation is typically a long-term strategy due to the
capital investment required, but it can be coupled with other long-term management
approaches to improve water quality, such as reduction in external nutrient loading,
such that its use can be curtailed over time.
The goal of VEM is to disrupt the successful growth of blue-green algae by physically
mixing the upper layer of the water column, such that blue-greens cannot out-compete
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diatoms or other algal species (e.g., cryptomonads, green algae) and form large seasonal
surface scums.
In general, blue-green algae are successful in warm stratified lakes and reservoirs
because they can regulate their buoyancy, moving from high to optimum light levels
during the day and migrating into deeper waters at night where nutrients are more
abundant. Buoyancy regulation for blue-green algae, whose largest colonies are the size
of a small pea, cannot be effective when the water is mixing vertically faster than they
can rise or sink, at a rough maximum of 0.1 cm/s. In contrast, diatoms, which grow
faster and are heavier than blue-green algae, thrive in waters that are mixing sufficiently
to keep them suspended in the photic zone. During late spring/early summer, as the
water column of most temperate lakes and reservoirs becomes thermally stratified, the
heavier diatoms sink out and blue-green algae can prosper.
Artificial mixing of surface waters using VEM shifts the balance away from blue-green
algae dominance. VEM is accomplished by introducing a column of bubbles at depth via
an air diffuser placed approximately 1 ft above the bottom sediments. The rising airbubbles cause water in the bottom waters to rise, pulling water upwards and toward
the surface and creating a mixing cell around the bubble column. Although described as
such, this type of mixing is only “vigorous” from the viewpoint of the algae; upward
water column velocities of 0.1 cm/s are barely noticeable by a recreational lake user.
While VEM is suitable to reduce nuisance scums of blue-green algae (e.g.,
Aphanizomenon, Anabaena) it may not reduce algae overall or even the percent of nonscumming (single filament) blue-greens (e.g., Cylindrospermopsis).
Cherry Creek Reservoir is a 13,960 ac-ft flood-control reservoir near Denver, Colorado,
that is owned and operated by the US Army Corps of Engineers. The reservoir has
experienced seasonal nuisance algal blooms and was not meeting the chlorophyll-a
water quality standard (15-ug/l) from 1996 through 2005. In 2008, a destratification
system was installed in the deepest portion of the reservoir near the dam to strongly
mix the water column and oxidize the deep bottom sediments to reduce the release of
nutrients from the sediments into the water column (Ruzzo 2008). Although not
referred to as “VEM”, a primary objective of the destratification system was to vertically
mix algae in surface waters to reduce production of blue-green algae, which is
consistent with the VEM approach.
Since installation of the destratification system, Cherry Creek Reservoir has exhibited a
shift in the algal species composition whereby blue-green algae have decreased from
41−76% of algal density (2006−2008) to 1−8% of algal density (2009−2012) (Leonard
Rice Engineers, Inc. et al. 2012). Cryptomonads, diatoms, and green algae have become
the numerically dominant algal types, accounting for the largest chlorophyll-a levels
observed in the reservoir. However, as a consequence of the efficient mixing, the
relatively constant supply of soluble nutrients to the algal community allows all types of
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algae to maximize their productivity. As a result, the Cherry Creek Reservoir exhibited
extremely high chlorophyll-a levels (19−30 ug/L) during June to September 2012
(Leonard Rice Engineers, Inc. et al. 2012).
Algae collected from Soulajule Reservoir surface waters during August 2013 included 32
species of algae in the following 7 groups:
• Bacillariophyta (diatoms)
• Chlorophyta (green algae)
• Cryptophyta
• Cyanobacteria (blue-green algae)
• Euglenophyta
• Haptophyta
• Pyrrhophyta (dinoflagellates)
Several miscellaneous species of algae were also identified (Brown and Caldwell and
Stillwater Sciences 2013, Appendix C).
The highest cell counts were observed in the Eastern Arm at site S-WQ1 and S-WQ2
(Figure 23) corresponding to dense blooms of algae at these locations. At all sites, bluegreen algae were the dominant algal group in the sample accounting for anywhere from
approximately 75 to 99% of total cell numbers (Figure 23). At all reservoir sites the most
abundant species was the filamentous blue-green algae Aphanizomenon flos-aquae,
while at the reservoir discharge and in Arroyo Sausal, the most abundant species was
the filamentous blue-green algae Pseudanabaena sp..
The following additional information is needed to rank and prioritize VEM
implementation in Soulajule Reservoir compared with other applicable reservoir
management approaches:
• Frequency of dense algal blooms in the shallow Eastern Arm of the reservoir as
compared with other locations in the reservoir.
Use of VEM to reduce blue-green algae dominance in the Eastern Arm of Soulajule
Reservoir would not affect reservoir storage or downstream flows and thus would be
compatible with current water supply objectives 1 and 2 (Section 1.1). Seasonal VEM
application in the Eastern Arm would likely destratify the entire water column, mixing
cooler bottom waters with warmer surface waters and eliminating any potential
thermal refugia for fish in this location. However, existing data (2012) suggest that this
potential refugia is likely to be a short-lived condition in the late spring/early summer,
where the thermocline progressively descends all the way to the bottom sediments by
mid-summer and the relatively shallow water column becomes fully mixed (Brown and
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Caldwell and Stillwater Sciences 2013). Further, the extent of this affect would be
limited to the Eastern Arm and would not affect water temperatures or dissolved
oxygen elsewhere in the reservoir or in downstream Arroyo Sausal. Therefore, VEM
application would be compatible with current water supply objective 3 (Section 1.1).
Figure 23. Cell counts and identified algal groups by site in surface water samples in Soulajule
Reservoir (S-WQ1 through S-WQ4, and S-WQ6), at the reservoir discharge (S-WQ5)
and Arroyo Sausal (AC-WQ1) during August 2012. Note the logarithmic scale on the
y-axis. Percent of cell count as blue-green algae for each sample is shown in
parentheses. Source: Brown and Caldwell and Stillwater Sciences (2013).
Permit requirements
While agency permits would not be required to install VEM within the reservoir
footprint, landowner agreements may be needed to allow periodic access to shorelines
along the Eastern Arm for equipment installation, staging and long-term operation and
maintenance of the VEM system.
Installation of a VEM system in the shallow Eastern Arm would require the following:
• Multiple 8-inch diameter diffusers placed along the channel thalweg, ranging
approximately 1,000–2,500 linear feet (LF), at a total combined length of 8,500 ft
(see Figure 24)
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• Assuming horizontal extent of aeration plume is a maximum of 15 times the
vertical water depth:
- At water depths < 25 ft and reservoir channel width 350 ft, two parallel
diffusers required
- At water depths > 25 ft and reservoir channel width 450 ft, one diffuser
required
• Onshore low-pressure air compressors with control valves for diffusers at different
depths
• Power source
• Vehicle access for equipment installation, staging and long-term operation and
maintenance
Based on equipment and installation costs for a proposed VEM system of approximately
the same size at Lake Hodges, San Diego, California, VEM implementation in the Eastern
Arm of Soulajule Reservoir would range $500,000 to $1,500,000.
30-50 ft water depth
350 ft wide channel
~1,500 LF
~2,200 LF
350 ft wide channel
~1,300 LF
450 ft wide channel
Figure 24. Conceptual placement of VEM system in the shallow Eastern Arm of Soulajule
Reservoir. Two diffuser lines are required in the channels < 25 ft deep, while one
line is required in the channel > 25 ft deep.
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Manage fisheries to reduce methylmercury in fish tissue
Existing data describing sediment total mercury and fish tissue mercury in reservoirs
throughout California illustrate the limited potential for improving conditions based on
mercury source reduction alone and point to the need for in situ methylation and
bioaccumulation controls (SWRCB 2013). Along these lines, the California Statewide
Mercury Control Program for Reservoirs includes food web
management/biomanipulation among the set of potential control options, with a focus
on changing fish growth rates through increasing productivity, intensive fishing, and/or
stocking (SWRCB 2013). As applicable to Soulajule Reservoir, the stocking approach is
discussed further below with respect to both direct and indirect controls on
bioaccumulation.
3.2.4.1
Stocking of sport fish un-impacted by mercury
Stocking of sport fish is a biomanipulation technique that typically aims to increase the
population of piscivorous fish in a waterbody in order to affect desired change in the
food web. In the case of reservoir mercury management, the waterbody would be
stocked with sport fish that do not contain elevated levels of mercury. As an isolated
management approach, stocking activities would need to occur on a long-term, seasonal
basis to maintain the desired effect, since methylmercury tissue concentrations in
stocked fish presumably would increase over time as a result of consuming mercury in
their prey. However, stocking can be coupled with other long-term management
approaches to reduce methylmercury production and bioaccumulation in the food web,
such that its use can be curtailed over time.
With respect to reservoir mercury management, the goal of sport fish stocking is to
dilute the average body burden of mercury in the broader sport fish population by
adding mercury-free individuals. While mercury levels in individual sport fish that have
consistently resided in Soulajule Reservoir would remain unchanged, this would be a
direct effect on average mercury concentrations in the sport fish population, where the
TMDL numeric target for average methylmercury concentration in fish consumed by
piscivorous birds is ≤ 0.05 mg/kg fish (wet weight), measured in whole fish 5−15 cm in
length, and ≤ 0.1 mg/kg (wet weight), measured in whole fish 15−35 cm in length
(Section 2.2).
As a classic top-down biomanipulation activity, sport fish stocking for mercury control
would also be expected to affect populations of organisms occupying lower trophic
levels, according to the ecological phenomenon of trophic cascade. For example, the
addition or removal of predators could result in reciprocal changes in abundance,
biomass, or productivity of organisms across multiple links in the food web (Cooke et al.
2005). Assuming that predation is the primary population control mechanism, a simple
model of trophic cascading effects in Soulajule Reservoir would indicate that increases
in sport fish abundance would increase predation of smaller fishes and insects, reducing
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their populations accordingly. The resulting decrease in planktivory from smaller fishes
and insects would then increase zooplankton populations and increase grazing pressure
on algal species, leading to an overall decrease in algal populations.
While the aforementioned trophic level effects would not be directly related to mercury
fish tissue concentrations, stocking of sport fish may indirectly affect mercury tissue
concentrations, in both sport fish and prey fish. This is because the increase in
zooplankton populations coupled with a decrease in competition for food resources
among prey fish may allow for increased growth rates within the prey fish populations
themselves. This could result in somatic growth dilution of mercury tissue
concentrations in prey fishes, where the latter occurs due to biomass growth rates that
are higher than the rate of methylmercury uptake (Section 1.1.1.1). The degree to which
somatic growth dilution would occur depends upon the magnitude of growth rate
changes in prey fish, as a response to increases in the sport fish population, as well as
the rate of methylmercury uptake.
Further, the reduction in algal populations, resulting from piscivorous sport fish
stocking, may indirectly reduce mercury methylation rates in Soulajule Reservoir.
Heterotrophic sulfate-reducing bacteria responsible for hypolimnetic mercury
methylation require an organic carbon source to fuel respiration (Section 2.4.1.1). Algae
that die and settle to the bottom of the reservoir provide a source of organic carbon,
such that reduced algal populations may reduce mercury methylation during
stratification periods. Ultimately, this potential effect depends on whether methylation
in Soulajule Reservoir is carbon limited, and the path of mercury bioaccumulation from
zooplankton to predator fish.
Our general review of the literature regarding control of mercury methylation and
bioaccumulation in lakes and reservoirs did not identify management case studies
related specifically to sport fish stocking as a means to reduce fish tissue mercury
concentrations. However, a number of field and laboratory studies support the
occurrence of somatic growth dilution through measured decreases in mercury
concentrations with increasing tissue growth (Foe and Louie 2014). Classic top-down
biomanipulation has also been used in multiple temperate lakes to reduce algal
populations through the addition of piscivorous fish (Cooke et al. 2005), and by
inference, could reduce average fish tissue mercury concentrations in piscivorous fish
through mass biodilution as well as in prey fish through somatic growth dilution.
However, the food web effects on mercury concentrations in biota due to top-down
biomanipulation are currently untested.
Algal species collected in Soulajule Reservoir in 2012 are described in Section 3.2.3.1.
Small prey fish (5–15 cm FL) collected during both August and December 2012 were
exclusively juvenile largemouth bass. Larger prey fish (15–35 cm FL) included black
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crappie and bluegill. Although fish surveys have not been conducted in Soulajule
Reservoir, the species observed during the 2012 surveys are similar to those observed at
several other District reservoirs (Figure 25).
Figure 25. District reservoir fisheries composition at individual lakes. Data from 2006.
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The following additional information is needed to rank and prioritize stocking of sport
fish in Soulajule Reservoir compared with other applicable reservoir management
approaches:
• community survey for relative abundance of sport fish
• required stocking rate (i.e., number of fish un-impacted by mercury) for meeting
TMDL numeric targets (0.1 mg/kg wet weight)
With respect to second bullet above, assuming a simple ratio between the existing
Soulajule average fish tissue methylmercury concentration of 0.54 mg/kg (wet weight)
for piscivorous fish (15–35 cm FL) and the Walker Creek TMDL numeric target of 0.1
mg/kg (wet weight), the sport fish population would need to be increased by roughly 5
times. An estimate of the current abundance of fish in Soulajule Reservoir would help to
determine whether this represents an unreasonably large number of fish to be stocked.
However, because determining the actual population size of fishes within a reservoir
requires an intensive sampling effort, the recommended approach is the following:
1. identify the relative abundance of fishes within the reservoir via community
survey
2. identify the target fish for stocking
3. adaptively manage stocking levels over time, based on mercury concentration in
fish tissue during subsequent surveys
Based on existing information, the target fish for stocking is likely to be largemouth
bass; however, this assumption should be tested through additional investigation (see
Section 6.3 − Pilot Study 3). Further, the determination of potential and indirect
cascading trophic effects of increased sport fish population on mercury cycling and
methylation requires additional information:
• current composition and distribution of prey fish
• mercury bioaccumulation pathway from zooplankton to piscivorous fishes
• stomach content assessment of fishes captured during survey
Stocking of sport fish to manage fish tissue mercury concentrations in Soulajule
Reservoir would not affect reservoir storage or downstream flows and thus would be
compatible with current water supply objectives 1 and 2 (Section 1.1). Designated
downstream beneficial uses include cold freshwater habitat, preservation of rare and
endangered species, fish spawning habitat, and wildlife habitat. While stocking would
not affect water temperature or dissolved oxygen in the reservoir or in downstream
Arroyo Sausal (water supply objective 3, Section 1.1), introduced sport fish species could
be washed downstream and could impact habitat beneficial uses by competing with or
preying upon native aquatic species, including central California coast Coho salmon
(Oncorhynchus kisutch) and steelhead trout (Oncorhynchus mykiss).
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Additionally, while potential stocking species such as largemouth bass can disturb
bottom sediments during spring spawning periods, typically their activity is limited to
shallow, littoral sediments. Given the relatively small area of disturbance, they are
unlikely to significantly increase methylmercury release from reservoir sediments.
Permit requirements
The California Fish and Game Code authorizes CDFW to issue permits for the stocking of
fish in private and public waters throughout the state. Under CDFW’s regulations, a
stocking permit must be issued once the department has determined that the proposed
stocking would be consistent with the department’s fisheries management plans and
would avoid the introduction of diseased or parasitized fish into California waters. The
permit must provide the following information:
• a list of species currently in the reservoir;
• species, number, and size of fish to be stocked; and,
• identification of the registered aquaculturist from whom the fish will be obtained.
