Microbial pollution of the water in Chichester Harbour

Microbial pollution of the water in
Chichester Harbour
Simon Kershaw, Carlos J.A. Campos, Owen C. Morgan
October 2011
Table of contents
1
Introduction ................................................................................................................................... 1
2
Sewage discharges ....................................................................................................................... 2
3
Results ........................................................................................................................................... 3
3.1
Changes in the levels of E. coli in surface water ..................................................................... 3
3.2
Levels of E. coli in surface water excluding results before step changes ............................... 4
4
Conclusions ................................................................................................................................... 9
5
Acknowledgements .................................................................................................................... 10
6
References ................................................................................................................................... 10
Microbial pollution in Chichester Harbour
Page ii
1 Introduction
Chichester District Council and Chichester Harbour Conservancy have monitored the levels of faecal
indicator microorganisms (total coliforms, enterococci and Escherichia coli) in recreational waters of
Chichester Harbour since June 2007. Surface water samples have been collected from eleven sites
within the harbour at frequencies varying from monthly to three times a month. Various other
environmental parameters (time of high tide, tidal range, wind direction and strength, general
weather conditions and percentage of cloud cover) have also been recorded at the time of sampling.
The results of this monitoring are interpreted via the Chichester DC internet site1, using EU Bathing
Water Standards as a guide.
Over the last 5 years, the Chichester Channel designated Shellfish Water (SW) has failed the
Guideline (G) faecal coliform shellfish flesh standard (Environment Agency, 2009). Thornham
Channel SW only achieved the G standard for faecal coliforms in shellfish flesh in 2005 and 2008,
although faecal coliform levels observed in the water column have been consistently low
(Environment Agency, 2009a). Emsworth Channel SW achieved the G standard for faecal coliforms in
shellfish flesh in 2004 and 2005 (Environment Agency, 2009b).
This report describes an assessment made by Cefas on the spatial and temporal variability of the
quality of surface waters with respect to E. coli. This bacterium is an indicator of contamination of
faecal origin and, as such, it is used in the official controls for bivalve shellfish intended for human
consumption in the European Union.
The assessment consisted of: cumulative sum (CUSUM)2 analysis of levels of E. coli to detect any
changes in the overall quality of the water, summary statistics and graphical display by means of
box-and-whisker plots3 of levels of the microbiological indicator.
1
Chichester District Council water quality results: http://www.chichester.gov.uk/index.cfm?articleid=5765.
CUSUM functions have been successfully applied as quantitative techniques for evaluating changes in water
quality related to human activity and environmental variability.
3
Box-and whisker plots summarise the distribution of a dataset, providing visual summaries of the median
(centre line of the box), variation or spread (inter-quartile range or box height), skewness (quartile skew or
size of box halves) and presence of unusual values (outliers represented by asterisks).
2
Microbial pollution in Chichester Harbour
Page 1 of 10
2 Sewage discharges
Figure 1 shows the locations of significant continuous and intermittent sewage treatment works
(STWs) discharging directly to Chichester Harbour, sampling points used by local authorities in their
water quality investigations and production area boundaries for native oysters (O. edulis). The larger
continuous STWs discharges are associated with the urbanised areas of Chichester, Bosham and
Emsworth.
Figure 1. Locations of significant sewage discharges to Chichester Harbour, classified native oyster
production areas and water sampling points. Please refer to Tables 1, 2 for sampling site names.
The classifications are effective from 1 September 2010.
The level of treatment at Bosham STW and Chichester STW was upgraded to ultraviolet disinfection
in March 2008 as part of a water company investment programme to improve water quality in the
catchment and endeavour to ensure compliance with Shellfish Waters guideline standards
(Environment Agency, 2009). There are a number of combined sewer overflows, emergency
Microbial pollution in Chichester Harbour
Page 2 of 10
overflows and overflows from pumping stations discharging directly to the harbour. Notifications
from Southern Water of the latest storm water discharges to the Harbour are reported on the
Chichester internet site4. Records of the discharge history for key intermittent discharges from 1st
April 2008 are posted on the Chichester Harbour Conservancy internet site5.
3 Results
3.1
Changes in the levels of E. coli in surface water
Step changes were detected in E. coli levels in the water sampled from four sampling points along
Chichester Channel (Table 1). Median E. coli levels computed for datasets before and after changes
indicate an overall improvement in the microbial quality of the water, particularly in upstream areas
of the channel.
Table 1. Results of cumulative sum analysis of levels of E. coli in seawater in Chichester Harbour.
