Iodine as a sensitive tracer for detecting influence of organic

Applied Geochemistry 60 (2015) 29–36
Contents lists available at ScienceDirect
Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
Iodine as a sensitive tracer for detecting influence of organic-rich shale
in shallow groundwater
Zunli Lu a,⇑, Sunshyne T. Hummel a, Laura K. Lautz a, Gregory D. Hoke a, Xiaoli Zhou a, James Leone b,
Donald I. Siegel a
a
b
Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USA
Reservoir Characterization Group, New York State Museum, Albany, NY 12230, USA
a r t i c l e
i n f o
Article history:
Available online 28 November 2014
a b s t r a c t
Public and regulatory agencies are concerned over the potential for drinking water contamination related
to high-volume hydraulic fracturing (hydrofracking) of the Marcellus shale in Pennsylvania and in New
York State (NYS), where exploitation of Marcellus gas has not yet begun. Unique natural tracers are
helpful for distinguishing the influence of formation water and/or flow-back water. Here we use halogen
concentrations, particularly bromine and iodine, to characterize natural variability of baseline water
chemistry in the southern tier of NYS. Majority of streams and drinking water wells have Br and I concentrations below 1 and 0.1 lM, respectively, a range typical for relatively pristine surface water and
shallow groundwater. Wells that have higher Br and I concentrations are likely affected by formation
waters. Br/I ratios indicate two different sources of formation waters in these wells, possibly controlled
by geologic settings. Our results suggest that iodine, combined with other halogens, may be a novel and
sensitive tool for fingerprinting trace levels of formation water signal in drinking water sources.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Shale gas extraction from the Marcellus formation in the Appalachian Basin has increased with technological improvements in
horizontal drilling and hydraulic fracturing (Kerr, 2010; Soeder,
2010). The increase in hydraulic fracturing has led to environmental concerns regarding potential shallow aquifer contamination.
Primary concerns are contamination from hydraulic fracturing fluids, and also from produced brines during their transport, drilling,
and disposal (Dresel and Rose, 2010; Rowan et al., 2011), including
stray gas migration to shallow aquifers (Osborn et al., 2011).
Hydrocarbon bearing formations in sedimentary basins typically contain water with high salinity, which can exceed 100 g/L,
due to seawater evaporation and halite precipitation during the
basin’s evolution (Dellwig and Evans, 1969; Dollar et al., 1991;
Hanor, 1994). With compaction, residual brines disperse into other
stratigraphic layers of the sedimentary basin as it evolves through
geologic time. Hydraulic fracturing of organic-rich shale produces
waters, called flow-back, with high salinity and high concentrations of dissolved solids, trace metals, and organic compounds.
Chloride and bromine have been used to trace groundwater and
surface water flow for decades and distinguish different sources of
⇑ Corresponding author.
E-mail address: zunlilu@syr.edu (Z. Lu).
http://dx.doi.org/10.1016/j.apgeochem.2014.10.019
0883-2927/Ó 2014 Elsevier Ltd. All rights reserved.
salinity, such as road salt and septic effluent in streams and
groundwater. For example, chloride and bromine were used to distinguish contamination of drinking water from oil-field brines,
halite dissolution, and other sources (Townsend and Whittemore,
2005). Other studies show that Cl/Br ratios can effectively distinguish saline water coming from road salt, halite dissolution brines,
and household water softeners (Alley et al., 2011; Davis et al.,
2004, 1998; Knuth et al., 1990; Panno, 2006; Walter et al., 1990).
Halogen ratios are potentially effective for distinguishing formation water from other sources of salinity because halite precipitate
excludes bromine from its crystal structure, causing the remaining
formation waters to have low Cl/Br ratios compared to the seawater they originated from (Herrmann, 1972).
Fresh water overlies saline water in many sedimentary basins
(Feth, 1970). In particular, water wells in valleys, where groundwater discharges from local to regional scale flow systems, can
develop naturally high salinity because of their close proximity
to the saltwater–freshwater interface (Panno, 2006; Warner
et al., 2012). This saline groundwater can have about the same
Cl/Br value as deeper groundwater in formations that commercially produce gas and oil (Haluszczak et al., 2013). For example,
bromine and chloride could not distinguish between Appalachian
Basin brines as a whole from Marcellus formation or older brines
(Warner et al., 2012). Another non-reactive solute, in addition to
bromine and chloride, needs to be considered that can potentially
30
Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36
distinguish and ‘‘fingerprint’’ brines from deep gas producing formations from those in shallow gas producing formations.