We assume that stocking practices would be consistent with CDFW stocking program
guidelines, policies, and requirements (e.g., http://www.dfg.ca.gov/fish/Hatcheries/
EIR/).
Conceptual infrastructure and cost considerations
Stocking of sport fish would require a local or regional source of fish un-impacted by
mercury (e.g., hatchery) and sufficient vehicle access during early spring through late fall
to allow for placement of fish directly into the reservoir from a stocking truck.
Based on the price of $10 per fish for 25–30.5 cm (10–12 in) fork length (FL) largemouth
bass (Professional Aquaculture Services [PAS], the annual stocking cost for providing
2,000–2,500 sport fish would be approximately $20,000−$25,000 (not including
transportation). The appropriate number of fish for stocking would be determined
based on the results of a fish community composition study (see Pilot Study 3—Section
6.3).
3.2.4.2
Stocking of prey fish un-impacted by mercury
Stocking of prey fish is another common biomanipulation technique that aims to
increase the population of planktivorous fish in a waterbody in order to affect desired
change in the food web. In the case of reservoir mercury management, the waterbody
would be stocked with prey fish that do not contain elevated levels of mercury. As an
isolated management approach, stocking activities would need to occur on a long-term,
seasonal basis to maintain the desired effect. However, as with the stocking of sport
fish, this approach can be coupled with other long-term management approaches to
reduce methylmercury production and bioaccumulation in the food web, such that its
use can be curtailed over time.
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As with the stocking of sport fish (Section 3.2.4.1), the goal of this biomanipulation
technique with respect to reservoir management is to dilute the average body burden of
the total prey fish population by directly adding mercury-free individuals. While mercury
levels in individual prey fish that have consistently resided in Soulajule Reservoir would
remain unchanged, this would be a direct effect on average mercury concentrations in
the prey fish population , where the TMDL numeric target for average methylmercury
concentration in fish consumed by piscivorous birds is ≤ 0.05 mg/kg fish (wet weight),
measured in whole fish 5−15 cm in length, and ≤ 0.1 mg/kg (wet weight), measured in
whole fish 15−35 cm in length (Section 2.2).
For prey fish stocking, trophic cascade effects would be expected to occur somewhat
differently than with sport fish stocking. Assuming that predation is the primary
population control mechanism in the waterbody of interest, a simple model would
presume that increases in prey fish would increase predation of zooplankton and thus
decrease their populations accordingly. The resulting decrease in grazing pressure on
algal species would lead to increased algal populations.
While the aforementioned trophic level effects would not be directly related to mercury
fish tissue concentrations, stocking of prey fish may indirectly affect mercury tissue
concentrations, in both sport fish and prey fish. An increase is prey fish and algal
populations may increase growth rates of both sport fish and zooplankton. Somatic
growth dilution of methylmercury may occur within these populations if biomass is
gained at a higher rate relative to methylmercury uptake (see also Section 3.2.4.1).
However, stocking of prey fish may have the opposite effect on overall rates of mercury
methylation in Soulajule Reservoir, with increased algal populations increasing available
carbon for mercury methylation. Again, this potential effect depends on whether
methylation in Soulajule Reservoir is carbon limited.
Prey fish stocking, as a way to reduce mercury levels in sport fish, was studied recently
in College Lake, a small (62-ac), shallow (20 ft maximum depth) lake on the Colorado
State University Campus (Lepak et al. 2012). Based on the dominant sport fish
population of northern pike (1,000 fish), approximately 9,000 rainbow trout with low
mercury concentrations were stocked in March 2009 (Lepak et al. 2012). Approximately
50 days later, the study found that northern pike total mercury concentrations were
reduced by up to 50% (20% on average), and average individual biomass increased by
about 15%. However, northern pike mercury concentrations and growth rates
rebounded to pre-experiment levels 1 year after stocking ceased.
Relevant information for Soulajule Reservoir is discussed in Section 3.2.4.1.
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The following additional information is needed to rank and prioritize stocking of prey
fish in Soulajule Reservoir compared with other applicable reservoir management
approaches:
• community survey for relative abundance of prey fish
• required stocking rate (i.e., number of fish un-impacted by mercury) for meeting
TMDL numeric targets (0.05 mg/kg wet weight)
With respect to second bullet above, assuming a simple ratio between the existing
Soulajule average fish tissue methylmercury concentration of 0.22 mg/kg (wet weight)
for prey fish (5–15 cm FL) and the Walker Creek TMDL numeric target of 0.05 mg/kg
(wet weight), the prey fish population would need to be increased by roughly 4 times.
An estimate of the current abundance of fish in Soulajule Reservoir would help to
determine whether this represents an unreasonably large number of fish to be stocked.
However, because determining the actual population size of fishes within a reservoir
requires an intensive sampling effort, the recommended approach is the following:
1. identify the relative abundance of fishes within the reservoir via community
survey
2. identify the target fish for stocking
3. adaptively manage stocking levels over time, based on mercury concentration in
fish tissue during subsequent surveys
Further, the determination of potential and indirect cascading trophic effects of
increased prey fish population on mercury cycling and methylation requires additional
information
• relative abundance of sport fish
• mercury bioaccumulation pathway from zooplankton to piscivorous fishes
• stomach content assessment of fishes captured during survey
As with sport fish stocking, the stocking of prey fish to manage fish tissue mercury
concentrations would not affect reservoir storage or downstream flows and thus would
be compatible with current water supply objectives 1 and 2 (Section 1.1). While prey
fish stocking would not affect water temperature or dissolved oxygen in the reservoir or
in downstream Arroyo Sausal (water supply objective 3, Section 1.1), introduced prey
fish species could be washed downstream and could impact habitat beneficial uses by
competing with or preying upon native aquatic species.
Additionally, likely prey stocking species such as bluegill are planktivorous fish that
reside in the littoral zone and are not likely to disturb bottom sediments, thereby
increasing the potential for methylmercury release from reservoir sediments.
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Permit requirements
Permit requirements for prey fish stocking are the same as those for sport fish stocking.
Conceptual infrastructure and cost considerations
Infrastructure requirements for prey fish stocking are the same as those for sport fish
stocking, but would also include distributing the smaller, less mobile fish throughout the
reservoir via boat.
Based on the price of $2.50 per fish for 5–10 cm (2–4 inches) FL largemouth bass
(Professional Aquaculture Services [PAS], the annual stocking cost for providing 3,000–
5,000 prey fish would be approximately $7,500−12,500 (not including transportation).
An alternative option may be to stock with a less expensive prey fish species such as
blue gill. Based on the price of $1.50 per fish for 5–10 cm (2–4 in) FL bluegill
(Professional Aquaculture Services [PAS], the annual stocking cost for 3,000–5,000 prey
fish would be approximately $4,500−7,500 (not including transportation). The
appropriate number of fish for stocking would be determined based on the results of a
fish community composition study (see Pilot Study 3—Section 6.3).
3.3
Summary of Conceptual-level Cost Estimates for Management Methods
Applicable to Soulajule Reservoir
Conceptual-level cost estimates for each of the seven applicable (or potentially
applicable) management methods for Soulajule Reservoir are presented in Table 5. The
cost estimates were used to help rank and prioritize the methods for further
implementation, including identification of appropriate pilot studies (Section 5).
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Table 5. Conceptual-level cost estimates for in-lake and watershed management methods
applicable to Soulajule Reservoir.
Management Method
Conceptual-level Cost Estimate
Reduce Sediment Mercury Loads
Erosion control for upland soils in Eastern Arm
$50–70K1
Cap reservoir sediments in Eastern Arm
$4.4–8.8M (8 ac)2,3
Dredge and dispose of sediments from Eastern Arm
$5.3M (8 ac)3,4
Manage Water Chemistry
Hypolimnetic oxygenation system (HOS)
$1.5–3M
Decrease Levels of Blue-Green Algae
Vigorous epilimnetic mixing (VEM)
$0.5M–$1.5M
Manage Fisheries
Stocking of sport fish un-impacted by mercury
$20–25K (annual)
Stocking of prey fish un-impacted by mercury
1
2
3
4
5
$4.5–7.5K (annual)
Cost is for an evaluation of the potential for mercury loading from upland soils in the Eastern Arm.
Cost does not include development of erosion control plan (estimated additional $50-100K) or
implementation of plan recommendations.
Cost of capping per acre is estimated to be $0.55M–$1.1M. The 8-ac estimated cost applies to the
area of sediments exhibiting 600 to > 1,000 ng/g total mercury. The estimated cost of capping 35
ac applies to the area of sediments exhibiting 300 to > 1,000 ng/g total mercury = $19.3 –38.5M.
Due to the unknown extent of sediments requiring capping or dredging, this estimate assumes a
cost range per acre, and includes a 40% contingency allowance and 25% allowance for
engineering design and permitting.
Estimated cost of dredging 8 ac applies to the area of sediments exhibiting 600 to > 1,000 ng/g
total mercury. Estimated cost of dredging 35 ac applies to the area of sediments exhibiting 300 to
> 1,000 ng/g total mercury = $23.2M.
District evaluation of the feasibility of increased use of Soulajule Reservoir for drinking water
supply is not evaluated as part of this Study Plan.
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4
WATER LEVEL FLUCTUATION
As a lake/reservoir management approach, water level fluctuation has been applied to
control fisheries and aquatic vegetation (e.g., invasive macrophytes such as water
milfoil), as well as algae. For example, fluctuating water surface elevations during the
spring can disrupt bass spawning in the shallow (e.g., < 1.5 ft water depth) margins of a
lake/reservoir, reducing overall bass populations. If timed properly, seasonally
fluctuating water levels can also reduce recruitment and maintenance of vegetated
areas along lake/reservoir margins, and could reduce the volume of water available for
supporting large algal blooms.
We are unaware of applications of water level fluctuation to manage mercury
methylation and bioaccumulation in reservoirs. However, as the District is currently
considering more frequent use of Soulajule Reservoir for drinking water supply, this
Study Plan considers the potential for effects related to changing magnitude, frequency,
and duration of reservoir volume and water level fluctuations on mercury methylation
and bioaccumulation. Because it represents a potential change in reservoir conditions
that may affect mercury methylation and bioaccumulation, rather than a targeted
mercury management approach, water level fluctuation is addressed below, separate
from the other management methods.
Currently, Soulajule Reservoir storage fluctuates on a seasonal basis between
approximately 8,000 and 10,500 ac-ft (roughly 25%) and 9 ft of depth (elevation
between 332 ft and 323 ft) (Figure 4) due to regulated flow releases (Table 1) and
evaporation loss. The District currently anticipates increasing volume withdrawals by
2,000−3,000 ac-ft on an annual basis, which would result in depth fluctuations ranging
18−23 ft (elevation between 314 ft and 309 ft) Drawdown would typically begin May 1
and continue for approximately three months. Withdrawals earlier in the season (e.g.,
January) could occur, if downstream storage capacity is available. Water would be
transferred from the area immediately downstream of the Soulajule Reservoir spillway,
through an existing approximately 3-mi pipeline, and into an unnamed intermittent
creek that flows roughly 1.3 mi before entering Nicasio Reservoir. Soulajule Reservoir
water could also be transferred to other District locations, as needed.
The anticipated increases in Soulajule Reservoir volume withdrawals may result in
annual storage fluctuations on the order of 40−50%. Readily available literature sources
suggest that increasing water level fluctuation in lakes/reservoirs would result in
increased bioaccumulation of mercury (Section 2.4.2.7). However, this effect may be
related to the ratio of littoral zone area to total reservoir volume. Steep-sided reservoirs
with organic-poor substrates may exhibit less efficient methylmercury production, lower
ambient methylmercury concentrations, and less bioaccumulation compared to
reservoirs possessing relatively large, shallow littoral zones with organic rich soils
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(Section 2.4.2.7). With the exception of the eastern and western arms, Soulajule
Reservoir shorelines tend to be steep and not well vegetated (Figure 4 and Figure 5).
Further consideration of a potential relationship between water level and mercury
methylation and bioaccumulation in Soulajule Reservoir would benefit from the
following additional information:
• Area of littoral zone;
• Presence/absence of benthic algae, macrophyte stands, or other significant
biomass in littoral zones of shallower arms;
• Soil grain size and nutrient content in the littoral zone;
• Understanding of the relative importance of littoral zones as a source of
methylmercury in the food web;
• Effect of anticipated future changes in volume and water level fluctuations on
seasonal stratification patterns, potentially changing:
- the volume of epilimnion available for seasonal algal growth; and,
- the volume of hypolimnion available for maintaining sufficient dissolved
oxygen concentrations and minimizing methylmercury production
throughout the stratification period.
Accordingly, Pilot Study 2 (Section 6.2) explores the potential for increased mercury
methylation and bioaccumulation in Soulajule Reservoir given the possibility of more
frequent water level fluctuation in future years. This study includes a preliminary
investigation of likely changes in epilimnion and hypolimnion volumes based upon existing
data (i.e., not a detailed modeling effort) and in relation to effects on algal growth and
mercury methylation. While confirmation of the link between water level fluctuation,
stratification patterns, and fish tissue mercury in Soulajule Reservoir requires a long-term
monitoring effort involving actual hydrology changes, Pilot Study 2 involves short-term
monitoring to establish baseline conditions with respect to littoral zone extent and
productivity and to develop an understanding of the likelihood and directionality of change
with respect to current mercury methylation and bioaccumulation rates.
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5
5.1
RANKING OF POTENTIAL MANAGEMENT METHODS AND
RECOMMENDED PILOT STUDIES
Ranking Approach
The in-lake and watershed management methods judged to have applicability or
potential applicability for controlling methylmercury production and bioaccumulation in
Soulajule Reservoir (Section 3) were subsequently ranked by the project team using 15
criteria weighted as either 1 (least important), 2 (moderately important) or 3 (most
important) (Table 6). Ratings by criterion used a scale of 1 to 5 (negative 1−2, neutral 3,
positive 4−5) and the weighted ratings were then summed and normalized to a scale of
1−100 (Appendix B).
Because water level fluctuation is being considered as a potential change in operating
conditions for Soulajule Reservoir, rather than a methylmercury-specific control action,
it was not ranked alongside the other potential management methods and is discussed
separately (Section 5.3).
5.2
Ranking Results
Results of the ranking exercise (Table 7) suggested three general rating ranges for
potential methylmercury control actions:
• 80–100—no negative ratings, most ratings positive
• 70–79—most ratings neutral or positive
• <69—several negative ratings
Results are discussed further in Sections 5.2.1—5.2.7.
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Table 6. Rating criteria and weighting factors for Soulajule Reservoir in-lake and watershed potential methylmercury control actions.