Site code
Site name
CHC1
CDC4
CHC2
CDC5
CDC6
North of Dell Quay
Dell Quay
Chichester Marina Beacon
Chichester Marina Jetty
Itchenor Quay
CDC2
CDC3
CHC3
Bosham Quay
Cobnor Sailing Club
Deep End, Cobnor Point
CDC1
CHC4
Thorney Island
East Head
CHC5
Emsworth Jetty
Date of step change
Chichester Channel
18 March 2008
3 March 2009
18 March 2008
13 May 2008
No step change detected
Bosham Channel
No step change detected
No step change detected
No step change detected
Thorney Channel
No step change detected
No step change detected
Emsworth Channel
No step change detected
Median
(before step
change)
Median
(after step
change)
3,300
1,000
220
560
-
50
70
10
70
-
-
-
-
-
-
-
The occurrence of step changes at North of Dell Quay and Chichester Marina Beacon is coincident
with UV installation at Bosham STW and Chichester STW. Modelling studies recently undertaken by
Cefas and CREH assessed the impact of these sewerage infrastructure improvements on fluxes of
4
5
Chichester DC news of storm water discharges: http://www.chichester.gov.uk/index.cfm?articleid=5765.
Chichester Harbour Conservancy discharge history information: http://www.conservancy.co.uk/page/WaterQuality/339/.
Microbial pollution in Chichester Harbour
Page 3 of 10
faecal indicator organisms from the Chichester catchment. These studies found substantial
reductions in total fluxes of faecal coliforms (-79%) and enterococci (-67%) after UV installation at
Bosham STW and Chichester STW (Crowther et al. 2011). The study highlights that while STW
discharges currently contribute significantly to the fluxes of indicators under base flow conditions,
fluxes at high flow and the total fluxes are almost entirely derived from intermittent discharges. The
upgrade in the hygiene classification of native oysters at Birdham Spit from C to B is however
consistent with the overall reductions in the delivery of faecal organisms from the catchment.
In contrast, no step changes were detected in the levels of the microbial indicator in sampling points
situated along Bosham and Thorney channels. This suggests that the improvements to wastewater
pipeline discharges may not have produced the expected improvements in the overall quality of the
water at these sites.
3.2
Levels of E. coli in surface water excluding results before step changes
Table 2 shows a relatively similar sampling effort at all sampling points.
Table 2. Summary statistics for levels of E. coli in seawater in Chichester Harbour after step
changes.
-1
Site
code
Site name
CFU E. coli 100ml
Minimum Maximum
Median
Date of first
Date of last
n
sample
sample
Chichester Channel
CHC1 North of Dell Quay
2 April 2008
20 July 2010
49
<10
170,000
CDC4 Dell Quay
17 March 2009
31 August 2010
39
<10
4,500
CHC2 Chichester Marina Beacon
2 April 2008
31 August 2010
60
<10
2,500
CDC5 Chichester Marina Jetty
27 May 2008
31 August 2010
57
<10
3,900
CDC6 Itchenor Quay
26 June 2007
31 August 2010
80
<10
3,700
Bosham Channel
CDC2 Bosham Quay
26 June 2007
31 August 2010
80
<10
1,100
CDC3 Cobnor Sailing Club
10 July 2007
31 August 2010
79
<10
910
CHC3 Deep End, Cobnor Point
26 June 2007
31 August 2010
79
<10
1,000
Thorney Channel
CDC1 Thorney Island
10 July 2007
31 August 2010
79
<10
740
CHC4 East Head
17 February 2008
31 August 2010
61
<10
840
Emsworth Channel
CHC5 Emsworth Jetty
8 January 2008
31 August 2010
64
<10
7,300
NB. Data from Chichester Marina Slipway were amalgamated with that from Chichester Marina Jetty.
Microbial pollution in Chichester Harbour
Page 4 of 10
50
70
10
70
10
40
10
10
20
5
105
The highest result obtained at North of Dell Quay (170,000 E. coli 100ml-1) coincided with a relatively
high result at Dell Quay (560 E. coli 100ml-1) on 16 February 2010 indicating deteriorated quality of
the water in the upper Chichester Channel on this date.
Median levels of E. coli suggest the following spatial pattern of contamination across the harbour:
Emsworth Jetty>
Dell Quay≈Chichester Marina Jetty>
North of Dell Quay>
Bosham Quay>
Thorney Island>
Chichester Marina Beacon≈Itchenor Quay≈Deep End (Cobnor Point)≈Cobnor Sailing Club>
East Head.
The high median levels of E. coli at Emsworth Jetty could be attributed to the proximity of this
sampling point to sources of contamination such as combined sewer overflows and runoff from
farmland and impervious surfaces in the urban area of Emsworth. The area also receives freshwater
inputs from two streams; one of them discharges to a tidal pond. The low levels of contamination in
Thorney Channel are inconsistent with the levels of E. coli that have been detected in native oysters
at this site between 2007 and 2009. This bed has however been recently upgraded to class B.