Iodine has been used to trace water movement in methane-rich
settings, because pore water iodine and methane mostly derive
from organic matter (Fehn et al., 2000; Lu et al., 2008a,b, 2011;
Martin et al., 1993; Moran et al., 1995; Muramatsu et al., 2007;
Scholz et al., 2010). Organic-rich rocks and associated pore waters
in marine sedimentary basins have elevated iodine concentrations
and low Br/I ratios (Worden, 1996). Therefore iodine is a
potentially sensitive tracer for sedimentary basin brines where it
is associated with geologic formations enriched in organic matter
deposited in marine environments, such as the Marcellus shale.
In this paper, we present the results of an initial geochemical
background study and show the utility of iodine as a tracer for fluids potentially associated with the Marcellus formation of the
Appalachian Basin of the eastern United States. Iodine concentrations, in conjunction with other halogens and isotopic systems,
may be able to distinguish the presence of deep fluids that migrate
upwards into shallow groundwater or spilled on the land surface
from other sources of salinity.
1.1. Iodine geochemistry
Similar to other halogens, iodine is a conservative solute and
iodine concentrations in freshwater are lower than those in the
marine environment (Fig. 1) because iodine is virtually absent from
terrestrial igneous rocks (Fig. 1a). Iodine concentrations in shallow
ground water and surface waters are generally <0.05 lM (Panno
et al., 2006), whereas the average iodine concentration in seawater
is ten times higher, about 0.5 lM (GERM, 2012). Some rare contaminated surface water and shallow ground water have iodine
concentrations of 10 lM derived from municipal landfill leachate
incorporating disinfectants, medical products, and animal feeds/
additives (Panno et al., 2006). Iodized salt is a potential anthropogenic source of iodine; however, elevated iodine concentrations in
ground water due to such contamination have not been reported,
to our knowledge.
Marine origin organic-rich sedimentary rocks and halite are the
two major natural sources of iodine in the terrestrial environment,
and these have distinctive Br/I ratios (Fig. 1a). Iodine is a biophillic
element, strongly enriched in marine organic matter, such as kelp,
and consequently accumulates in marine sediments (Elderfield and
a
I
10
10
10
10
10
10
Br
Cl
1.2. Geologic settings
The Appalachian Basin formed as elongated foreland basin west
of the ancestral Appalachian Mountains with relatively
un-deformed sedimentary rocks deposited throughout the Paleozoic era (Colton, 1970). Repeating sequences of carbonates, shales,
siltstones, and sandstones occur through the Paleozoic section
reflecting several marine transgressions and regressions. The black
shales present throughout the Appalachian Basin contain high
levels of organic matter (Roen, 1983).
A stratigraphic cross-section for the southern tier NYS is shown
in Fig. 2. The lithofacies most relevant for halogens are the shales
and evaporites of Ordovician to Devonian ages. The Marcellus
sub-group is Middle Devonian organic-rich black shale of the Hamilton Group. The remainder of the Hamilton group consists of interbeded shale, mudstone, sandstone, and limestone, deposited
during several cycles of sea-level and redox changes (Sageman
et al., 2003; Straeten et al., 2011). The Geneseo, Middlesex and
Rhinestreet are additional black shale formations in the southern
NYS (Fig. 2). The Steuben County overlaps with all of these shale
formations, while other counties towards the east overlaps mainly
with the Marcellus and Geneseo. The Middlesex and Rhinestreet
are both black shale formations of the Frasnian age (Rickard,
1975). No euxinic condition was suggested for the deposition of
Geneseo formation which has lower TOC than the Marcellus and
(Sageman et al., 2003). The Upper Silurian Salina group consists
of inter-bedded shale, dolomites, and salt deposits (Fig. 2). Utica
Shale is an organic-rich, thermally-mature, black to gray shale,
b
4
10
3
10
μM/l)
10
Truesdale, 1980; Muramatsu and Wedepohl, 1998). When marine
organic matter is buried, microbial/thermal decomposition produces methane and releases iodine. This process leaches out iodine
from the sediments, and concentrates it in interstitial fluids, along
with methane. These methane-rich and iodine-rich fluids escape to
the surface, in modern seeps/gas fields, or get trapped within sedimentary strata which are now located in terrestrial settings. High
concentrations of iodine in groundwater occur in oil/gas field
brines (Moran et al., 1995). Iodine concentrations are enriched in
the Appalachian Basin brines (Fig. 1b), similar to those found in
other oil/gas fields (Bottomley et al., 2002; Dresel and Rose,
2010; Moran et al., 1995; Osborn et al., 2012).