Criteria
Weighting
Factor
Compatibility with water
supply objectives
3
Effectiveness (MeHg
water)
3
Effectiveness (MeHg
fish)
3
Risk of failure
3
Cost
3
Multiple benefits
2
Landowner acceptability
2
Longevity of treatment
2
Engineering
requirements
Operation and
maintenance (O&M)
requirements
Rating Descriptions
Neutral
3
Negative
1
2
Not compatible or limited
compatibility
Likely or potentially likely to
result in an increase in water
column MeHg
Likely or potentially likely to
result in an increase in fish
tissue MeHg
Compatible
No effect on water
column MeHg
No effect on fish tissue
MeHg
Positive
4
5
Enhances ability to meet
objectives
Somewhat effective or extremely
effective at decreasing water
column MeHg
Somewhat effective or extremely
effective at decreasing fish tissue
MeHg
High or moderately high
Moderate
Limited or none
High (>$5M) to moderately
high (>$3M to $5M)
Would or may interfere with
other benefits
Surrounding landowners
would not or may not
support
Short-term treatment (0 to 1
yr)
Moderate (>$1M to
$3M)
Moderately low ($500K to $1M)
to low (<$500K)
Potential for or certainty of
multiple benefits
No additional benefits
Weighted
Rating
= 3x rating
= 3x rating
= 3x rating
= 3x rating
= 3x rating
= 2x rating
No effect on
surrounding landowners
Surrounding landowners would
potentially or definitely support
= 2x rating
Moderately long
treatment (> 1 to 5 yrs)
Long-term treatment
(> 5 yrs)
= 2x rating
2
Intensive
Moderate
Limited or none
= 2x rating
2
Intensive
Moderate
Limited or none
= 2x rating
Permitting
2
Intensive
Moderate
Limited or none
= 2x rating
Implementation time
1
Long (> 3yrs to 5 yrs) or very
long (> 5 yrs)
Moderate (2-3 yrs)
Short (1-2 yrs) or very short (<1
yr)
= 1x rating
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Criteria
Weighting
Factor
Time to effectiveness
1
Recreation effects
1
Need for pilot studies
1
Rating Descriptions
Neutral
3
Negative
1
2
Long (> 3yrs to 5 yrs) or very
long (> 5 yrs)
Would have or may have a
negative effect
Expensive and/or timeintensive studies
Moderate (2-3 yrs)
No effect
Moderately expensive
and/or time-intensive
studies
Total Score (sum of all weighted ratings)
Positive
4
5
Short (1-2 yrs) or very short (<1
yr)
May have or would have a
positive effect
Limited cost and/or timeintensive studies or none
Weighted
Rating
= 1x rating
= 1x rating
= 1x rating
Sum
Normalized
sum
Overall Rating (scale of 100)
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Biomanipulation—sport fish stocking
Sport fish stocking was the only potential management method that received no
negative ratings (Table 7). This is largely because adding sport fish un-impacted by
methylmercury is the most direct way to reduce average sport fish tissue
methylmercury concentrations, it carries a relatively low risk of failure with respect to
disturbance of the existing food web, it is relatively inexpensive, and it requires little to
no engineering or operation/maintenance requirements.. Sport fish stocking also has
the potential to reduce algal biomass in the reservoir via top-down cascading trophic
interactions (Section 3.2.4.1). While stocked sport fish could be washed downstream
and interfere with habitat beneficial uses in Arroyo Sausal by competing with or preying
upon native aquatic species, this would represent an incremental effect only, since
largemouth bass are already in the reservoir.
A key assumption related to this method is that it occurs in conjunction with another inlake approach (or approaches) focused on controlling methylmercury production and
water column concentrations. We also assume that sport fish stocking would be
implemented using an adaptive management approach with respect to annual stocking
rates, with the goal of demonstrating near-term improvements in average sport fish
tissue concentrations and reducing stocking over the long-term as other in-lake
approaches successfully reduce methylation in the reservoir. Results of Pilot Studies 3
and 4 would inform the selection of appropriate annual stocking rates, which fish
species would be the best choice, and which age-size classes should be stocked. Results
of Pilot Study 1 would indicate whether methylmercury bioaccumulation is spatially
distributed in Soulajule Reservoir and whether a stocking strategy should account for
such a pattern.
Hypolimnetic oxygenation system
Installation of an hypolimnetic oxygenation system (HOS) was the only potential
management method that received the highest rating for compatibility with water
supply objectives because it would reduce blue-green algae dominance and associated
taste and odor compounds, as well as nutrient release (ortho-P, NH4+, Mn2+, Fe2+) from
anoxic sediments during periods of stratification, and thus would enhance overall water
quality in the reservoir (Table 7). By increasing dissolved oxygen in deeper waters, HOS
also would increase the availability of daytime refugia habitat for zooplankton, which
could increase grazing pressure on algae and further improve water quality. Fish habitat
would extend into deeper waters, potentially enhancing sport fish recreational
opportunities. In addition, HOS may provide a meaningful offset of the potential effects
of changing reservoir stratification patterns (e.g., earlier seasonal stratification, longer
period of stratification) associated with climate change projections for the northern
coastal California region (Section 2.5).
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With respect to mercury control, HOS directly and effectively would decrease water
column methylmercury production, assuming that a Speece cone would be placed near
the dam. This type of HOS application carries a low risk of failure, as it is a proven
approach to oxygenating bottom waters and sediments in seasonally stratified lakes and
reservoirs (Section 2.4.2). HOS scored lower with respect to costs, engineering
requirements, and ongoing operation and maintenance requirements as, compared
with other methylmercury control options considered, it is a relatively expensive and
more technically intensive approach. Results of Pilot Study 5 would inform those aspects
of HOS design that involve system size, extent, and in-reservoir location. Results of Pilot
Study 1 would indicate whether an oxygen plume extending into the eastern and
western arm is needed to reduce methylmercury bioaccumulation outside of the
reservoir area nearest the dam.
Erosion control for uplands soils in the Eastern Arm
Overall, erosion control for soils in the vicinity of the mine sites was rated relatively
high, driven primarily by low implementation costs and presumed effectiveness with
respect to methylmercury in water and fish tissue (Table 7). However, our evaluation
assumes that results of Pilot Study 6 would indicate that upland erosion is a significant
external source of mercury to Soulajule Reservoir. We also assume that landowners of
the mine sites would be amenable to recommended erosion control measures and that
cattle grazing near mine sites would not interfere with erosion control measures, both
of which must be confirmed as part of Pilot Study 6. Lastly, because reduction of
mercury concentrations in the reservoir water column and/or fish tissue due to control
of an upland source may not be easily observable in the short-term, we assume that
upland erosion control measures (if needed) would occur in conjunction with another
in-lake approach (or approaches) to more rapidly address methylmercury production
and bioaccumulation.
VEM in the Eastern Arm
Vigorous epilimnetic mixing (VEM) in the Eastern Arm of the reservoir received a high
rating for compatibility with water supply objectives because it would reduce blue-green
algae dominance and associated taste and odor compounds and thus enhance overall
water quality in the reservoir. Reducing turbidity associated with large blue-green algal
blooms may also support photo-demethylation of mercury through increased water
clarity, which would decrease methylmercury concentrations in water. However, while
VEM has been applied successfully to control algal species in other studies, it is currently
untested for methylmercury control. Results of Pilot Study 1 would indicate whether
reductions in blue green algae in the Eastern Arm are likely to result in concomitant
reductions in methylmercury in zooplankton and small fish in the Eastern Arm and,
potentially, elsewhere in the reservoir. Like HOS, VEM may provide a meaningful offset
of the potential effects of changing reservoir stratification patterns (e.g., earlier
seasonal stratification, longer period of stratification) and a longer growth season for
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blue-green algae, associated with climate change projections for the northern coastal
California region (Section 2.5).
Biomanipulation—prey fish stocking
Prey fish stocking, while similar to sport fish stocking in terms of relatively low costs and
ease of implementation (e.g., no engineering or operation and maintenance
requirements), was not rated as high as sport fish stocking due to potential food web
impacts (Table 7). Assuming that predation is the primary population control
mechanism in the waterbody of interest, an increase in prey fish would lead to an
increase in algal biomass (Section 3.2.4.2), which would reduce water quality. As with
sport fish stocking, we assume that this method would occur in conjunction with
another in-lake approach (or approaches) that is focused on controlling methylmercury
production and water column concentrations. Stocking would be implemented using an
adaptive management approach and would be reduced over the long-term. As with
sport fish stocking, results of Pilot Studies 5 and 6 would inform the selection of
appropriate annual stocking rates, which fish species would be the best choice, and
which age-size classes should be stocked.
Dredging of reservoir sediments in the Eastern Arm
Dredging of Eastern Arm sediments exhibiting elevated total mercury was ranked
relatively low (Table 7). Assuming that sediments in the Eastern Arm are an important
pathway for bioaccumulation in Soulajule Reservoir, this method would decrease
methylmercury in water and fish tissue by removing the mercury source. However,
assuming that sediments in the Eastern Arm are not likely to be exposed given typical
reservoir conditions, nor are they likely to be exposed if annual reservoir fluctuations
increase by the anticipated 2,000-3,000 ac-ft (Section 4), dredging costs for the
potentially broad extent of elevated sediments in the Eastern Arm are anticipated to be
extremely high. Further, permits for this type of work are often difficult to obtain.
Additionally, engineering requirements and implementation time for dredging projects
are high. Accordingly, dredging is not considered further for mercury control in Soulajule
Reservoir.
Capping of reservoir sediments in the Eastern Arm
Capping of Eastern Arm sediments exhibiting elevated total mercury was ranked the
lowest of the potentially applicable management methods (Table 7). Assuming that
sediments in the Eastern Arm are an important pathway for bioaccumulation in
Soulajule Reservoir, this method would reduce methylmercury in water and fish tissue
by isolating the source of mercury. However, cost, implementation time, and
requirements for engineering, ongoing operation and maintenance, and permitting of
reservoir sediment projects are typically high. As for dredging, we assume that
sediments in the Eastern Arm are not likely to be exposed given typical reservoir
conditions, nor are they likely to be exposed for easier access if annual reservoir
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fluctuations increase by the anticipated 2,000-3,000 ac-ft (Section 4). Additionally, the
risk of failure for capping is moderately high, based on potential issues with increasing
reservoir water level fluctuation, low-water grazing impacts, and production of methane
or other gases in reservoir sediments. Accordingly, this method is not considered further
for mercury control in Soulajule Reservoir.
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Table 7. Ranking summary and key assumptions for Soulajule Reservoir in-lake and watershed methylmercury control actions.
Methylmercury (MeHg) Control
Action
Weighted Rating
(Scale 1-100)
Key Assumptions
•
•
Biomanipulation—sport fish
stocking
81
•
•
•
•
system (HOS)
79
•
•
•
Upland erosion control in
Eastern Arm
74
Vigorous epilimnetic mixing
(VEM) in Eastern Arm
73
•
•
•
•
•
Results of Pilot Studies 5 and 6 would inform selection of appropriate annual stocking rates,
species, and age-size class
Results of Pilot Study 1 would indicate whether methylmercury bioaccumulation is spatially
distributed and whether any stocking strategy should account for a pattern
Adaptive management approach on annual stocking rates, to demonstrate improvements in
average sport fish tissue concentrations and reduced stocking over time
Would occur in conjunction with other in-lake approaches to address methylmercury
production
Results of Pilot Study 5 would inform those aspects of HOS design that involve system size,
extent, and in-reservoir location
Results of Pilot Study 1 would indicate whether algae, zooplankton, and small fish near the
dam exhibit highest levels of methylmercury bioaccumulation and accordingly where to focus
HOS
Liquid oxygen (LOX) could be trucked, delivered and stored adjacent to the existing pump
station on District property. If LOX delivery is not feasible due to space/transportation
constraints, onsite oxygen generation is also feasible, though more expensive and involves
more equipment.
A Speece cone and supporting equipment would be located near the dam, with extensions
into reservoir arms if results of Pilot Study 1 indicate that methylation and bioaccumulation in
the arms is important
Results of Pilot Study 6 would indicate whether upland erosion is a significant external source
of mercury
Landowners at mine sites would be amenable to recommended erosion control measures
Cattle grazing near mine sites would not interfere with erosion control measures
Occurs in conjunction with other in-lake approaches to address methylmercury production
and bioaccumulation
Results of Pilot Study 1 would indicate whether algae, zooplankton, and small fish in the
Eastern Arm exhibit relatively high levels of methylmercury bioaccumulation
Note VEM works in theory but is yet untested for methylmercury control
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Methylmercury (MeHg) Control
Action
Weighted Rating
(Scale 1-100)
Key Assumptions
•
•
Biomanipulation—prey fish
stocking
71
•
•
•
Dredging of reservoir sediments
in Eastern Arm
65
•
•
•
Capping of reservoir sediments
in Eastern Arm
•
59
•
•
Results of Pilot Studies 5 and 6 would inform selection of appropriate annual stocking rates,
species, and age-size class
Results of Pilot Study 1 would indicate whether methylmercury bioaccumulation is spatially
distributed and whether any stocking strategy should account for a pattern
Adaptive management approach on annual stocking rates, to demonstrate improvements in
average prey fish tissue concentrations and reduced stocking over time
Would occur in conjunction with other in-lake approaches to address methylmercury
production
Results of Pilot Study 6 would indicate whether upland erosion is a significant and continuing
external source of mercury
Results of an additional study (not proposed in this Study Plan) would indicate volume of
sediments with elevated mercury
Results of Pilot Study 6 would indicate whether upland erosion is a significant and continuing
external source of mercury
Results of an additional study (not proposed in this Study Plan) would indicate areal extent of
sediments with elevated mercury
Eastern Arm sediments are non-mobile
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5.3
Water Level Fluctuation
Currently, Soulajule Reservoir storage fluctuates on a seasonal basis between
approximately 8,000 and 10,500 ac-ft (roughly 25%) and 9 ft of depth (elevation
between 332 ft and 323 ft) (Section 4,). The District is currently considering more
frequent use of Soulajule Reservoir for drinking water supply, which could alter the
future magnitude, frequency, and duration of volume and water level fluctuations. For
example, increasing volume withdrawals by 2,000−3,000 ac-ft on an annual basis would
result in total storage fluctuations on the order of 40−50%.
Readily available literature sources suggest that increasing water level fluctuation in
lakes/reservoirs would result in increased bioaccumulation of mercury (Section 4,).
However, this effect may be related to the ratio of littoral zone area to total reservoir
volume. Steep-sided reservoirs with organic-poor substrates may exhibit less efficient
methylmercury production, lower ambient methylmercury concentrations, and less
bioaccumulation compared to reservoirs possessing relatively large, shallow littoral
zones with organic rich soils (Section 4). With the exception of the eastern and western
arms, Soulajule Reservoir shorelines tend to be steep and not well vegetated (Figure 4
and Figure 5).
Pilot Study 2 explores the potential for increased mercury methylation and
bioaccumulation in Soulajule Reservoir given the possibility of more frequent water level
fluctuation in future years. While confirmation of the link between water level fluctuation
and fish tissue mercury in Soulajule Reservoir requires a long-term monitoring effort
involving actual hydrology changes, the pilot study involves short-term monitoring to
establish baseline conditions and to develop an understanding of the likelihood and
directionality of change with respect to current mercury methylation and bioaccumulation
rates.
5.4
Prioritization of Pilot Studies
Based on results of the management method ranking effort, six pilot studies are priority
studies (Table 8). These are listed in the same general rank order as the management
methods, with the first two studies (pilot studies 1 and 2) providing information relevant
to all management method. Potential pilot studies associated with capping and dredging
activities in the Eastern Arm are no longer considered as these two management
methods were ranked relatively low.
