B05006/B05007: Evaluation of the impact on shellfisheries of runoff from land receiving organic waste

Study Duration: July 1999 to November 2002

Contractor: ADAS Ltd and Centre for Environment, Fisheries and Aquaculture Sciences (CEFAS)

Bivalve molluscan shellfish have been identified as being high-risk foods, with contamination mainly associated with human faecal contamination. The potential contamination of food and water by human and animal faecal bacteria is assessed by the detection and enumeration of faecal indicator bacteria such as faecal coliforms (including E. coli), faecal streptococci and clostridia. Public health controls on commercial shellfish production principally use faecal coliforms and/or E. coli to assess the degree of faecal contamination. However, such bacteria may arise from either human or animal sources and the degree of risk, and measures for reducing contamination, may differ depending on the source.

The first objective of this work will be to investigate whether there is a link between the diffuse spreading of livestock wastes to land and the level of indicator bacteria (E. coli) in shellfish production areas. The second objective of this study will be to prototype a general tool for catchment scale assessment of the relative importance of different sources of indicator bacteria with respect to the contamination of shellfish, with the potential for application to any catchment in England and Wales.

A 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:

• calculations of faecal coliform loadings to agricultural land, fresh and stored manures;
• simulation of faecal coliform survival on/in soil;
• simulation of faecal coliform losses to rivers by leaching and absorption to eroded soil particles;
• simulation of combined sewer overflow discharges from urban areas;
• simulation of continuous discharges from sewage treatment works;
• simulation of river transport and decay of faecal coliforms.

To yield predicted daily time-series of faecal coliform concentrations at any site on a river system.

• There was a good correspondence between the monthly profiles of predicted concentrations of faecal coliforms at the tidal limit in two selected 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 tissue and thus may need to be considered when undertaking further model development or interpreting model outputs.
• 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. However, 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.

Final report is available from the FSA Library and Information centre. To obtain a copy, please contact the Enquiry Desk, Dr Elsie Widdowson Library and Information Services, Food Standards Agency ( Tel: +44 (0) 20 7276 8181/8182 or by e-mail to: InfoCentre@foodstandards.gsi.gov.uk ).

Contact: Email: science@foodstandards.gsi.gov.uk