2
1
10
10
10
10
0
10
-1
10
10
-2
10
10
-3
10
Marine Marine
organic sediments
maers
Salt
Terrestrial Igneous
sediments rock
7
6
5
4
3
2
1
0
-1
-2
-3
Oil/gas Appala. Sea Ground Surface Contam
field brines water water water -inated
brines
shallow
water
Fig. 1. Comparison of halogen concentrations in solid materials and fluids (Bottomley et al., 2002; Davis et al., 1998; Dresel and Rose, 2010; Lu et al., 2008a; Moran et al.,
1995; Muramatsu et al., 2007; Muramatsu and Wedepohl, 1998; Osborn et al., 2012; Panno et al., 2006; Warner et al., 2012; Worden, 1996).
31
Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36
W
E
Devonian
Steuben County
Chemung County
Tioga County
Broome County
Present day Sea Level
Sandstone and Shales
Rhinestreet Shale
Tully Limestone
Middlesex Shale
Geneseo Shale
LODI
G E NE S E O
T UL L Y
Grey Hamilton Group and Marcellus Subgroup some black shale and limestone beds
Marcellus Shale
Silurian
MO T T E VIL L E _MB R
S T AF F O R D
MAR C E L L US
O AT K A_C R E E K
C HE R R Y _V AL L E Y
UNIO N_S P R ING S
O NO ND AG A
Onondaga
Limestone
Silurian Salt of the Salina Group Interbedded with Shales
Lockport Limetone
Ordovician
Clinton Group
Medina Sandstone
Queenston Group
T HR
UWA
YT_D
RE
ISNT
C
O NF
N
OR
MIT
Y
D
O
LG
EV
ILE
U
Trenton Limestone
3000 Meters Below Present day Sea Level
Fig. 2. Simplified geologic cross section across southern tier NYS.
derived from the erosion of the Taconic Mountains during the
Ordovician, overlying the Trenton Group strata (SGEIS, 2011).
2. Methods
2.1. Sampling strategy and protocol
We completed a systematic sampling of landowner drinking
water wells and streams from four counties located in the southern
tier of NYS (Fig. 3). Waters from 60 drinking water wells and 20
headwater streams were sampled. An aerial grid of 49 km2 was
placed over the counties of interest (Steuben, Tioga, Broome, and
Chemung) with the explicit goal of collecting one sample within
each grid cell to optimize uniform spatial coverage. The NYS water
well contractor program database, containing data on all water
wells drilled since April 2000 in a GIS format, was combined with
GIS-based county cadastral data to identify potential sampling targets. We targeted deep and maximum penetrating wells within
each grid cell. Wells sampled for this study range in depth from
20 m to 122 m. In addition to shallow groundwater samples, surface water samples were collected from headwater streams in
the counties of interest with upstream drainage areas between
50 and 100 km2.
Standard quality control procedures were followed for field
measurements and sampling. All well waters were collected
upstream of water treatment equipment at either outdoor spigots
or at the water pressure tank. Before sampling, water was purged
from the well until the temperature stabilized. Field blanks were
also taken daily. Water samples were collected in prewashed HDPE
bottles that were triple-rinsed with sample and filled until there
was no headspace. Water samples were filtered with a 0.45-lm
Fig. 3. Sampling locations for stream waters and drinking water wells. NYS formation water wells from Osborn et al. (2012). D60 is the deep well drilled into Marcellus
interval. Marcellus shale rock samples were collected from a nearby core in Yates County. The symbols for Type I to III waters are assigned based on Br/I ratios in Fig. 5.
32
Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36
nylon filter and split into aliquots for various analyses. All samples
were stored at 4 °C before analyses.
2.2. Analytical methods
Chloride concentrations were measured at Syracuse University
via ion chromatography (IC) using a Dionex ICS-2000 with column
AS18 for anions. The IC was calibrated with five internal laboratory
standards and the calibration quality was validated using three
external United States Geological Survey Standard Reference Samples (N103, M178, and P50, http://bqs.usgs.gov/srs/). Percent error
for chloride is less than 3%. Total dissolved iodine and bromine
concentrations were measured on a Bruker Aurora M90 ICP-MS
at Syracuse University. Fresh iodine calibration standards were
prepared before the measurements. Ultrapure potassium iodide
powders were weighed on a microgram balance and diluted in a
tertiary amine matrix with cesium as internal standards. Blanks
were monitored every three samples and calibration standards
were run every six samples. Instrumental error for iodine and bromine concentrations are typically below one percent and are not
reported individually.