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Table 8. Priority pilot studies for Soulajule Reservoir mercury control actions.
Pilot Study
Study 1—Additional Characterization of
Methylmercury in Water and Biota
Study 2—Water level fluctuation
Study 3—Soulajule Reservoir Fish
Community Composition
Study 4—Soulajule Reservoir Food Web
Structure
Study 5— Evaluation of Reservoir Seasonal
Oxygen Demand and Sediment Response to
Hypolimnetic Oxygenation
Study 6—Upland Mercury Source Loading
Characterization
Rationale
Would provide information relevant to all management
methods considered, and in particular the two highest
ranked actions (i.e., biomanipulation − sport fish stocking,
HOS)
Would inform the likelihood and directionality of change
for methylation and bioaccumulation rates given
increased water level fluctuation and for all management
methods considered
Would inform implementation of biomanipulation − sport
fish stocking
Would inform implementation of biomanipulation − sport
fish stocking
Would inform conceptual design and detailed costing for
HOS
Would indicate whether erosion control of uplands in
vicinity of the Cycle and Franciscan mines is needed
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6
PILOT STUDIES TO FILL DATA GAPS
Pilot studies to further inform future management methods for Soulajule Reservoir
involve multiple scales, depending on information needs, as follows.
• Bench-scale laboratory tests, such as experimental sediment core chambers,
which can be subjected to varying physical and chemical conditions such as water
velocities or water column dissolved oxygen concentrations.
• Experimental in-lake enclosures or mesocosms, where variations in nutrient
availability to algae and zooplankton grazers can be tested against methylmercury
uptake.
• Reduced-scale in-lake or out-of-lake assemblies deployed in a portion of the total
treatment area to test effectiveness.
• Monitoring of water quality, sediment, and/or biota to determine patterns and
trends under in situ conditions.
• Field reconnaissance surveys to determine current conditions, as required for
further consideration of potential management approaches.
The following sections describe pilot studies to fill data gaps identified for each of the
potential mercury control methods discussed in Section 3.
6.1
Pilot Study 1—Additional Characterization of Methylmercury in Water and
Biota
Objectives
The objective of Pilot Study 1 is to build on existing data describing methylmercury
concentrations in water, zooplankton, and small fish in Soulajule Reservoir to better
understand the role of conditions in the Eastern Arm and patterns of mercury
bioaccumulation throughout the reservoir. The monitoring focus is on small prey fish as
indicators of methylmercury exposure and bioaccumulation on a seasonal timescale (e.g.,
weeks to months) and at a localized spatial scale (e.g., reservoir arm or near the dam).
Larger fish typically move throughout the reservoir and live for multiple years, and as such,
they are broader spatial and temporal integrators of mercury bioaccumulation factors and
would be unlikely to demonstrate patterns of mercury bioaccumulation at the scale of
particular locations in the reservoir.
Hypotheses
The pilot study will test the following hypotheses:
Seasonal algal blooms are concentrated in the Eastern Arm of Soulajule Reservoir
and are dominated by blue green algae.
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During algal blooms and periods of stratification, DO in the shallow Eastern Arm
varies with water depth and time of day such that anaerobic conditions occur at or
near the sediment water interface.
Algal, zooplankton, and small (5−15 cm FL, TL3) prey fish methylmercury
concentrations and bioaccumulation factors (BAFs) are higher in the mining
impacted shallow, Eastern Arm of Soulajule Reservoir than the main body of the
reservoir or the Western Arm.
Algal, zooplankton, and small (5−15 cm FL, TL3) prey fish methylmercury
concentrations and BAFs are higher in the fall than in the spring, independent of
location in the reservoir.
Measured BAFs are less than the 1,300,000 (L/kg) value assumed in the Walker
Creek TMDL for development of the water column allocation (0.04 ng/L annual
average dissolved methylmercury) (SFBRWQCB 2008a), such that an allocation
based upon measured BAFs would be greater than 0.04 ng/L.
Sampling design
6.1.3.1
Sampling sites and monitoring constituents
Methylmercury concentrations will be monitored in surface water and in algae,
zooplankton, and small (5–15 cm FL, TL3) prey fish at three sites in Soulajule Reservoir
(Table 9, Figure 26). In situ water quality parameters (water temperature, DO, pH,
conductivity, oxidation-reduction potential [ORP]) will be monitored at the same three
locations over varying time scales, depending on location. Water temperature, dissolved
oxygen, methylmercury, and dissolved sulfides in water will be monitored at the dam
outlet to Arroyo Sausal.
Table 9. Sampling sites.
Site ID
Description
Latitude
Longitude
EA-1
Eastern arm, in vicinity of historical
mining locations
38° 8'45.53"N
122°45'19.34"W
WA-1
Longitudinal center line of western
arm, potentially unaffected by
historical mining locations
38° 8'43.37"N
122°47'11.26"W
D-1
In vicinity of dam, ideally between
the original and the current dam
38° 9'5.62"N
122°46'58.69"W
A-11
At dam outlet to Arroyo Sausal
38° 9'15.43"N
122°47'1.56"W
1
Water sample only.
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A-1
D-1
WA-1
EA-1
Figure 26. Soulajule Reservoir sampling sites for additional characterization of methylmercury
in water and biota.
This pilot study involves four spatially-replicated sampling events in Soulajule Reservoir,
including one fully-mixed winter/early spring event (January/February 2016) when algal
productivity is low; just after the onset of stratification and during the spring algal bloom
(April/May 2016); during stratification and the late summer algal bloom (July/August 2016);
and, just prior to fall turnover when hypolimnion methylmercury concentrations and
anoxia peak (October/November 2016). The following activities will take place during each
sampling event:
• Collect vertical water column profiles for in situ parameters
• Collect 48-hr continuous measurements for in situ parameters in bottom waters of
Site EA-1 in the Eastern Arm
• Collect individual surface water grab samples for analysis of dissolved
methylmercury
• Collect multi-individual algal grab samples for methylmercury analysis, cell count,
species identification, and spatial extent (via satellite imagery)
• Collect multi-individual zooplankton composite samples for methylmercury
analysis
• Collect individual small (5-15 cm FL, TL3) prey fish for methylmercury analysis
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Beginning in January 2016, the District will also collect monthly grab samples at site A-1
for in situ measurements of water temperature, dissolved oxygen, and analysis of
dissolved methylmercury and dissolved sulfides. Sampling frequency will increase to
every two weeks surrounding reservoir destratification in the late fall and the onset of
stratification in spring.
The number of methylmercury samples within each site, across sites, and across surveys
is presented in Table 10. Sample sizes have been selected to support determination of
statistical significance (e.g., α=0.05, p≤0.05) between sites.
Table 10. Number of water, algae, zooplankton, and fish methylmercury samples.
Water1
Algae2
Zooplankton3
Fish4
Within-site sample replication
3
3
3
15
Number of sites
4
3
3
3
Number of samples per survey
12
9
9
45
Number of surveys
4
4
4
4
Total number of samples
48
36
36
180
1
2
3
4
Individual grab samples
Multi-individual composite samples filtered from 1 L of water
Multi-individual composite samples from 1-2 vertical tows
Individual prey fish (5 – 15 cm)
6.1.3.2
Field collection methods
Water quality
A YSI 600 series or YSI 6920 water quality Sonde will be used for vertical water column
profiles of in situ parameters. A (separate) YSI 6920 Sonde will be used for continuous
measurements of in situ parameters. The probe will be deployed approximately 0.1 m (0.5
ft) above the sediment water interface. The Sondes will be calibrated at elevations
consistent with the associated monitoring site. After the 48-hour monitoring period,
continuous data will be downloaded onto a Toughbook © for further analysis.
Pilot Study 1 will also include an investigation of the potential for cost-effective collection
of in situ chlorophyll-a concentrations and blue-green algae relative biomass along with the
standard in situ parameters (water temperature, DO, pH, conductivity, oxidation-reduction
potential [ORP]). This data would provide a continuous indicator of relative algal biomass
during probe deployment and would the supplement cell count and algal species
identification data that will be collected as seasonal grab samples (see below). In addition,
collection of in situ chlorophyll-a concentrations and blue-green algae relative biomass may
facilitate future use of probes without laboratory confirmation. However, the simultaneous
use of three optical probes (including dissolved oxygen) would require a different water
quality Sonde and may result in a non-trivial increase in monitoring costs for Pilot Study 1.
Therefore, as an initial step, the cost of using in situ chlorophyll-a and blue-green algae
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probes will be investigated and discussed with the District during fall 2015. Deployment of
the probes in 2016 will be based on the outcome of this investigation and the budget will
be adjusted, as needed.
Water samples
Surface water grab samples will be collected from a boat. Samples for analysis of dissolved
methylmercury concentrations will be field-filtered using capsule filters and pre-cleaned
tubing and placed in pre-cleaned bottles with preservative supplied by the analytical
laboratory. All water samples will be kept on ice while in the field, and stored at less than
12°C until analysis. Trace metal clean sampling techniques as prescribed by US EPA Method
1669 will be used for collection, handling and analysis of all methylmercury samples.
Algae samples
Algae samples will be collected from a boat in the epilimnion using a depth-integrated
sampler (5–8 cm diameter). Algae samples will not be collected near the bottom, since
cyanobacteria growth does not occur outside of the photic zone. Algal samples will be
stored in ice coolers and preserved within 4 hrs of collection using one part Lugol’s solution
for each 100 parts of sample for sample preservation. Algal samples will be analyzed for
methylmercury, cell count, and species identification. The spatial extent of spring and
summer algae blooms will be confirmed by satellite imagery for 1−2 days prior, during, and
after the field sampling event.
Zooplankton samples
Zooplankton samples will be collected from a boat using a non-metallic zooplankton net
and repeated vertical tows. The tow net (64 um mesh size, 90 cm long) will possess a 30-cm
polycarbonate ring and a nonmetallic bridle. During periods of high algal biomass in the
photic zone, zooplankton sampling will be conducted just beneath the photic zone to avoid
sampling algae. After each zooplankton tow, the net will be rinsed by hosing with lake
water from the outside of the net, so that material adhering to the inside of net collects in
the sample bottle. The sample bottle will be removed from the net and poured into a 1 L
laboratory sample bottle. If the sample is not highly concentrated, it will first be dewatered
by pouring the contents into netting held by strainer. Dewatered material will then be
removed using a Teflon spatula and/or nylon forceps to the laboratory sample bottle for
mercury analysis. Prior to collection, all Teflon ware will be acid-washed in concentrated
nitric acid, rinsed with nanopure water, and stored in acid-washed polypro bags, double
bagged. Sample bottles will be individually labeled and placed on ice for transport to the
laboratory.
Fish samples
Gill nets for small (5−15 cm FL, TL 3) prey fish will be placed at each sampling site in shallow
areas (3–5 ft depth) and adjacent to submerged aquatic macrophytes or under
overhanging woody vegetation. Gill nets will be oriented perpendicular to the shoreline.
With respect to mercury, protocols for fish handling and processing will generally follow
methods described in USEPA (2000). After fish are captured, total length (TL, mm), fork
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length (FL, mm), and weight (g) will be recorded. In order to confirm that small prey fish
individuals have accumulated methylmercury for no more than one year (i.e., one
stratification period), up to 10 scale samples will be taken as part of the spring survey
(April/May 2016) fish collection. If a size threshold is identified above which fish are older
than one year, then the quantities of fish collected above the threshold size will be reduced
in future data collection, and a subset of the data will be identified as biosentinels that can
be used to measure future effectiveness of management methods.
Fish will be identified by species, individually labeled, and placed in zip lock plastic bags on
wet ice for transport from the field and transfer to a commercial freezer for storage.
Following laboratory protocol, frozen fish will be shipped overnight on wet ice to the
analytical laboratory. At the laboratory, fish specimens will be analyzed using whole-body
individual homogenization.
6.1.3.3
Laboratory analysis
Methylmercury water and tissue samples will be sent to a lab capable of achieving ultralow detection limits (e.g., Brooks Rand Laboratories). Samples will be analyzed using USEPA
1630. Tissues will be digested in a KOH/methanol solution. Samples will then be analyzed
by ethylation, Tenax trap pre-concentration, gas chromatography separation, pyrolytic
combustion and atomic fluorescence spectroscopy (CV-GC-AFS).
Data analysis and reporting approach
In situ profiles collected at each site will be used to evaluate seasonal and spatial
stratification patterns during the study period. Water column aqueous methylmercury,
as well as algal, zooplankton, and small (5−15 cm FL, TL 3) fish methylmercury
concentrations and BAFs in the shallow, Eastern Arm of Soulajule Reservoir will be
compared to those in the deeper portion of the reservoir near the dam and the Western
Arm for all sampling events to identify spatial and seasonal trends. BAFs will be
calculated using the average methylmercury tissue concentration for each trophic level
and surface water methylmercury concentrations in the following equation:
BAF = log10 (MeHgwater/MeHgtissue)
Aqueous methylmercury in water and mercury in fish tissue will be compared to criteria
given in the Walker Creek TMDL (SFRWQCB 2008a) and evaluated within the context of
other in situ and analytical water quality data collected during the study. Results will be
summarized in a technical memorandum and provided to the District. In addition, a data
report will be prepared for use with permitting authorities that includes: date, time, and
location of sampling activities; species and number of species collected; and a copy of
field data sheets.
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Implementation schedule
Ideally, the first sampling event would occur in late summer to early fall 2015,
corresponding with a secondary seasonal algal bloom. The second event would occur
following reservoir destratification and mixing in winter 2015/early spring 2016, and the
third sampling event would occur in late spring/early summer 2016 following the onset of
stratification and the first algal bloom of the season. Overall, we anticipate that the study
will take 12−16 months to complete.
Estimated cost
We estimate the cost of this study to be $256,700, including four field surveys, analytical
costs, and reporting (Table 11).
Table 11. Estimated cost of Pilot Study 1—Additional Characterization of Methylmercury in
Water and Biota.
Task
Permitting
$2,000 1
Field effort
$95,300 2
Algal imagery
$9,000 3
Laboratory analysis
$84,000 4
Data analysis and reporting
$66,400 5
Total
$256,700
1
2
3
4
5
6.2
Estimated Costs
Assumes coordination with CDFW for letter of authorization
for reservoir sampling activities.
Assumes 3-day field effort x 4 surveys during 2016 for water
and biota sampling in the reservoir, plus 5-hr field effort x 12
events during 2016 for water sampling at the dam outlet.
Equipment included (e.g., nets, water quality meters,
sampling bottles/coolers, GPS, boat rental).
Based on quote from Blue Water Satellite Inc. for 6 images.
Assumes MeHg in water ($164/sample, including filtration for
dissolved fraction); dissolved sulfide in water ($65/sample,
including filtration); MeHg in algae ($196/composite sample);
MeHg in zooplankton ($196/composite sample), MeHg in fish
($172/individual); algal enumeration and species identification
($225/composite sample); zooplankton enumeration and
species identification ($200/composite sample). MeHg quote
provided by Brooks Rand, April 2015.
Assumes one review cycle on draft and final report.