Figure 2 indicates the generally right-skewed distribution of E. coli levels at most sites. The relatively
Log10 CFU E. coli 100ml-1
symmetric distributions of E. coli levels at Dell Quay and Chichester Marina Jetty are also evident.
100,000
10,000
1,000
100
10
Em
o
sw
rth
t
y
y
y
ad land
on e tty
in
ub uay
t ty
ua
ua
ua
Cl
ac
Po
Je
J
He
Q
Q
Q
Q
e
Is
g
r
t
ll
ll
a
r
B
n
o
s
m
o
y
ili
rin
De f De
bn
Ea
ne
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en
Sa
ina M a
o
r
or
Co
os
ar
ch
o
t
h
r
,
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I
T
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En
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N
ich
es
ep
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ich
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C
Figure 2. Box-and-whisker plots of levels of E. coli in seawater in Chichester
Harbour after step changes. Sites ordered from west to east.
Microbial pollution in Chichester Harbour
Page 5 of 10
The outlier E. coli results in Chichester Marina Beacon, Cobnor Sailing Club, Deep End, East head,
Itchenor Quay and North of Dell Quay reflects the proximity of these sites to intermittent point
sources of pollution.
The microbial quality of the water improves significantly towards the mouth of Chichester Channel,
at the confluence with Bosham Channel and at the mouth of the harbour. Chichester Harbour is a
meso-tidal (tidal range is 2–4m) estuary receiving low freshwater inputs. The River Lavant is the
most significant freshwater input to Chichester Channel. The lower reaches of the harbour are ebb
dominant (Futurecoast, 2002). Overall, the prevailing winds influencing Chichester Harbour are from
the southwestern sector. For more than 90% of the time, winds are 8 ms-1 or less, resulting in a low
wave energy environment (ABP Research & Consultancy Ltd, 2001).
At the harbour entrance, the ebb tidal currents are of shorter duration, but significantly greater
velocity, than flood currents (Futurecoast, 2002; SCOPAC, 2004). Flood and ebb tidal streams
increase significantly towards the mouth of the harbour (UK Hydrographic Office, 2000). Stronger
tidal currents at the mouth (maximum averaged currents = 1.5–3ms-1; HR Wallingford, 1997) are
likely to promote the dispersion of microbial contaminants towards the sea. In contrast, faecallycontaminated surface waters may result in settlement of contaminated silt deposited in less flushed
areas in the upper reaches of the channels. These areas would be vulnerable to contaminated runoff
and/or re-suspended seawater following rainfall events. Furthermore, there is evidence that
complex hydrodynamic structures such as large back eddies occur in the lower reaches of the main
harbour channels (Chichester Harbour Conservancy, 1999).
Median levels of E. coli increase markedly in the winter relative to those in the summer at some of
the sampling sites. Differences between these seasons exceed 1Log10 in Dell Quay, North of Dell
Quay, Chichester Marina Beacon, Deep End and East Head (Figures 3–4). These seasonal differences
were statistically significant in Dell Quay (F3,35=4.77; p=0.007).
Microbial pollution in Chichester Harbour
Page 6 of 10
Chichester Marina Jetty
Log10 CFU E. coli 100ml-1
Log10 CFU E. coli 100ml-1
Dell Quay
10,000
1,000
100
10
Spring
Summer
Autumn
1,000
100
10
Winter
Spring
1,000
100
10
Spring
Summer
Autumn
Winter
Autumn
Winter
100
Spring
Summer
Thorney Island
Log10 CFU E. coli 100ml-1
Log10 CFU E. coli 100ml-1
Autumn
1000
Winter
1,000
100
10
Summer
Winter
10,000
Bosham Quay
Spring
Autumn
North of Dell Quay
10,000
Log10 CFU E. coli 100ml-1
Log10 CFU E. coli 100ml-1
Emsworth Jetty
Summer
Autumn
1,000
100
10
Winter
Spring
Summer
Figure 3. Box-and-whisker plots of levels of E. coli in seawater in six sampling points at Chichester
Harbour after step changes.
Itchenor Quay
Log10 CFU E. coli 100ml-1
Log10 CFU E. coli
Chichester Marina Beacon
1,000
100
10
Spring
Summer
Autumn
1,000
100
10
Winter
Spring
100
10
Spring
Summer
Autumn
Winter
Autumn
Winter
Cobnor Sailing Club
1,000
Log10 CFU E. coli 100ml-1
Log10 CFU E. coli 100ml-1
Deep End, Cobnor Point
Summer
Autumn
Winter
Autumn
Winter
1,000
100
10
Spring
Summer
Log10 CFU E. coli 100ml-1
East Head
1,000
100
10
Spring
Summer
Figure 4. Box-and-whisker plots of levels of E. coli in seawater in five sampling points at Chichester
Harbour after step changes.