To better constrain the source rock halogen composition, two
shale samples were collected from the Marcellus interval in the
Morton Salt core drilled near our study area (Fig. 3). Iodine and
bromine associated with the organic matter were extracted with
an alkaline solution from well-homogenized shale powder. The
solutions were then diluted for concentration measurements on
ICP-MS.
Spatial patterns in iodine concentrations and ratios were compared to the geologic variability of the source rocks using GIS.
Available GIS resources included the Finger Lakes sheet of the
1:250,000 geologic map of NYS (NYS Museum, 1999). Contour lines
were derived from 10 m resolution National Elevation Dataset data
(http://ned.usgs.gov) and isopach maps for the Marcellus shale.
Contour lines for total organic carbon (TOC), depth, thickness,
and thermal maturity of Marcellus shale were obtained from the
NYS Museum and overlaid over the study area. The values TOC,
depth, thickness, and thermal maturity at water wells were estimated by measuring the distance from well to nearest contour line.
3. Results
Histograms for iodine and bromine in all of the samples are
shown in Fig. 4. Samples with concentrations <1 lM for bromine
and <0.1 lM of iodine are considered to represent relatively pristine surface water and shallow groundwater in the region. 35 out
20
of 56 drinking water wells fall in this category and are grouped
as Type I water regardless of their geographic locations (Fig. 5a).
Rest of the wells have elevated concentrations and are grouped
into Type II and III waters with distinct Br/I ratios. All stream
waters have bromine concentrations below 1 lM, except for two
samples (Fig. 5a). Iodine concentrations in stream waters range
between 0.01 and 0.04 lM, a range slightly higher than iodine concentrations reported in an earlier study in western NY (Rao and
Fehn, 1999).
From our groundwater analyses, two separate linear relationships in bromine versus iodine are apparent in the samples with
elevated iodine and bromine concentrations (Fig. 5a). These two
linear trends broadly correspond to two geographic regions. Nine
of the 10 wells located in Steuben County fall on a line defined
by higher Br/I ratios (17.5–26.9), which we classify as Type II
waters. Nine of 11 wells located throughout the remaining three
counties, Chemung, Tioga, and Broome, form the other linear relationship, which is defined by lower Br/I ratios of 1.7 to 6.9 which
we classify as Type III waters. In three wells, Br/I ratios differ from
that found in their geographic grouping (Figs. 3 and 5a). Linear
regressions defined Type II and Type III water average Br/I ratios
(Fig. 5a) as 19.3 and 5.6, respectively.
4. Discussion
4.1. Mixing endmembers and trends
To set up the context for interpreting the significance of Type II
and III water signatures, it is necessary to examine the potential
endmembers in different mixing scenarios. The sources/endmembers for halogens include halite, formation waters and organic rich
shale (Fig. 5a). Due to water–rock interaction, the halogen compositions in formation waters can be affected by halite dissolution
and leaching from shale, thus the compositions may vary vertically
and horizontally within a large basin.
Only a few deep wells in the New York State have been reported
for their bromine and iodine concentrations. The Br/I ratios are
between 17.5 and 49, in formation waters sampled from Middle
to Upper Devonian formations, including the Marcellus shale
(Osborn et al., 2012). Halogen extractions done on Marcellus shale
in this study (Fig. 5a) had an average Br/I ratio of 0.59, much lower
than the ratio in formation waters. Salt deposits are common in the
Appalachian Basin (i.e. Salina Formation; Fig. 2) due to evaporation
of seawater (Hanor, 1994). Halite has low iodine concentrations
(0.1–10 mg/kg) compared to bromine (12.6–1584.9 mg/kg), and a
high Br/I ratio (Schijf, 2007). Water chemistry data from the Salina
a
b
15
Counts
Counts
15
10
10
5
5
0
0
0.0
0.1
0.2
0.3
I (µM)
0.4
0.5
0
1
2
3
4
5
6
Br (µM)
Fig. 4. Histograms for iodine and bromine concentrations in all samples of this study. Majority of the values are below 1 lM for Br and below 0.1 lM for I.