Pilot Study 2—Assessment of Littoral Zone Extent and Productivity as
Related to Increased Water Level Fluctuation
Objectives
The objective of the study is to explore the potential for increased mercury methylation
and bioaccumulation in Soulajule Reservoir given the possibility of more frequent water
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level fluctuation in future years. While confirmation of the link between water level
fluctuation and fish tissue mercury in Soulajule Reservoir requires a long-term monitoring
effort involving actual hydrology changes, this study will involve short-term monitoring to
establish baseline conditions and to develop an understanding of the likelihood and
directionality of change with respect to current mercury methylation and bioaccumulation
rates.
Hypotheses
The study will test the following hypothesis:
The littoral zone in Soulajule Reservoir is relatively small and steep and supports low
levels of benthic algal production and macrophytes.
This study will also provide data to inform the following hypotheses, although they can
only be fully tested through a long-term study:
Increasing magnitude and frequency of water level fluctuations will result in
increased mercury concentrations in fish tissue.
Increased water level fluctuations will reduce spawning habitat for sport fishes,
thereby reducing the sport fish population and subsequently increasing the prey and
algae populations and increasing available carbon for mercury methylation (see
also, Section 2.3.2.2).
Approach
The areal extent of the littoral zone in Soulajule Reservoir will be assessed initially as a
desktop GIS analysis, using aerial imagery, reservoir bathymetry, and existing data
characterizing storage and water elevation and storage in Soulajule Reservoir. Groundtruthing will be completed during a late summer benthic algae and macrophyte survey,
when water level is low and primary productivity is high. The approximate location and
area of the littoral zone will be estimated by boating and/or walking the reservoir
perimeter and shallow areas, where accessible.
In addition, baseline information on sediment grain size and nutrient content will be
assessed at up to six transects in the littoral zone, including up to two transects in the
Eastern Arm, two in the Western Arm, and two between the dam and the Eastern Arm in
the deeper portion of the reservoir. It is anticipated that three sediment samples per
transect will be collected (up to 18 samples).
The effect of water level fluctuations on mercury concentrations in fish tissue will be
difficult to definitively determine if other management methods are underway, particularly
in the long-term. However, collection of baseline data will be invaluable if reservoir storage
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management changes in the future. Tissue samples for small prey fish (5−15 cm FL, TL3)
collected as part of Study 3 (Section 6.1) can be used as baseline data for water-level
fluctuation current conditions in Soulajule Reservoir, with continued sampling in the future
if the management of Soulajule Reservoir is changed.
Note that the food web structure study (Study 4) also addresses the question of littoral vs.
pelagic food webs in Soulajule Reservoir. If the littoral zone is identified as an important
pathway for mercury bioaccumulation, then additional water level studies may be
required.
Lastly, a literature review of studies involving reservoir water level fluctuation to manage
fish spawning will be conducted, in order to explore the potential indirect effect of water
level management on sport fish populations and mercury bioaccumulation.
The ratio of littoral zone area to Soulajule Reservoir volume (see also Section 2.4.2.7) will
be calculated and, along with field verification of benthic algae and macrophyte
presence/absence, will be used to determine if the littoral zone is likely to be an important
pathway for mercury bioaccumulation in Soulajule Reservoir. Results will be summarized in
a report, including the literature review of potential effects on fish spawning and any
relevant adaptive management recommendations related to future water level
management in the reservoir.
The littoral zone GIS analysis will be undertaken in late summer/ early fall 2015. Ground
truthing and the survey for benthic algae and macrophyte survey will be conducted
during spring 2016. Overall, we anticipate this study to require 9 months to complete.
Estimated cost
We anticipate this effort to take 1–2 months to complete at an estimated cost of
$22,000 (Table 12), including a 1- to 2-day ground-truthing survey effort and 1 day of
sediment collection in mid- to late summer 2016.
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Table 12. Estimated Cost of Pilot Study 2—Assessment of Littoral Zone Extent and Productivity
as Related to Increased Water Level Fluctuation.
Task
Estimated Cost
GIS analysis
Ground truthing field survey
Sediment collection and analysis1
Report
Total
$3,600
$3,600
$6,000
$8,800
$22,000
1 Includes collection and analysis of approximately 18 samples for grain size
and nutrient content. Assumes approximately $230 per sample ($50 for
nutrient content [C,N,P], $180 for grain size).
6.3
Pilot Study 3—Assessment of Reservoir Fish Community Composition
Objectives
The objective of the study is to determine the relative abundance, distribution, and agesize class distribution of fishes within Soulajule Reservoir as a means of informing potential
mercury bioaccumulation management approaches involving biomanipulation. This study
will involve monitoring.
Hypotheses
The fish community composition study objective is descriptive and thus there are no
associated hypotheses.
Sampling design
Reservoir sampling will be conducted using a combination of beach seines, boat
electrofishing, and gill nets; sampling will occur during the spring and fall (Table 13). Spring
sampling targets the spawning population, whereas fall sampling includes the year-1 (i.e.,
young-of-year) fish. Details of these methods are provided below. To assess fish age
composition, up to 20 scale samples of each fish species will be collected from a subsample within a reservoir reach.
Spring sampling will identify fish populations that piscivorous birds (e.g., kingfishers,
herons) are most likely to consume before and after their breeding season. Fall sampling
will identify fish populations when reservoir levels are relatively low, resulting in the
highest sampling efficiency.
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Table 13. Fish sampling methods and reaches.
Sample Method
Beach seine
Boat electrofishing
Gill net
Gill net—vertical
array
Fish
Population
Sampling Reach
(see also Figure 26)
Habitat
Sample Period
Juvenile
W1, M1, M2,
E1, E2, E3, E4
Shallow shoreline
Fall and Spring
W1, M1, M2,
E1, E2, E3, E4
Shallow locations
<10 ft (cove or
shallow arms)
Fall and Spring
W1, M1, M2,
E1, E2, E3, E4
Near shore and
deep water
Fall and Spring
Near dam
Throughout water
column
Fall
Adult
Juvenile
Adult
Juvenile
Adult
Juvenile
Soulajule Reservoir will be divided into seven reaches to assess fish populations and
distribution (Figure 27). The Eastern Arm will be divided into four reaches: the reach
downstream of the Franciscan Mine (E1), the reach between the two mines that also
exhibits the highest total mercury in sediments (E2), the reach upstream and to the east of
Cycle Mine (E3), and reach upstream and to the south of Cycle Mine (E4) (Figure 27). The
Western Arm will be represented by one reach (W1). The main body of the reservoir will be
divided into two reaches (M1 and M2), where M1 represents the deepest portion of the
reservoir, near the dam. A vertical array will assess the fish population distribution
throughout the water column near the deepest location in the reservoir (Figure 27).
One sampling event by beach seine (where feasible), adult gill net, juvenile gill net, and
electrofishing will be conducted within a reservoir reach to determine the spatial
population distribution of adult and juvenile fish within the reservoir (Table 13). Within
reach site selection will occur in the field.
Figure 27. Approximate fish sampling reaches (rectangles) in Soulajule Reservoir. The red
rectangle marks the area with high total mercury levels in sediments. The orange
line represents the approximate location of the vertical gill net array.
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6.3.3.1
Fish Sampling
Beach seines
Beach seines will be used at locations, where practical, in near-shore areas with shallow
depths, gradual slopes, and small substrates lacking large cover objects (Table 13). Up to
seven sites will be seined to assess near-shore habitat use. Sampling stations will be
documented using global positioning system (GPS). Fish data collected at each site will
include species identification, total length (TL, mm), fork length (FL, mm), weight (g), and, if
applicable, notes on general condition. Scales may be collected from a subset of fishes to
confirm length/age relationships.
General information recorded will include impoundment name, GPS coordinates of sample
location, and in situ data (i.e., water temperature, dissolved oxygen, pH, and conductivity).
Boat electrofishing
Boat electrofishing will be conducted in coordination with gill-netting efforts, using
standard methods (Reynolds 1996) to sample reservoir near-shore habitats. Electrofishing
will target shallow locations (<10 ft deep), such as a cove or shallow arm. This method is
particularly helpful if there are cover objects such as logs, stumps, rocks, that would
otherwise prohibit use of a beach seine. Sampling will include one site within each of the
seven reaches, if suitable habitat is available.
Electrofishing and gillnet sampling sites will be located sufficiently far apart to avoid
interference and frightening fish into or away from sampling sites. Electrofishing stations
approximately 100 m in length will be established around each reach, targeting a diversity
of near-shore habitats. Sampling stations will be documented using global positioning
system (GPS). Electrofisher “time on” will be recorded for each sampling site and a
consistent effort and pace will be employed at all sites. Fish data and general information
on site conditions will be collected as described in the ‘Beach Seines’ section, above.
Adult and juvenile gill nets
To address fish species composition and distribution, one variable-mesh “adult” gill net (1to 4-in. mesh) and one variable-mesh “juvenile” gill net (<1-in. mesh) will be deployed in up
to seven locations along the shoreline in Soulajule Reservoir, occupying near-shore and
deep-water habitats (Figure 28). Variable-mesh gill nets are 100 ft long and 6–8 ft deep and
consist of 4, 25-ft panels of variable mesh sizes; each panel consists of a different mesh size
(e.g., 1 in., 1½ in., 2 in., and 3 in.) so that a gradient of sizes is represented across the net.
Juvenile mesh gill nets are 30 ft long, 6 ft deep and consist of three 10-ft panels of variable
mesh sizes; each panel consists of a different mesh size (e.g., 1/2 in., 5/16 in., and 3/4 in.).
The time of deployment and locations of each gill-net set will be recorded. In order to
reduce the potential for mortality, the gill nets will be set for two, 12-hr net-set periods,
including one day and one night period, over an approximate 24-hr period to facilitate
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good coverage and to separate diel periods. Fish data and general information on site
conditions will be collected as described in the ‘Beach Seines’ section, above.
Vertical gill net arrays
To characterize the vertical distribution of fishes and life stages in Soulajule Reservoir, one
gill net array will be set in the deepest area feasible to sample in the vicinity of the dam
(Figure 28). The array will include four adult variable-mesh gill nets: one floating net at the
surface away from the shoreline; one net suspended at the thermocline, if present, or
middle water column, if absent; one gill net suspended at approximately 90% of the depth;
and one gill net located on the reservoir floor. The array also includes three juvenile
variable-mesh gill nets: one floating net at the surface away from the shoreline; one net
suspended at the thermocline, if present, or middle water column, if absent; and one gill
net suspended at approximately 90% of the depth (Figure 28).
Shallow nets
Shoreline nets
Mid-column nets
Deep nets
Figure 28. Conceptual gill net array in the deepest area of the reservoir and along the
shoreline of Soulajule Reservoir (juvenile nets in blue, adult nets in gray).
Captured and processed fish will be allowed to recover, and will be released after the
sampling is complete or in an area away from the sampling location. Fish data and general
information will be collected using the methods as described in the ‘Beach Seines’ section,
above. In addition, at each gill net sample location, the minimum, maximum, and mean
water depths will be recorded, and in situ profiles will be collected at the approximate net
placement locations.
Fish collection data (e.g., fish species and number captured, length, weight) will be entered
into an Excel spreadsheet for reduction, tabulation, and summary. Analyses will include
quantifying and describing fish composition and distribution by life stage and by time
period when fish were captured (i.e., daytime versus nighttime). Length-frequency
histograms will be developed for all fish species observed in the reservoir, and by reservoir
reach; select scales will be analyzed to ascertain the age-class structure of the reservoir
fisheries.
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Fish capture results will be reported both as total catch and in terms of catch per unit
effort. Catch per unit effort (CPUE) for fishes captured by beach seine and electrofishing
will be dividing number of fish of each species captured by the total area of water sampled
multiplied by the length of time fished [e.g., fish/(ft2 x second)]. CPUE for fishes captured
by gill net will be calculated by dividing number of fish captured by the dimensions of the
gill net multiplied by the length of time fished [e.g., fish/(ft2 x hour)]. CPUE will be
summarized by reservoir location and species.
Habitat descriptions for each site will include a summary of general water chemistry
conditions, the total sample area of seine sites, and minimum, mean, and maximum
depths.
Fish community composition studies will take place in the fall of 2015 and spring of 2016
(Table 13). Overall, we anticipate that the study will take approximately 12 months to
complete.
Estimated cost
We anticipate this effort to take 1 year to complete at an estimated cost of $80,000,
including fall and spring surveys (Table 14).
Table 14. Estimated cost of Pilot Study 3—Assessment of Reservoir Fish Community
Composition.
Task
Estimated Cost
$37,000 1
$3,000 2
$40,000 3
$80,000
Field effort
Laboratory analysis
Total
1
2
3
6.4
Assumes 3-day field effort x 2 surveys during 2015-2016. Equipment
included (e.g., e-fishing boat, additional boat if needed, nets, scales,
coolers, GPS). Assumes permitting is conducted as part of Pilot Study 1.
Includes scale age analysis.
Assumes one review cycle on draft and final report.
Pilot Study 4—Assessment of Reservoir Food Web Structure
Objectives
The objective of the study is to characterize the food web structure in Soulajule Reservoir
as a means of informing potential mercury bioaccumulation management approaches
involving biomanipulation and (potentially) water level fluctuation.
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Hypotheses
The study will test the following hypotheses:
The food web structure in Soulajule Reservoir is dominated by pelagic rather than
littoral primary productivity.
The relative dominance of blue-green algae in Soulajule Reservoir phytoplankton
limits mercury bioaccumulation in the pelagic portion of the food web because bluegreen algae are disproportionately not consumed by planktivorous biota (e.g.,
zooplankton).
The relative dominance of blue-green algae in Soulajule Reservoir phytoplankton
exacerbates mercury bioaccumulation in the profundal portion of the food web
(e.g., crayfish, catfish) because detritivores do not selectively exclude senescent blue
green algae from their diets.
Approach
The study of food webs structure in Soulajule Reservoir will occur in two phases. In Phase I,
the food web structure of Soulajule will be assessed using stomach content sampling of
larger TL3 (15-35 cm FL) and TL4 (>15 cm FL) fish. Because fish stomach content varies by
season (Power et al. 2002, Clarke et al. 2005) and is difficult or impossible to perform on
smaller fish and invertebrates (Vander Zanden et al. 2000), this sampling technique is not
sufficient to address the study hypotheses. Alternately, stable isotope analysis (SIA)
measures ratios of carbon and nitrogen isotopes (δ13C/δ12C, δ15N/δ14N) in biota at different
trophic levels in order to determine spatial dietary niches in an aquatic ecosystem.
Organisms fractionate carbon and nitrogen differently during cellular metabolism, resulting
in predictably enriched or depleted isotope ratios. For example, free-floating
phytoplankton tend to be enriched in δ13C, while attached benthic algae tend to be δ13C
depleted. Ratios of δ13C/δ12C in fish muscle tissue tend to be similar to those of the prey
consumed (Vander Zanden and Rasmussen 1996), while δ15N is enriched from prey to
predator by approximately 3-4 %, indicating the trophic position of the fish (DeNiro and
Epstein 1981, Cabana and Rasmussen 1994, Vander Zanden and Rasmussen 1996).
Combined isotopic signatures of both δ13C and δ15N can yield a time-integrated diet
estimate based on spatial and trophic feeding niches (Christensen and Moore 2009), thus
SIA and can be used to address the study hypotheses presented above. However, because
possible diet sources should be identified before SIA is conducted, initial stomach content
sampling will be conducted in order to provide a “snapshot” of immediate feeding trends
at higher trophic levels and set the stage for the more comprehensive analysis of food web
structure using SIA.
