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Evaluation of the impact on shellfisheries production of runoff from land receiving organic wastes

Project Code: B05006

30/11/2002

ADAS Gleadthorpe Research Centre
Rose, S ;
ADAS, Wolverhampton
Anthony, S; Lyons, H;
ADAS Mamhead Castle
Bailey, G;
Centre for Environment, Fisheries & Agriculture Science (CEFAS) Weymouth
Lee, R; Morgan, O; Stockley, L;
Centre for Environment, Fisheries & Agriculture Science (CEFAS) Lowestoft
Tatterstall, G

Twenty-two commercial shellfish production areas in England and Wales were initially selected for study on the basis of the characteristics of the catchment areas draining to the shellfishery, the shellfisheries themselves and the extent of historical bacteriological data available for the shellfisheries. Data on potential sources of contamination available to ADAS and CEFAS were accumulated, together with information on land-cover, livestock, soil types, drainage, rainfall and human population density.

Risk indices were produced for the 22 areas based on estimates of the faecal bacteria indicator loadings contributed by both agricultural sources and point-source discharges. The indices were compared to the geometric mean E. coli concentrations in shellfish sampled over several years at single points in each shellfishery. No clear relationship was observed and therefore this approach was discontinued.

Two of the shellfisheries, Devon Avon and Poole, showed some evidence of a relationship between rainfall and the shellfish E. coli results, with the latter tending to be increased following rain. These two areas were selected for further work. The approaches consisted of the following:

development of a catchment tool (Coliform Source Apportionment Tool – CSAT) predicting the concentration of faecal coliforms at the tidal limit(s) in the river(s) impacting on the estuaries where the shellfisheries were located;
estuary-level modelling in order to extrapolate the predictions from the catchment modelling to concentrations in seawater at the shellfish monitoring points;
basal and storm condition sampling of river waters within the catchments and seawater and shellfish from within the estuaries, in order to provide additional data for comparison with predictions;
analysis of both historical and new data with respect to the effect of environmental factors.

E. coli concentrations in the historical data set of shellfish samples from the Devon Avon tended to peak in the summer and those from Poole Harbour in the winter. In each area, profiles by month of predicted faecal coliform concentrations at the tidal limit and E. coli concentrations in shellfish flesh showed a similar pattern. A time series comparison of predicted high results at the tidal limit and results in shellfish flesh was not undertaken due to the relatively small number of results available at each sampling point.

Results from the estuary-level modelling extrapolating the predictions to the shellfish monitoring point showed that in the Devon Avon, predicted concentrations in the shellfish at high river flows were similar to those observed in routine monitoring. However, predicted concentrations under low river flow conditions were much lower than those observed in the shellfish and this could have been due to restrictions within the estuarine modelling, principally the absence of drying areas from the model and the limited number of runs over which simulations were made.

Extrapolation of the CSAT predictions in Poole Harbour yielded predicted concentrations that were approximately twice those observed in the seawater and shellfish. These minor discrepancies can be due to any number of factors, including timing of sampling, variations in point source loadings, and a poor descriptor of the estuary environment. The discrepancies were also observed when actual riverine data was used as the input to the estuarine module. The differences could have arisen from incorrect assumptions as to the effective dilution at the experimental shellfish site, obtained from earlier work, and/or the omission of bacterial die-off within the estuary.

Further comparison of model predictions with observations made at sites distributed along the lengths of the study rivers indicated that the CSAT was able to reproduce the spatial variability of observed faecal coliform concentrations in river waters.

Analysis of the effect of environmental factors on the observed concentrations of E. coli in shellfish showed that, as well as the effects of season and rainfall, the tidal cycle could also have a significant effect, particularly with M. edulis. The latter may be more prone to such effects than C. gigas because of their greater rates of accumulation and depuration of contaminants. In the Devon Avon, shellfish samples were always taken at low tide and therefore analysis with respect to the high/low tidal cycle was not possible. However, higher concentrations were observed in the shellfish on spring tides than on neap ones. The estuary modelling had shown the more rapid transit of contamination from the tidal limit to the shellfishery during spring tides and this could have been a factor. However, the effect could also have been due to a greater influx of contamination from exterior to the estuary.

In Poole Harbour, the spring/neap tidal cycle did not have a significant effect but the high/low cycle, with M. edulis showing significantly higher contamination on an outgoing tide. This would be consistent with the river catchments being significant causes of contamination of the shellfishery, rather than the relatively large sewage discharges to the east of the Harbour.

Key findings

A simple risk index of contamination of shellfish harvesting areas by agricultural inputs and sewage discharges did not show a significant relationship to observed geometric mean E. coli concentrations in shellfisheries.
A catchment level model (Coliform Source Apportionment Tool, CSAT) was developed which incorporated:
- Calculation of faecal coliform loadings to agricultural land from fresh and stored manures, including the effects of die-off in storage;
- Simulation of faecal coliform survival on soils, including the effects of soil pH, radiation, temperature and soil moisture;
- Simulation of coliform losses to rivers by leaching and adsorped to eroded soil particles, including the effect of landscape retention;
- Simulation of CSO discharges from urban areas in response to rainfall, including the effect of sewer storage and flow to treatment;
- Simulation of continuous discharges from sewage treatment works, including effects of differing levels of treatment;
- Simulation of river transport and decay, including sedimentation and resuspension of sediment adsorped coliform during storm events. to yield predicted daily time-series of faecal coliform concentrations at any site on a river system.
The was a good correspondence between the monthly profiles of predicted concentrations of faecal coliforms at the tidal limit in two select study areas and the contamination seen in the shellfish.
The use of estuary models to extrapolate the predictions to concentrations in seawater and shellfish at the harvesting area gave predicted values within an order of magnitude of those observed.
Season and tidal cycle may affect the E. coli concentrations observed in shellfish flesh and thus may need to be taken into account when undertaking further model development or interpreting model output.
The available data sets of observed E. coli concentrations in river water, seawater and shellfish were not extensive enough to permit comparison of predicted individual peak events at the tidal limit with observed peak concentrations at the shellfishery.
The CSAT has calculated that point sources are responsible for more than 95% of the annual faecal coliform load exported from two study catchments on the south coast of England. During storm events, diffuse (manure-related) sources may contribute up to 80% of the instantaneous loading. The impact of such storm flows on shellfish beds is dependent on the volume of estuarine water, the location of the bed relative to the freshwater plume and the state of the tide.

Recommendations

The performance of the CSAT and integrated estuary models should be validated in a small number of additional catchments or sub-catchments, with monitoring throughout the year.
A protocol should be established for the use of the system, including the following:
- survey requirements for a study catchment in order to identify potentially unknown sources of contamination
- data required for establishing and running the models
- data required for the validation of the models
- interpretation of the output
- The models should be expanded in scope in order to encompass possible direct impacts within an estuary or coastal area.
In order to improve the reliability of the models, information is required on the actual spill frequencies and volumes from CSOs and the associated E. coli concentrations in the effluent.
Further work should be undertaken on the input parameters for the particle modelling component of the estuary models and determination of the appropriate means of summarising the output.
Work should be undertaken to improve understanding of the uptake and depuration of E. coli by shellfish in the natural environment and of the relationship between E. coli in shellfish and the surrounding seawater in order to improve the extrapolation of seawater concentration to those in shellfish.

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