33
Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36
19
R² .32x
= 0 -0.
.89 08
ma
tio
n(D
60
)
or
sf
llu
Ma
rce
sig nge
na r h
tu ali
re te
Type II
8000
6000
Upper
4000
2000
14)
ian (D
Devon
ter (D60)
Marcellus wa
0
0
4.0
0.1
0.2
0.3
0.4
0.5
0.6
I (µM)
Type III
.28
0.01%
2.0
y=
0
7x 5.5 0.88
=
²
R
ic
1.0
14000
Halite
0.02%
3.0
12000
c
10000
Cl (µM)
r organ
Stronge re
signatu
Br (µM)
St
ro
0.03%
5.0
b
10000
Streams
y=
Uppe
r
6.0
12000
Type I
Cl (µM)
(D14
)
0.05%
Devo
nian
Halite
7.0
14000
a
Halite
Known NY Formation Waters
8.0
8000
6000
4000
ractions
Marcellus Shale ext
Upper and
Middle
Devonian
2000
0.0
0
0
0.1
0.2
0.3
0.4
0.5
I (µM)
0.6
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
Br (µM)
Fig. 5. Bivariate plots for halogen concentrations. All of the samples are grouped into stream waters, Type I, II and III drinking waters, based on the Br, I concentrations and
ratios. To compared with these groups, calculated mixing trends are shown for halite dissolution, Appalachian Basin formation waters in NYS (shaded region) (Osborn et al.,
2012), Marcellus flowback water from PA, and halogen extractions from Marcellus shale rock sample. Two NYS formation waters wells with highest and lowest Br/I ratios are
marked D14 and D60 (Osborn et al., 2012). D60 represents the Marcellus formation water and the ticks on this mixing trend mark the percentages of formation water in the
two-endmember mixing scenario.
Formation, as major halite bed of Silurian age (Dresel and Rose,
2010), was used to derive the halite dissolution mixing line. All
of the formation waters have Br/I ratios between the halite and
shale extractions, indicating that the halogens in the formation
waters were ultimately derived from halite, organic matters and
also the seawater. Stronger organic signature would result in lower
Br/I ratios in formation waters, because iodine is highly enriched in
marine organic matters (Fig. 5a). Stronger halite signature would
increase the Br/I ratios since halite is enriched in bromine relatively to iodine.
Among the four groups of water samples (Type I, II and III well
waters and streams), the stream waters are more influenced by the
halite end-member, relative to pristine shallow groundwaters,
Type I, (Fig. 5a), but the stream waters do not show perfect halite
dissolution mixing. The stream water halogen system is likely
influenced by residual road salt applied during winter. Both Type
II and III water groups have Br/I ratios between that of halite and
shale extractions (Fig. 5a). Type III waters have a stronger organic
signature compared to Type II, and Type II waters have Br/I ratios
similar to the NYS formation waters (Fig. 5a).
The I/Cl ratios in Type II and Type III water also indicate that
halite dissolution cannot be the sole source of elevated iodine level
and are more consistent with the I/Cl ratios in formation waters
(Fig. 5b). Compared to Type I groundwater samples, stream waters
have I/Cl ratios more similar to halite dissolution, rather than
formation waters. A plot of Cl versus Br does not contradict the
observations made in the Br–Cl and I–Cl plots (Fig. 5c).
4.2. Formation waters
To potentially pinpoint the source of excess halogens in Type II
and III shallow groundwater, it is important to know the formation
water compositions at various depths and also the spatial heterogeneity of groundwater chemistry in the same geologic formation
across the study area. Iodine historically has not been used as a
natural tracer in deep groundwater in NY State. Only one study
reported iodine and bromine concentrations in formation waters
from several deep wells in New York State (Osborn et al., 2012).
Mixing between these formation waters and freshwater would
result in mixing relationships bracketing the Type II group, warrant
more detailed discussions. Type III group has a significantly lower
Br/I ratios and cannot be explained by these known formation
water compositions (Fig. 5a).