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Implementation of Phase II SIA will be dependent on the results of the stomach content
sampling and any pertinent information gained from characterization of the extent of the
littoral zone (Study 2, Section 6.2) and/or the fish community composition study (Study 3,
Section 6.3). The below methods are focused on Phase I.
6.4.3.1
Field collection methods
Gastric lavage (i.e., stomach pumping) samples will be taken from larger TL3 (15-35 cm FL)
and TL4 (>15 cm FL) fish collected during the fish community composition survey (Study 3,
Section 6.3); if possible, some fish <15 cm FL may also be lavaged. A hose line will be
inserted into the fish and attached to a container filled with trace metal water. The water,
under slight pressure, will provide a steady stream from the hose end. While held upside
down, the fish’s mouth will be positioned so the hose can reach through the mouth and
into the esophagus. Water will be streamed into the stomach cavity causing the stomach
contents to extract out the mouth and into a collection container. Gentle stroking (lavage)
of the stomach from the posterior end to the anterior end will facilitate extraction.
Individual fish samples will be frozen for Phase II SIA (and/or mercury) analysis.
If a stomach content sample is able to be collected, it will be filtered through a fine-mesh
net and preserved for storage. Fish-specific information such as length, weight, species,
date, and capture location will be recorded for subsequent analysis.
6.4.3.2
Laboratory analysis
Stomach content analysis will enumerate each food item and type (i.e., invertebrate,
mollusk, fish) and obtain a combined wet weight to the nearest 0.1 g of the combined
stomach contents. Each preserved sample will be retained for further potential analysis for
up to 6 months.
In Phase I, results of the stomach content analysis will be used to develop a food web
conceptual model for Soulajule Reservoir. The results and conceptual model will be
summarized in a technical memorandum, with a primary focus on the implications for
mercury bioaccumulation in the reservoir. The memorandum will also include
recommendations for implementation of the Phase II SIA study and any additional
sampling required.
Food web sampling will take place in association with fish community composition surveys,
which will occur in the fall of 2015 and spring of 2016 (Table 13). Overall, we anticipate that
the study will take approximately 12 months to complete.
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Estimated cost
We anticipate the Phase I effort to take 1 year to complete at an estimated cost of
$20,000–30,000. Because fish collection would occur as part of Study 3 (Section 6.3), the
estimated cost of Phase I includes only actual stomach sampling, data analysis and
reporting.
Table 15. Estimated cost of Pilot Study 4—Assessment of Reservoir Food Web Structure.
Task
Field effort
$8,000 1
Laboratory analysis
$4,800
$10,000
Total
$22,800
1
6.5
Estimated Costs
Assumes one additional field day for each of two Pilot
Study 3 surveys during 2015-2016. Lavage equipment
included. Assumes permitting is conducted as part of
Pilot Study 1.
Pilot Study 5—Evaluation of Reservoir Seasonal Oxygen Demand and
Sediment Response to Hypolimnetic Oxygenation
Objectives
The objective of the study is to fill data gaps related to the design and implementation of a
hypolimnetic oxygenation system (HOS) in Soulajule Reservoir. The study will involve
monitoring and bench-scale laboratory tests to determine water column oxygen demand,
sediment oxygen demand, and sediment flux rates of redox sensitive compounds under
varying redox conditions. Results will inform those aspects of HOS design that involve
system size, extent, and in-reservoir location for the successful control of seasonal
methylmercury production.
Approach
The following monitoring and laboratory testing activities will be undertaken in order to
provide additional information relevant to the design of an HOS in Soulajule Reservoir.
6.5.2.1
Reservoir bathymetry
The District is conducting a bathymetric survey in 2015 to map the lake bottom, particularly
near the dam, in order to identify suitable locations for installing a HOS. This survey work
also will develop an updated hypsographic curve (water surface elevation versus reservoir
volume), which will be used in conjunction with recent dissolved oxygen profiles to
calculate the expected oxygen demand in the deeper waters near the dam, as well as in the
shallow reservoir arms.
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6.5.2.2
Water column oxygen demand
The District collects dissolved oxygen profiles approximately quarterly near the dam, and
DO profiles were collected at numerous locations in the reservoir during April, August, and
December 2012 (Brown and Caldwell and Stillwater Sciences 2013). These profiles would
be used, along with quarterly profile data collected by the District during 2013−2015, to
provide a more accurate estimate of water column oxygen demand for preliminary sizing of
HOS equipment. Additionally, Pilot Study 1 includes the collection of in situ water quality
profiles (water temperature, dissolved oxygen, pH, conductivity, oxidation-reduction
potential [ORP]) at sites near the dam, in the Eastern Arm, and in the Western Arm during
late summer/early fall 2015, winter/early spring 2015/2016 and spring 2016 (Section 6.1).
At the Eastern Arm site, continuous in situ measurements would also be collected in
bottom waters over a 48-hr period during each seasonal survey. The vertical profile and
continuous in situ data collected during Pilot Study 1 by the consultant team would be used
in combination with quarterly data collected by the District to undertake preliminary HOS
sizing, particularly with respect to the extent of seasonal dissolved oxygen depletion in the
reservoir arms.
6.5.2.3
Laboratory bench-scale sediment tests
Pilot Study 5 includes sediment testing to assess sediment oxygen demand (SOD) and
sediment release rates of redox sensitive compounds (i.e., ammonia, nitrate, phosphate,
sulfate, sulfide, dissolved organic carbon, iron, manganese, total mercury and
methylmercury) under oxic versus anoxic conditions. This testing will be conducted using
laboratory bench-scale chambers in a dedicated facility at the Washington State University,
Pullman, Washington. Results will help to determine the design oxygen delivery rate for an
HOS and confirm that maintenance of an oxygenated sediment-water interface will
improve water quality by suppressing the sediment release of redox sensitive compounds.
SOD incubations for HOS support
SOD chamber samples will be collected at up to four profundal sites in Soulajule Reservoir,
including two sites near the dam, one in the Eastern Arm proximal to the Cycle Mine, and
one in the Western Arm. For each site, materials to fill three SOD sediment chambers will
be collected either from a boat using an Ekman dredge or by divers. Sediments will be
collected when the water column is fully mixed (i.e., following destratification in the late
fall/early winter or prior to stratification in late spring/early summer).
SOD incubations will consist of three phases. For Phase 1, chambers will be incubated
under quiescent conditions and dissolved oxygen in chamber water overlaying sediment
will be monitored with time. For Phase 2, chambers will be installed onto a mixing
apparatus and mixed at a mean water velocity of ~1 centimeter per second (cm/s) and
chamber water overlaying sediment will be monitored with time. For Phase 3, chambers
will be installed onto a mixing apparatus and mixed at a mean water velocity of ~4 cm/s
and chamber water overlaying sediment will be monitored with time. Chambers are made
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of Plexiglas and are 9.5 cm in diameter and 10 cm in height. All chambers will be incubated
in the dark at in situ summertime bottom water temperature (12–14 °C).
Sediment flux incubations for HOS support
Flux chamber samples will be collected at up to four sites in Soulajule Reservoir, including
two sites near the dam, one in the Eastern Arm proximal to the Cycle Mine, and one in the
Western Arm. For each site, materials to fill four sediment chambers will be collected
either from a boat using an Ekman dredge or by divers. Sediments will be collected when
the water column is fully mixed (i.e., following destratification in the late fall/early winter
or prior to stratification in late spring/early summer).
Sediment flux testing will consist of three phases. For Phase 1, chambers will be topped
with filtered lake water and incubated under oxygenated conditions. For Phase 2,
chambers will be topped with filtered lake water and incubated under anaerobic
conditions. Periodic water quality samples will be collected and analyzed for the
aforementioned redox sensitive constituents (i.e., ammonia, nitrate, phosphate, sulfate,
sulfide, dissolved organic carbon, iron, manganese, total mercury and methylmercury). For
Phase 3, chambers will be exposed to varying conditions, corresponding to diurnal
dissolved oxygen concentrations measured over a 48-hr period in the Eastern Arm as part
of Pilot Study 1. Additional sediment samples may be required for Phase 3; if so, they will
be collected prior to reservoir stratification in spring 2016. Chambers are made of
polycarbonate and are 9.5 cm in diameter and 20 cm in height. Chambers will be incubated
in the dark at in situ summertime bottom water temperatures (12–14 °C).
Analytical methods
Dissolved oxygen in SOD chambers will be measured using Hach HQ40 meters and LDO
(luminescent dissolved oxygen) probes. In flux chambers, total mercury in chamber water
will be quantified using USEPA Method 1631. Methylmercury in chamber water will be
analyzed by USEPA Method 1630. Standard methods will be used for monitoring the redox
sensitive water quality constituents.
Dissolved oxygen profile data (e.g., 2012−2015) will be combined with 2015 bathymetry
data to determine water column oxygen demand in reservoir bottom waters during periods
of stratification. Results of the laboratory sediment chamber studies will be analyzed to
determine how variations in oxygen at and near the sediment water interface affect
concentrations of ammonia, nitrate, phosphate, sulfate, sulfide, dissolved organic carbon,
iron, manganese, total mercury and methylmercury in the overlying water column, and
whether maintenance of an oxygenated sediment-water interface suppresses the release
of redox sensitive compounds from reservoir sediments. Combined, results of the water
column and sediment testing will be used to determine the design oxygen delivery rate for
an HOS. Results will be summarized in a technical memorandum, including
recommendations for HOS configuration and implementation.
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Conducting the required investigations and developing the technical memorandum is
expected to take approximately 4−6 months to complete, beginning in the fall of 2015. A
generalized schedule is outlined below and included in Table 19.
• Preliminary DO demand calculations using existing data (May−June 2015)
• Bathymetry survey (June−August 2015)
• Quarterly DO profiles (March, June, September, December each year)
• Bench-scale sediment chamber testing (October−December 2015)
• Revisions of preliminary DO demand calculations (December 2015)
• Data analysis and technical memorandum (January 2016)
Estimated cost
Based on the effort described above, we anticipate the cost of the study effort to be on the
order of $75,000.
Table 16. Estimated Cost of Pilot Study 5—Evaluation of Reservoir Seasonal Oxygen Demand
and Sediment Response to Hypolimnetic Oxygenation.
Task
Sizing calculations
Laboratory bench-scale sediment tests
Data analysis and technical
memorandum
Total
Estimated Cost 1
$5,000
$45,000
$25,000
$75,000
1 Required
bathymetry and additional DO profile acquisition will be performed
separately by the Districts and are not included in the estimated cost.
6.6
Pilot Study 6—Evaluation of Potential for Mercury Loading from Upland
Soils in the Eastern Arm
Objectives
The primary objective of this pilot study is to fill data gaps associated with potential upland
mercury sources to the Eastern Arm of Soulajule Reservoir. This study will involve field
reconnaissance, sampling, and analysis, and will include the following activities:
a. Observe and document (photograph, map) the area surrounding the Franciscan
and Cycle Mines for disturbed soil that may be mercury tailings.
b. Observe and document (photograph, map) potentially erodible, unvegetated, or
disturbed soils in the vicinity of the Franciscan and Cycle Mines.
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c. Evaluate and photo document the slope, slope length, vegetative cover and other
factors that may affect soil erosion or sediment transport in the vicinity of the
Franciscan and Cycle Mines.
d. Collect and analyze soil samples in the vicinity of the Franciscan and Cycle Mines.
Soil samples will be analyzed for:
i.
Total mercury
ii.
Grain size analysis (soil texture)
e. Collect and analyze soil samples from background locations with similar slope,
aspect and proximity to but not affected by the Franciscan and Cycle Mines. Soil
samples will be analyzed for total mercury and soil texture.
f. Collect and analyze soil samples from background locations with similar slope,
aspect and proximity to but not affected by the Franciscan and Cycle Mines. Soil
samples will be analyzed for total mercury and soil texture.
g. Based on site reconnaissance and soil analytical results, provide a preliminary
assessment of the likelihood that eroding upland soils in the vicinity of the
historical mines are likely to represent a significant source of mercury loading to
Soulajule Reservoir.
Hypotheses
The study will test the following hypotheses:
Upland soils are visibly erodible and are likely to represent a significant source of
mercury loading to Soulajule Reservoir.
Upland soils in the vicinity of the Franciscan and Cycle Mines have a higher total
mercury concentration than background upland soils unaffected by mining or other
anthropogenic activity.
Sampling design
6.6.3.1
Sampling locations
In order to characterize potential upland sources of mercury to Soulajule Reservoir,
surface soil samples will be collected and analyzed for total mercury at multiple
locations in the watershed, with a focus on the Franciscan and Cycle Mines located in
the reservoir’s Eastern Arm (Table 17).
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Table 17. Upland soil mercury characterization soil sampling locations.
Sample
Location
Soil Type
Number of
Samples
Description
Disturbed areas
10
Surface soil samples (0−12 in)
Background soil
samples
4
Surface soil samples (0−12 in) in locations
unaffected by mining or other anthropogenic
activity
Disturbed areas
10
Surface soil samples (0−12 in)
Background soil
samples
4
Upland areas
Erodible upland
areas
≤4
Major creeks
entering the
reservoir
Creek
sediments
2–4
Arroyo Sausal
downstream of
Soulajule Dam
Creek
sediments
2
Franciscan Mine
Cycle Mine
6.6.3.2
Surface soil samples (0−12 in) in locations
unaffected by mining or other anthropogenic
activity
Surface soil samples (0−12 in) from any highly
erodible soils identified in the Eastern Arm of
Soulajule Reservoir. These samples are
contingent on identifying highly erodible areas
during the site reconnaissance.
Sediment of the main creeks that enter
Soulajule Reservoir. These data will provide
additional watershed background values and
will allow an evaluation of relative mercury
contributions compared to the former mine
sites.
These samples will characterize sediments in
two locations downstream of Soulajule Dam,
which may or may not be impacted by
discharges from the reservoir.
Field sampling and handling protocols
Soil samples will be collected using disposable trowels and clean plastic containers.
Sampling techniques described by US EPA Method 1669 (developed for handling water
samples) will be used for collection, handling and analysis of total mercury soil samples.
6.6.3.3
Laboratory soils analysis
Soil total mercury will be quantified using USEPA Method 7471A with a sufficiently low
method detection limit (MDL < 10 ng/g) given the assumed watershed background
soil/sediment concentration of 200 ng/g (SFBRWQCB 2008a). Method 7471A involves a
digestion at room temperature using aqua regia and analysis by cold-vapor atomic
absorption (CVAA).
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The results of the reconnaissance effort and soil sampling will be presented to the
District as a Technical Memorandum (TM). The TM will consist of:
• A summary of the reconnaissance photographic and mapping efforts to identify
tailings or potentially erodible soils.
• A presentation of total mercury concentrations in upland surface soils from the
Franciscan and Cycle Mines compared to background soils adjacent to each mine.
Note: statistical comparisons between soils adjacent to the mines and background
are possible but not recommended.
• A preliminary evaluation of the relative importance of upland soil erosion (from
existing estimates of sediment delivery in the greater Tomales Bay watershed
[e.g., Stillwater Sciences 2007]) compared with creek sediment mercury
concentrations, in order to compare order of magnitude estimates of mercury
loading to Soulajule Reservoir.