Microbial pollution in Chichester Harbour
Page 7 of 10
Increased levels of contamination in the winter could be associated with the increase in water levels
in the River Lavant during the period November–December (CEH-NERC, 2008). In fact, the increase
in E. coli levels at this site during the period September–January closely mirrors the rising limb of the
River Lavant hydrograph. It has also been hypothesised that surcharges in effluent volumes from
Chichester STW when the water table is high during the period November–March could also impact
upon water quality in the upper channel (I. Dunhill, pers. comm., 2010). Levels of faecal coliforms in
final effluent discharges from Bosham STWs and Chichester STWs monitored by Southern Water
during the period July 2008–June 2011 were analysed to test this hypothesis6. These data provide no
indication of significant seasonal differences in the microbial quality of effluent discharges from
these assets. Median levels of the indicator in effluent discharges from Chichester STW do however
increase markedly during the autumn in relation to those during the spring.
Levels of E. coli at sampling points situated in other areas of the harbour (e.g. Emsworth Jetty,
Bosham Quay) do not show appreciable seasonal differences.
It would be necessary to obtain targeted data and undertake further analyses to understand
whether:
There is an association between peak E. coli results and periods of deteriorated quality of
rainfall-dependent discharges (assuming these exist); and
To what extent the above influences the seasonality of E. coli contamination in Chichester
Harbour.
Further work is envisaged on the effects of tidal state and sewage spills on E. coli results.
6
The dataset did not contain information on volumes discharged.
Microbial pollution in Chichester Harbour
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4 Conclusions
Levels of E. coli quantified in surface water samples from eleven sites across Chichester Harbour
during the period June 2007–August 2010 were analysed. Shifts in median levels of the
microbial indicator were detected in North of Dell Quay, Dell Quay, Chichester Marina Beacon
and Chichester Marina Jetty.
Median E. coli levels decreased significantly after the changes indicating an overall
improvement in the microbial quality of the water in the upper Chichester Channel.
The improvement in the microbial quality of the waters as indicated by North of Dell Quay and
Chichester Marina Beacon was temporally coincident and thought to be a result of upgrades in
treatment level at Bosham STW and Chichester STW.
The high median E. coli at Emsworth Jetty is likely to be attributed to the combined impact of
pollution sources impacting upon the the upper reaches of Emsworth Channel.
Levels of E. coli in surface waters in the upper channel increase markedly during the winter. This
is likely to be attributed to increased freshwater inputs from the River Lavant during winter
months.
Microbial pollution in Chichester Harbour
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5 Acknowledgements
We are indebted to Duncan Keir (Chichester District Council) for the provision of the water quality
data and Ian Udal (Environment Agency) for the provision of UV efficacy data for Bosham STWs and
Chichester STWs and commenting on the draft report. We also extend our thanks to Chichester
Harbour Conservancy and Chichester District Council staff who collected the water samples.
6 References
ABP Research & Consultancy Ltd, 2001. Geomorphological analysis of East Head and Chichester
Harbour. Report No. R.893 to Chichester Harbour Conservancy, English Nature and National Trust.
CEH-NERC,
2008.
41023
Lavant
at
http://www.ceh.ac.uk/data/nrfa/data/station.html?41023.
Graylingwell.
Chichester Harbour Conservancy, 1999. Racing and racing
http://www.conservancy.co.uk/page/Racing-and-Racing-Tips/330/.
tips.
Available
Available
at:
at:
Crowther, J., Kay, D., Campos, CJA, Morgan, OC. 2011. Sanitary profiles of selected shellfish water
catchments pre- and post-improvements in sewerage infrastructure. CREH and Cefas report to
Defra. Project WT1001 - Factors affecting the microbial quality of shellfish.
Environment Agency, 2009. Directive (2006/113/EC) on the quality required of shellfish waters.
Article 5 Programme. Chichester Harbour (Chichester Channel).
Environment Agency, 2009a. Directive (2006/113/EC) on the quality required of shellfish waters.
Article 5 Programme. Chichester Harbour (Thornham Channel).
Environment Agency, 2009b. Directive (2006/113/EC) on the quality required of shellfish waters.
Article 5 Programme. Chichester Harbour (Emsworth Channel).
Futurecoast, 2002. Department of Environment, Food and Rural Affairs, Halcrow Group Ltd, 3 CD set.
HR Wallingford, 1997. East Solent Shoreline Management Plan. Volume II, Management Unit 6:
Sandy Point to the Inn on the beach, Hayling Island.
Standing Conference on Problems Associated with the Coastline (SCOPAC), 2004. Sediment
Transport Study - Portsmouth Harbour Entrance to Chichester Harbour Entrance. Available at:
http://www.scopac.org.uk/scopac%20sediment%20db/pchi/pchi.htm.
UK Hydrographic Office, 2000. England – South Coast: Langstone and Chichester Harbours 3418.
Microbial pollution in Chichester Harbour
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