One of the formation water wells (Osborn et al., 2012), D60 was
drilled into the Marcellus shale and is located 15–45 km from the
Type II group samples in Steuben County (Fig. 3). The calculated
mixing line between D60 and freshwater is nearly identical to
the linear relationship shown for Type II group (Fig. 4a). It is possible that the elevated bromine and iodine in the Type II waters are
influenced by Marcellus formation waters. Assuming Marcellus
water as the source for this trend, a two-component mixing model
indicates that only a trace amount of formation water (<0.05%) has
entered the shallow groundwater aquifer in this area. However, it
is not clear whether formation waters associated with the
Rhinestreet, Middlesex and Utica shale (Fig. 2) may have Br/I ratios
similar to that of the Marcellus.
There are noticeable depth and spatial patterns of halogen
ratios in the formation waters sampled within the New York State
(Osborn et al., 2012). Formation waters appear to have decreasing
Br/I and Br/Cl and increasing I/Cl with depth (Fig. 6). Type II shallow groundwater has average Br/I and Br/Cl ratios similar to D60,
the deep well sampled Marcellus formation water. The Type III
group has lower Br/I (4.0 ± 1.7) and Br/Cl ratios (1.2 ± 1.7), but
higher I/Cl (0.50 ± 0.89) ratios than Type II and any of the known
NYS formation waters. These differences suggest Type III samples
may have solute contributions from formation waters deeper than
the Marcellus shale (Fig. 6).
On the other hand, the Br/I ratios in formation water decrease
from west to east across NY State (Fig. 7); D60 is the deepest and
easternmost sample of NY formation waters with reported iodine
values. It is located geographically close to the Type II group of
samples (Steuben County) and west of the Type III group. So the
current knowledge about formation water halogen compositions
do not allow us to argue whether the Type III group is affected
34
Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36
55
Br/I (μM/μM)
10.0
20.0
30.0
40.0
50.0
50
60.0
0
300
D10
400
500
D6
600
700
800
De
e
lo per
we s
r B ou
r/ rce
I?
Depth (m)
Br/I (µM/µM)
4.0±1.7
Type III
200
D14
0.1
0.15
0.2
0.25
Type II
100
Type III
ce
ur
so ?
er Cl
ep r I/
De ghe
hi
Depth (m)
0.50±0.89
D10
300
D14
500
600
D6
700
D17
800
900
D60
1000
Br/Cl (x 103 μM/μM)
3.0
4.0
5.0
0
Type II
1.2± 1.7
D10
600
700
De
low eper
er so
Br urc
/C
l? e
Depth (m)
400
c
Type III
300
500
-79
-78.5
-78
-77.5
Type III
-77
-76.5
0.3
b
200
100
Type II
Fig. 7. West-east trend in Br/I in formation waters. Symbols as Fig. 6.
0
200
25
Longitude
I/Cl (x 103 μM/μM)
2.0
30
10
-79.5
1000
400
35
15
D60
0.05
40
20
D17
900
Ea
low stern
er so
Br/ urc
I? e
45
a
Type II
100
800
900
D14
D6
D17
D60
1000
Fig. 6. Depth profiles of halogen ratios in NYS formation waters (Osborn et al.,
2012). Type II groundwaters are plotted with the average well depths versus the
average and standard deviation of its Br/I. Type III waters, marked by arrows and
numbers, have ratios beyond the range of know NYS formation waters.
by a deeper source or an eastern source, compared to the Type II
group.
The thickness of the Marcellus shale increases from approximately 50 feet in the west to greater than 550 feet in the east
(Fig. 8a), although organic rich part of the Marcellus shale in the
east only thickens to about 150 ft. Nine of the ten wells in the Type
II group are located where the thickness ranges between 50 and
100 feet in Steuben County. The thickness of the Marcellus shale
underlying the Type III water group varies from 50 to over 550 feet.
Nine of the eleven wells identified as Type III water are located
where the Marcellus thickness is greater than 100 feet in Chemung,
Tioga, and Broome Counties.
The TOC of the Marcellus is highest in western Chemung/Eastern Steuben counties and decreases to the east (Fig. 8b). We
multiplied the thickness of the shale by TOC as a rough index for
the size of sedimentary organic halogen pool which potentially
can influence the Type II and III waters. Type III waters with lower
Br/I ratios appear to be associated with a larger organic halogen
pool (Fig. 9). There is a greater chance for a larger organic source
to release more iodine into the overlaying groundwaters, as
observed in the Type III waters (Fig. 5a). In addition, the thermal
maturity of the Marcellus shale is highest in the eastern section
of the study area (Fig. 8c).