• Development of recommendations regarding the need for more quantitative
erosion-related analyses, such as the use of the USDA erosion model Revised
Universal Soil Loss Equation 2 (RUSLE2) to provide a preliminary estimate of
sediment loading mass to Soulajule Reservoir
• Development of recommendations regarding the necessity of a soil erosion
control plan.
The primary tasks associated with upland mercury source loading characterization study
include mobilizing for the field effort, field reconnaissance and sampling, analyzing soil
samples and evaluating data, and developing recommendations. We anticipate this
effort to take 3–4 months to complete. Site reconnaissance during the spring would be
ideal to more easily identify eroding areas, but this timing is not essential.
Estimated cost
Based on the level of effort described above, we anticipate that the upland mercury
source loading characterization study would cost roughly $30,000. Development and
implementation of a soil erosion control plan for the Eastern Arm would be budgeted
separately, based on pilot study results.
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Table 18. Estimated Cost of Pilot Study 6—Evaluation of Potential for Mercury Loading from
Upland Soils in the Eastern Arm.
Task
Site reconnaissance
Laboratory analysis
Report
Total
1
2
6.7
Estimated Cost
$7,000 1
$8,000 2
$15,000
$30,000
Includes 1-day field reconnaissance and soil collection effort for 2 staff.
Assumes District will arrange site access, boat rental, and any necessary
District staff support.
Includes analysis of approximately 38 samples for TotHg and grain size
(Table 10). Assumes approximately $205 per sample ($25 for TotHg, $180
for grain size).
Summary of Pilot Study Schedule and Estimated Costs
For planning purposes, the approximate schedule for each of the aforementioned pilot
studies is summarized in Table 19. Estimated costs for the pilot studies are summarized
in Table 20, including refinements that have incorporated as part of further pilot study
development since the ranking effort. Schedule and estimated costs will be further
refined as needed, in coordination with the District.
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Table 19. Approximate schedule for pilot studies. Anticipated timing for study elements are shown in dark grey shading. Light grey shading
indicates tasks that would provide data for the pilot study, but will be completed separately. Black indicates ‘as needed’ efforts.
Ta s k
Pi l ot Study Study 1—Addi ti ona l
Cha ra cteri za ti on of
Methyl mercury i n Wa ter
a nd Bi ota
2015
2016
2017
Ma y June Jul y Aug. Sept. Oct. Nov. Dec. Ja n. Feb. Ma r. Apr. Ma y June Jul y Aug. Sept. Oct. Nov. Dec. Ja n. Feb. Ma r.
Apr.
Fi el d effort
Di s s ol ved MeHg d/s of da m
La bora tory a na l ys i s
Report
Study 2 — As s es s ment of GIS a na l ys i s
Li ttora l Zone Extent a nd Fi el d effort
Producti vi ty
Report
Study 3—As s es s ment of Fi el d effort
Res ervoi r Fi s h Communi ty La bora tory a na l ys i s
Compos i ti on
Report
Study 4—As s es s ment of
Res ervoi r Food Web
Structure
Study 5—Eva l ua ti on of
Res ervoi r Sea s ona l
Oxygen Dema nd a nd
Sedi ment Res pons e to
Hypol i mneti c
Oxygena ti on
Fi el d effort
La bora tory a na l ys i s
Report
Prel i mi na ry DO dema nd
ca l cul a ti ons
Ba thymetry
Qua rterl y DO profi l es
Bench-s ca l e tes ti ng
Revi s e oxygen dema nd
ca l cul a ti ons
Tech memo
Study 6—Eva l ua ti on of
Potenti a l for Mercury
Loa di ng from Upl a nd
Soi l s i n the Ea s tern Arm
Si te reconna i s s a nce a nd
s oi l s a mpl i ng
Model i ng
Report
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Table 20. Estimated costs for pilot studies. General cost assumptions provided in the “Estimated cost” section for each pilot study.
Pilot Study
Study 1
Study 2
Study 3
Study 4
Study 5
Additional
Characterization
of Methylmercury
in Water and
Biota
Assessment of
Littoral Zone
Extent and
Productivity
Assessment of
Reservoir Fish
Community
Composition
Assessment of
Reservoir Food
Web Structure
Evaluation of Reservoir
Seasonal Oxygen Demand
and Sediment Response to
Hypolimnetic Oxygenation
GIS analysis
-
$3,600
-
-
-
-
Sizing calculations
-
-
-
-
$5,000
-
Field effort (includes permitting
for sample collection)
$97,000
$5,400
$37,000
$8,000
-
$7,000
Algal aerial imagery
$9,000
-
-
-
-
-
Laboratory analysis
$84,000
$4,200
$3,000
$4,800
$45,000
$8,000
$66,400
$8,800
$40,000
$10,000
$25,000
$15,000
Total
$256,700
22,000
$80,000
$22,800
$75,000
$30,000
Cost
Study 6
Evaluation of
Potential for
Mercury
Loading from
Upland Soils in
the Eastern Arm
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7
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Appendices
Final Study Plan
Appendix A
General Applicability of 17 In-lake and 5 Watershed
Management Methods to Soulajule Reservoir and Arroyo
Sausal Methylmercury Control
Final Study Plan
Table A-1. General applicability and associated rationale for each of the 17 potential in-lake and 5
watershed management methods discussed at the Soulajule Reservoir screening workshop.
Method
1
2
3
4
5
6
7
8
9
10
11
12
13
Applicability
Rationale
In-lake/Reservoir Method
Would remove legacy mercury
Potentially
Dredging
accumulation in Eastern Arm
applicable
sediments, but cost could be high.
More oxidized littoral sediments may
Water level fluctuation
Uncertain
result in less MeHg but subsequent
flooding could produce more MeHg.
Primary method for reducing blueMixing and/or
green algae and increasing UV-driven
Applicable
destratification
demethylation.
Macrophyte harvesting
Not applicable
Not enough present for any effect.
Specialized wetlands and additional
Wetland filters (fringe)
Not applicable
circulation would be needed to
reduce methylation at lake scale.
Difficult to achieve high harvest
Algae harvesting
Not applicable
efficiency at scale of reservoir.
Currently, hypolimnion withdrawals
support cool downstream water
Selective withdrawal of
temperatures in summer months,
Not applicable
hypolimnion
would not want to withdraw warmer
surface waters.
Insufficient clean water available and
Dilution/flushing
Not applicable
would not address source of
methylation.
Could be effective for legacy mercury
Sediment sealing/capping
Potentially
in Eastern Arm sediments, but not
(fabrics)
applicable
new inputs. Likely to be a high cost.
May be effective but would require
Algaecides/herbicides or
very large amounts of algaecide at a
Not applicable
molluscicides
high cost
Hypolimnetic no-bubble plume pure
oxygenation would be best way to
eliminate methylation by providing a
dedicated oxygen plume over the
sediments. Increased water clarity
Oxygenation/aeration/nitrate
would also increase demethylation
Applicable
addition
potential. For nitrate addition, is
likely less efficient than pure oxygen
and there is a risk of increasing
anoxia and methylation if algal
growth is stimulated by nitrogen.
Typically a short-term effect, would
require very large amounts of dye,
Shading/dyes
Not applicable
and would decrease natural
demethylation.
Sediment sealing (chemical,
Not clearly proven with respect to
Not applicable
alum, etc.)
mercury control.
Stillwater Sciences
September 2015
A-1
Final Study Plan
Method
Applicability
14
Pathogens/diseases of algae
Not applicable
15
Grazers (on algae or
macrophytes)
Not applicable
16
Nutrient/toxicant harvesting
from fish/weeds
Not applicable
17
Biomanipulation
Potentially
applicable
Rationale
Difficult to implement successfully
and algae can build up resistance.
Blue-green algae, the primary species
in Soulajule, are generally inedible so
grazing unlikely to be successful.
Would harvest only a tiny fraction of
methylmercury and would not
address source of methylation.
Top down – stock predator fish or
harvest prey fish. Would decrease
phytoplankton but increase potential
SAV MeHg production.
Top down – remove predator fish or
stock prey fish for somatic growth
dilution at higher trophic levels.
Bottom up – adding nutrients would
dilute MeHg but may increase
bottom anoxia & MeHg production.
Watershed Method
1
Land re-sculpture & stream
re-routing
Not applicable
2
Standard erosion control
measures (BMPs)
Potentially
applicable
3
Unit process natural
treatment (UPNT) wetlands
Not applicable
4
Volatilization in “dry”
cultivated wetlands
Not applicable
5
Volatilization in crops
Not applicable
Other than re-routing of a stream,
land re-sculpture methods are not
likely to be possible due to cost.
Standard erosion control measures
may be able to stabilize hillsides near
historical mines, if necessary.
Must be designed to sequester
bioavailable MeHg – no open or
accessible water (dry surface =
surface or sub-surface or aerated
surface flow design), so required area
would be too large to treat whole
reservoir.
Required area would be too large to
treat whole reservoir.
Volatilization crop needs hollow
stems & high stem cutting during
season, required area would be too
large to treat whole reservoir.
Stillwater Sciences
September 2015
A-2
Final Study Plan
Appendix B
Ranking Detail for Potential Management Methods
Final Study Plan
Biomanipulation - sport fish stocking
Criteria
Weighting
Factor
Negative
1
2
Rating Descriptions
Neutral
3
Positive
4
Rating
Weighted
Rating
5
Notes/Comments
Compatibility with water supply
objectives
3
Not compa ti bl e or l i mi ted
compa ti bi l i ty
Compa ti bl e
Enha nces a bi l i ty to meet objecti ves
3
9
Effectiveness
(MeHg water)
3
Li kel y or potenti a l l y l i kel y to res ul t
i n a n i ncrea s e i n wa ter col umn
MeHg
No effect on wa ter col umn
MeHg
Somewha t effecti ve or extremel y
effecti ve a t decrea s i ng wa ter col umn
MeHg
3
9
Coul d decrea s e MeHg i n wa ter through reducti on i n a l ga e a nd a va i l a bl e
orga ni c ca rbon for methyl a ti on, whi ch woul d i ncrea s e ra nki ng to 4.
Res ul ts of Pi l ot Studi es 5 & 6 woul d be i nforma ti ve.
Effectiveness
(MeHg fish)
3
i n a n i ncrea s e i n fi s h ti s s ue MeHg
No effect on fi s h ti s s ue
MeHg
effecti ve a t decrea s i ng fi s h ti s s ue
MeHg
5
15
Di rect effect on a vera ge MeHg concentra ti on i n s port fi s h.
12
Res ervoi r food web a ppea rs rel a ti vel y ba l a nced now, s o a ddi ng s port fi s h
i s not l i kel y to di s rupt the food web i n a bi g wa y. Propos e a da pti ve
ma na gement a pproa ch to s tocki ng l evel s , i n order to a voi d s port fi s h
a bunda nce es ti ma tes . As s umes tha t s howi ng i mprovement i n fi s h ti s s ue
concenta ti on i s s uffi ci ent for TMDL.
3
Hi gh or modera tel y hi gh
Modera te
Li mi ted or none
3
Hi gh (>$5M) to modera tel y hi gh
(>$3M to $5M)
Modera te (>$1M to $3M)
Modera tel y l ow ($500K to $1M) to l ow
(<$500K)
5
15
As s umi ng $25K a nnua l cos t ra ther tha n the l ower numbers i n Ta s k 3 TM.
As s ume goa l i s to reduce s tocki ng over ti me. NPV for 5 yea rs = $100K.
As s ume thi s a pproa ch i n ta ndem wi th other i n-l a ke ma na gement
a pproa ches for ma xi mum benefi t.
Cost
Multiple benefits
2
Woul d or ma y i nterfere wi th other
benefi ts
No a ddi ti ona l benefi ts
Potenti a l for or certa i nty of mul ti pl e
benefi ts
4
8
Top-down bi oma ni pul a ti on reduces a l ga e. Coul d i nterfere wi th na ti ve
s peci es i n Arroyo Sa us a l , a l though onl y a s a n i ncrementa l effect, s i nce
l a rgemouth ba s s a re a l rea dy i n the res ervoi r, s o ra ti ng drops from a 5 to 4.
2
Surroundi ng l a ndowners woul d not
or ma y not s upport
No effect on s urroundi ng
l a ndowners
Surroundi ng l a ndowners woul d
potenti a l l y or defi ni tel y s upport
3
6
2
Short-term trea tment (0 to 1 yr)
Modera tel y l ong trea tment
(> 1 to 5 yrs )
Long-term trea tment (> 5 yrs )
4
8
Engineering requirements
2
Intens i ve
Modera te
Li mi ted or none
5
10
O&M requirements
2
Intens i ve
Modera te
Li mi ted or none
5
10
Permitting
2
Intens i ve
Modera te
Li mi ted or none
3
6
Need to work w/CDFW re: s peci es
Implementation time
1
Long (> 3yrs to 5 yrs ) or very l ong (> 5
yrs )
Modera te (2-3 yrs )
Short (1-2 yrs ) or very s hort (<1 yr)
5
5
Woul d s ee i mmedi a te res pons e b/c l owers a vera ge concentra ti on i n fi s h
a s s oon a s s tocki ng occurs .
1
yrs )
5
5
Ra nki ng ba s ed on i mmedi a te effecti venes s but note tha t broa der food
web benefi ts woul d ta ke l onger to rea l i ze. Res ul ts of Pi l ot Studi es 5 & 6
woul d be i nforma ti ve.
Recreation effects
1
Woul d ha ve or ma y ha ve a nega ti ve
effect
No effect
Ma y ha ve or woul d ha ve a pos i ti ve
effect
5
5
Increa s e i n s port fi s hi ng/res ervoi r no l onger l i s ted
1
Expens i ve a nd/or ti me-i ntens i ve
s tudi es
Modera tel y expens i ve
a nd/or ti me-i ntens i ve
s tudi es
Li mi ted cos t a nd/or ti me-i ntens i ve
s tudi es or none
3
3
Requi res Pi l ot Studi es 5 & 6 ($85-110K combi ned) a nd potenti a l l y el ements
of Study 7 ($130-150K)
Risk of failure
4
As s umes ongoi ng trea tments a re s i mi l a r to oxygena ti on, VEM. Propos i ng a
goa l of ta peri ng off, a s s umi ng other techni ques i n pl a ce, a s s ume a round 5
yrs
126
81
Stillwater Sciences
September 2015
B-1
Final Study Plan
Hypolimnetic oxygenation system (HOS)
Criteria
Weighting
Factor
Negative
1
2
Rating Descriptions
Neutral
3
Positive
4
Rating
Weighted
Rating
5
Notes/Comments
objectives
3
compa ti bi l i ty
Compa ti bl e
5
15
Effectiveness
(MeHg water)
3
MeHg
MeHg
MeHg
5
15
Speece cone i s hi ghl y effecti ve; a s s umes pl a cement nea r da m i n deepes t
porti on of res ervoi r
Effectiveness
(MeHg fish)
3
MeHg
MeHg
4
12
As s umes fi s h res pons e ba s e on wa ter col umn res pons e. However, we
don't know i f Ea s tern Arm i s more cri ti ca l for control l i ng bi oa ccumul a ti on.
Res ul ts of Study 7 woul d be i nforma ti ve.
Risk of failure
3
Modera te
Li mi ted or none
5
15
As s umes oxygen pl ume wi l l extend i nto Ea s tern a nd Wes tern a rms i f
s ys tem i s overs i zed; however, we don't know yet i f rea chi ng thes e
l oca ti ons i s cri ti ca l for control l i ng bi oa ccumul a ti on. Res ul ts of Study 7
woul d be i nforma ti ve.
Cost
3
(>$3M to $5M)
(<$500K)
3
9
LOX cos t $20-30K for 5 yea rs ; NPV = $100-200K for ten yea rs , di s count ra te
0.08.
Multiple benefits
2
benefi ts
benefi ts
5
10
Addi ti ona l benefi ts = reduce BG a l ga e, geos mi n; reduce rel ea s e of orthoP, Mn, Fe2+, NH4+ from a noxi c s edi ments
2
l a ndowners
3
6
As s umes LOX a t exi s ti ng pump s ta ti on on Di s tri ct property
2
(> 1 to 5 yrs )
5
10
As s umes ongoi ng trea tment
2
Intens i ve
Modera te
Li mi ted or none
2
4
Wi l l a l s o need di ver s urveys , s ome geotech, need to cons i der s ubmerged
ori gi na l da m i n des i gn.
O&M requirements
2
Intens i ve
Modera te
Li mi ted or none
3
6
Need to run compres s ors , oxygen del i very, etc.
Permitting
2
Intens i ve
Modera te
Li mi ted or none
2
4
Wi l l need CEQA, coordi na ti on w/Regi ona l Boa rd
Implementation time
1
yrs )
4
4
Cone i s pre-fa bri ca ted, other components a re rel a ti vel y s i mpl e
1
yrs )
5
5
Recreation effects
1
effect
No effect
effect
5
5
As s umes decrea s ed BG a l ga e a nd better ha bi ta t for s port fi s h
1
s tudi es
s tudi es
s tudi es or none
3
3
Pi l ot Studi es 3 ($200-250K) & 7 ($130-150K)
123
79
Stillwater Sciences
September 2015
B-2
Final Study Plan
Upland erosion control in Eastern Arm
Criteria
Weighting
Factor
Negative
1
2
Rating Descriptions
Neutral
3
Positive
4
Rating
Weighted
Rating
3
9
4
12
As s umes tha t eros i on i s a n externa l s ource of Hg. Requi res Pi l ot Study 1.
Woul d requi re s i mul ta neous i n-l a ke a pproa ch to be effecti ve.
4
12
As s umes tha t eros i on i s a n externa l s ource of Hg. Requi res Pi l ot Study 1.
Woul d requi re s i mul ta neous i n-l a ke a pproa ch to be effecti ve.
Ma y be ca ttl e gra zi ng i mpa cts to BMP i ns ta l l a ti ons . Steep s l opes , ha rder
for BMPs to be effecti ve. We don't thi nk thi s wi l l "fa i l " a nd crea te a wors e
Hg probl em, but i t ma y not enough to s ol ve probl em.
5
Notes/Comments
objectives
3
compa ti bi l i ty
Compa ti bl e
Effectiveness
(MeHg water)
3
MeHg
MeHg
Effectiveness
(MeHg fish)
3
MeHg
Risk of failure
3
Modera te
Li mi ted or none
3
9
Cost
3
(>$3M to $5M)
(<$500K)
4
12
Multiple benefits
2
benefi ts
benefi ts
3
6
Sma l l a rea i n mi ne vi ci ni ty rel a ti ve to res t of wa ters hed, s o a ny potenti a l
gra zi ng i s s ues s houl d be mi ni ma l .
2
l a ndowners
3
6
Need to confi rm tha t there i s l i ttl e to no ca ttl e gra zi ng nea r the mi ne s i tes .
If there IS gra zi ng, ra te thi s a s a 2 b/c of potenti a l l a ndowner concerns re;
gra zi ng l i mi ta ti ons /need for fenci ng/etc.
2
(> 1 to 5 yrs )
5
10
2
Intens i ve
Modera te
Li mi ted or none
3
6
O&M requirements
2
Intens i ve
Modera te
Li mi ted or none
4
8
Permitting
2
Intens i ve
Modera te
Li mi ted or none
4
8
Implementation time
1
yrs )
4
4
1
yrs )
5
5
Recreation effects
1
effect
No effect
effect
3
3
1
s tudi es
s tudi es
s tudi es or none
4
4
MeHg
MeHg
As s umes tha t there i s l i ttl e or no ca ttl e gra zi ng. If there IS gra zi ng, ra te
thi s a s a 3.
Some coordi na ti on wi th Regi ona l Boa rd l i kel y, but s houl d be rel a ti vel y
ea s y to a pprove
Requi res Pi l ot Study 1, cos t i s $75-100K, rel a ti vel y s tra i ghtfowa rd
114
74
Stillwater Sciences
September 2015
B-3
Final Study Plan
Vigorous epilimnetic mixing (VEM) in Eastern Arm
Criteria
Weighting
Factor
Negative
1
2
Rating Descriptions
Neutral
3
Positive
4
Rating
Weighted
Rating
4
12
Decrea s es BGs a nd geos mi n, i mprovements i n DO, but not a s much a s
oxygena ti on vi a Speece cone
4
12
Supports photodemethyl a ti on of MeHg (a nd pos s i bl y vol i ta l i za ti on)
4
12
As s umes wa ter col umn MeHg decrea s e. Woul d benefi t from res ul ts of
Pi l ot Studi es 6 & 7
3
9
VEM for mercury control i s yet untes ted - i n theory thi s s houl d work, but we
don't ha ve da ta to demons tra te s ucces s
5
Notes/Comments
objectives
3
compa ti bi l i ty
Compa ti bl e
Effectiveness
(MeHg water)
3
MeHg
MeHg
Effectiveness
(MeHg fish)
3
MeHg
Risk of failure
3
Modera te
Li mi ted or none
Cost
3
(>$3M to $5M)
(<$500K)
3.5
10.5
Multiple benefits
2
benefi ts
benefi ts
4
8
Addi ti ona l benefi ts = reduce BG a l ga e, but coul d i ncrea s e overa l l chl -a
a nd producti vi ty; potenti a l to reduce rel ea s e of ortho-P, Mn, Fe2+, NH4+
from a noxi c s edi ments
2
l a ndowners
3
6
No rea l i mpa ct b/c pi pes woul d be on bottom of res ervoi r a nd woul d
ori gi na te a t the da m; ca n s oundproof; ma y requi re s ma l l s tructure ons hore
wi th pres s ure regul a tors , or coul d put thi s equi pment underwa ter
2
(> 1 to 5 yrs )
5
10
2
Intens i ve
Modera te
Li mi ted or none
3
6
Requi rements s omewha t l es s tha n Speece cone
O&M requirements
2
Intens i ve
Modera te
Li mi ted or none
3
6
Requi rements s omewha t l es s tha n Speece cone
Permitting
2
Intens i ve
Modera te
Li mi ted or none
3
6
Implementation time
1
yrs )
4
4
1
yrs )
5
5
Recreation effects
1
effect
No effect
effect
3
3
1
s tudi es
s tudi es
s tudi es or none
3
3
MeHg
MeHg
Woul d a l s o need ba thymetry & di ver s urveys , not refl ected i n current cos t
es ti ma tes
Woul d benefi t from res ul ts of Pi l ot Study 7 ($130-150K)
112.5
73
Stillwater Sciences
September 2015
B-4
Final Study Plan
Biomanipulation - prey fish stocking
Criteria
Weighting
Factor
Negative
1
2
Rating Descriptions
Neutral
3
Positive
4
Rating
Weighted
Rating
3
9
5
Notes/Comments
3
compa ti bi l i ty
Compa ti bl e
Effectiveness
(MeHg water)
3
MeHg
MeHg
MeHg
3
9
Coul d ha ve a nega ti ve effect vi a trophi c ca s ca de a nd s ubs equent
i ncrea s edi n BG a l ga e a nd a va i l a bl e orga ni c ca rbon for methyl a ti on.
Woul d be i nformed by Pi l ot Studi es 5 & 6. Mi ght go to 2, dependi ng on
pi l ot s tudy res ul ts .
Effectiveness
(MeHg fish)
3
MeHg
MeHg
4
12
Potenti a l to reduce prey a nd preda tor fi s h concentra ti ons , a l though need
food web s tudy
6
Res ervoi r food web a ppea rs rel a ti vel y ba l a nced now, not s ure how a ddi ng
prey fi s h woul d a ffect food web. Ada pti ve ma na gement a pproa ch to
s tocki ng l evel s . As s umes tha t s howi ng i mprovement i n fi s h ti s s ue conc i s
s uffi ci ent, even i f not meeti ng TMDL ta rget ri ght a wa y.
objectives
Risk of failure
3
Cost
2
Modera te
Li mi ted or none
3
(>$3M to $5M)
(<$500K)
5
15
As s umi ng $25K a nnua l cos t ra ther tha n the l ower numbers i n Ta s k 3 TM.
As s ume goa l i s to reduce s tocki ng over ti me. NPV for 5 yea rs = $100K.
As s ume thi s a pproa ch i n ta ndem wi th other i n-l a ke ma na gement
a pproa ches for ma xi mum benefi t.
Multiple benefits
2
benefi ts
benefi ts
2
4
Food web effect not cl ea r, coul d i ncrea s e a l ga e a nd chl -a .
2
l a ndowners
3
6
2
(> 1 to 5 yrs )
4
8
2
Intens i ve
Modera te
Li mi ted or none
5
10
O&M requirements
2
Intens i ve
Modera te
Li mi ted or none
5
10
Permitting
2
Intens i ve
Modera te
Li mi ted or none
3
6
Implementation time
1
yrs )
5
5
1
yrs )
3
3
Longer res pons e ti me for l ower MeHg concentra ti ons i n pi s ci vorous fi s h
Recreation effects
1
effect
No effect
effect
4
4
Woul d i mprove s port fi s h MeHg concentra ti on, mi ght ha ve better-fed
preda tors , but food web effects uncl ea r
1
s tudi es
s tudi es
s tudi es or none
3
3
Requi res Pi l ot Studi es 5 & 6 ($85-110K combi ned) a nd potenti a l l y el ements
of Study 7 ($130-150K)
As s umes ongoi ng trea tments a re s i mi l a r to oxygena ti on, VEM. Propos i ng a
goa l of ta peri ng off, a s s umi ng other techni ques i n pl a ce, a s s ume a round 5
yrs
Need to work w/CDFW re: s peci es
110
71
Stillwater Sciences
September 2015
B-5
Final Study Plan
Dredging of reservoir sediments in Eastern Arm
Criteria
Weighting
Factor
Negative
1
2
Rating Descriptions
Neutral
3
Positive
4
Rating
Weighted
Rating
3
9
4
12
4
12
5
Notes/Comments
objectives
3
compa ti bi l i ty
Compa ti bl e
Effectiveness
(MeHg water)
3
MeHg
MeHg
Effectiveness
(MeHg fish)
3
MeHg
Risk of failure
3
Modera te
Li mi ted or none
5
15
Cost
3
(>$3M to $5M)
(<$500K)
1
3
Multiple benefits
2
benefi ts
benefi ts
3
6
2
l a ndowners
3
6
2
(> 1 to 5 yrs )
5
10
2
Intens i ve
Modera te
Li mi ted or none
2
4
O&M requirements
2
Intens i ve
Modera te
Li mi ted or none
5
10
No l ong-term O&M (i .e., dewa teri ng a nd s edi ment tra ns fer a re s hort-term
O&M/pa rt of cons tructi on)
Permitting
2
Intens i ve
Modera te
Li mi ted or none
1
2
Di ffi cul t to permi t
Implementation time
1
yrs )
2
2
1
yrs )
5
5
Recreation effects
1
effect
No effect
effect
3
3
1
s tudi es
s tudi es
s tudi es or none
1
1
MeHg
MeHg
Potenti a l for s ma l l i ncrea s e i n res ervoi r vol ume (~8-35 a cre-ft)
As s umes Ea s tern Arm s edi ments a re non-mobi l e. As s umes tha t upl a nd
eros i on i s not a conti nuous externa l s ource of Hg.
Requi res Pi l ot Study 2, cos t i s $50-70K, Pi l ot Study 7 ($130-150K), pl us
ba thymetry, di vers
100
65
Stillwater Sciences
September 2015
B-6
Final Study Plan
Capping of reservoir sediments in Eastern Arm
Criteria
Weighting
Factor
Negative
1
2
Rating Descriptions
Neutral
3
objectives
3
compa ti bi l i ty
Compa ti bl e
Effectiveness
(MeHg water)
3
MeHg
MeHg
Effectiveness
(MeHg fish)
3
MeHg
Positive
4
Rating
Weighted
Rating
3
9
4
12
As s umes tha t el eva ted Hg s edi ments i n i n Ea s tern Arm a re i mporta nt
pa thwa y for bi oa ccuml a ti on. Requi res res ul ts of Pi l ot Study 7.
4
12
As s umes tha t el eva ted Hg s edi ments i n i n Ea s tern Arm a re i mporta nt
pa thwa y for bi oa ccuml a ti on. Requi res res ul ts of Pi l ot Study 7.
Potenti a l i s s ues wi th res ervoi r wa ter l evel fl uctua ti on, ca ttl e i ntera cti ng
wi th l i ner a t l ow wa ter l evel s . Wi th fa bri c l i ner, ma y need to cons i der
producti on of metha ne or other ga s es i n s edi ments . Ma y wa nt to cons i der
s a nd/cl a y l i ner (e.g., Ononda ga ).
5
MeHg
MeHg
Risk of failure
3
Modera te
Li mi ted or none
2
6
Cost
3
(>$3M to $5M)
(<$500K)
1
3
Multiple benefits
2
benefi ts
benefi ts
3
6
2
l a ndowners
3
6
2
Modera tel y l ong trea tment (>
1 to 5 yrs )
5
10
2
Intens i ve
Modera te
Li mi ted or none
2
4
O&M requirements
2
Intens i ve
Modera te
Li mi ted or none
3
6
Permitting
2
Intens i ve
Modera te
Li mi ted or none
2
4
Implementation time
1
yrs )
2
2
1
yrs )
5
5
Recreation effects
1
effect
No effect
effect
3
3
1
s tudi es
Modera tel y expens i ve a nd/or
ti me-i ntens i ve s tudi es
s tudi es or none
3
3
Notes/Comments
Rel a ti vel y s ma l l a rea , ca ppi ng i s not l i kel y to confer a ddi ti ona l benefi ts
re: ortho-P or other nutri ent rel ea s es .
As s umes Ea s tern Arm s edi ments a re non-mobi l e. As s umes tha t upl a nd
eros i on i s not a conti nuous externa l s ource of Hg.
Potenti a l i s s ues wi th res ervoi r wa ter l evel fl uctua ti on, ca ttl e i ntera cti ng
wi th l i ner a t l ow wa ter l evel s , potenti a l for growth of l i ttora l veg tha t
woul d i mpa ct l i ner s ta bi l i ty.
Requi res Pi l ot Study 2, cos t i s $50-70K, Pi l ot Study 7 ($130-150K), pl us
ba thymetry, di vers
91
59
Stillwater Sciences
September 2015
B-7

Soulajule Reservoir Mercury Occurrence and Bioaccumulation Study

Transcription

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