It is plausible that other organic-rich shale formations may have
similar spatial patterns as the Marcellus. To further test whether
Marcellus formation water may be responsible for the Type III signature, we obtained Marcellus flow-back waters (Chapman et al.,
2012) from Bradford County, PA, which borders Broome County,
NY. These flow-back waters have Br/I ratios (23–27), similar to that
of the Marcellus formation water in NYS (Fig. 5a), much higher
than the ratios in Type III group. If the Br/I ratios in these PA
flow-back waters are representative for the Marcellus formation
water in Broom County, then the Type III signature would probably
indicate a source of halogen other than the Marcellus. However it is
unclear whether the Br/I ratios in flow-back water were affected by
the initial compositions in hydraulic fracturing fluids and by the
fracturing process. Currently, the Type III signature cannot be conclusively tied to any specific shale formation.
4.4. Pros/cons for iodine as a groundwater tracer and future work
4.3. Potential geological controls
The Br/I ratios indicate stronger organic signature in Type III,
relative to Type II (Fig. 5a). Such a hypothesis can be tested by
examining the spatial difference in potential source formations,
like thickness of source rock, total organic carbon (TOC) and thermal maturity of the source rock (Fig. 8). Here we use the Marcellus
as an example to demonstrate the approach, although we fully
acknowledge the possibility that shale other than the Marcellus
may be the source of excess halogens in Type II and III waters.
This study demonstrates the potential of iodine being an important natural tracer in groundwater for the influence of organic rich
formations, particularly relevant for the debate about hydraulic
fracturing. Iodine and bromine concentrations can be rapidly
analyzed on ICP-MS at very reasonable cost, suitable for large scale
hydro-geochemical studies. Because iodine is the most biophilic
halogen, Br/I ratios have unique advantage tracking the influence
of marine source rocks, compared to Br/Cl ratios. For example,
the Br/I ratios in Type II group has a standard deviation
Z. Lu et al. / Applied Geochemistry 60 (2015) 29–36
35
needs to be further tested when the salinity derives from different
inorganic sources (e.g. halite formations of various ages).
Correct interpretations of iodine data in shallow groundwater
cannot be achieved without comprehensive knowledge about the
deep formation waters. The available iodine measurements in formation waters do not cover multiple shale units in the New York
State and do not allow any estimate of spatial variability in the
same unit. Once such a data set becomes available, the Br/I ratios
may be a key tracer to definitively pinpoint to a specific source
formation.
Currently, there is not sufficient information to discuss the
hydrological mechanisms for the migration of excess halogens into
shallow drinking water wells. Possible scenarios include, but not
limit to: (1) slow diffusion of halogen (possibly methane) from
deep formations; (2) preferential conduits such as naturally
existing faults and fracture zones introducing trace amount of deep
formation waters into shallow aquifers; (3) groundwater recharge
picking up halogens from uplifted shale units. Denser sampling
across the southern tier in future work will allow us further
evaluate these hypothesis.
5. Conclussions
Distinct iodine and bromine concentrations and ratios occur in
shallow groundwater wells located in southern NYS. According to
the available information about formation waters, elevated iodine
and bromine concentrations in Steuben County may be related to
trace quantity of solutes derived from Marcellus formation water
(Type II water). In Chemung, Tioga, and Broome Counties (Type
III water), the source for excess halogen probably is not the Marcellus shale. Varying shale thickness, TOC, and thermal maturity of
the source rock across southern NYS may cause the differences in
Br/I ratios between Type II and III groups. Further study of more
formation waters from greater depth range and spatial coverage
may allow iodine to be used as a sensitive tracer for quantifying
degrees and sources of groundwater contamination associate with
organic-rich shale.
Fig. 8. Sampling locations of Type II and Type III waters are shown on contour maps
for Marcellus shale (a) Thickness, (b) total organic carbon, and (c) thermal maturity.
Funding for this project was provided by a seed grant from Syracuse University and from the National Science Foundation (EAR
1313522). We thank all the volunteer homeowners for allowing
us to sample their well water. Egan Waggoner was instrumental
in organizing the sampling campaign and Natalie Teale and Max
Gade helped collect water samples during the summer of 2012.
30.0
Type II
Br/I (µM/µM)
25.0
Acknowledgements
Type III
20.0
15.0
References
10.0
5.0
0.0
0
50
100
150
200
250
300
TOCxThickness
Fig. 9. Br/I plotted against TOC shale thickness for Type II and Type III waters.
TOC and thickness values were estimated from Fig. 8.
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