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United States Department of Agriculture

Agricultural Research Service

Research Project: DEVELOPMENT OF ALTERNATIVE PRACTICES FOR IMPROVED WATERSHED MANAGEMENT

Location: Cropping Systems and Water Quality Research

Title: Modeling of Escherichia coli fluxes on a catchment and the impact on coastal water and shellfish quality

Authors
item Bougeard, Morgane -
item Le Saux, Jean-Claude -
item Perenne, Nicolas -
item Baffaut, Claire
item Robin, Marc -
item Pommepuy, Monique -

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 13, 2010
Publication Date: April 1, 2011
Citation: Bougeard, M., Le Saux, J., Perenne, N., Baffaut, C., Robin, M., Pommepuy, M. 2011. Modeling of Escherichia coli fluxes on a catchment and the impact on coastal water and shellfish quality. Journal of the American Water Resources Association. 47(2):350-366.

Interpretive Summary: Sea water bacterial contamination in an estuary is affected by tides, wind, and movement of bacteria in streams draining the nearby land catchment. These factors can be accounted for with models. However, current models devoted to coastal areas only take into account dilution and dispersion and little has been done to consider the possible influence of inputs from the catchment. The introduction of daily loadings directly linked with meteorological data and anthropogenic inputs into a coastal model represents a new approach. We coupled an upstream catchment model with a coastal model of the Daoulas’ catchment and estuary, Brittany, France, to estimate daily variations in bacteria loadings caused by manure spreading and wastewater treatment plant discharge and to assess how they affected coastal water and shellfish quality. Input parameters representing the current land use, management practices, and discharges were determined. The daily simulated E. coli loadings were then introduced in the coastal model to calculate E. coli concentrations in estuarine water and in shellfish. Results were validated using stream measurements and a large database of shellfish quality. Two scenarios evaluated the effect of agricultural practices on river and shellfish quality and on the regulatory classification of swimming and shellfish growing areas. The results indicated the rapid and large fluctuations in E. coli movement from the watershed caused by rainfall events, increasing 1000x in less than 24 hours. Seawater quality degraded within one to two days after a rainfall event. The effects of rainfall and agricultural practices were greater but comparable to tidal effects on shellfish and seawater quality. After additional validation, these models could be efficient tools to test different management scenarios and evaluate their impact on coastal water quality. Resource managers, fisheries, and regulatory agencies who manage coastal areas would benefit from this integrated approach that relates estuarine water and shellfish quality to weather conditions and activities in the catchment.

Technical Abstract: We coupled the Soil and Water Assessment Tool (SWAT) with a hydrodynamic model in the Daoulas’ catchment and estuary to estimate daily variations in Escherichia coli fluxes due to catchment activities (manure spreading and wastewater treatment plants discharge) and to assess their impact on coastal water and shellfish quality. SWAT input parameters representing the current land use, management practices and discharges were determined based on seven years of continuous flow data and one year of weekly water quality measurements. Results indicate that simulated flow agree with measured values (calibration: r² = 0.84, Ens= 0.79 – validation: r²=0.84, Ens=0.82). E. coli concentrations in the river were calibrated on the basis on their frequency of occurrence (r²=0.99). The daily simulated E. coli fluxes were then introduced in the MARS 2D hydrodynamic model to calculate E. coli concentrations in estuarine water and in shellfish. A large database from a network survey of shellfish quality demonstrated the efficiency of the modeling in term of E. coli frequency curves (r²=0.89). Two scenarios evaluated the effect of agricultural practices on river and shellfish quality and on the classification of bathing and shellfish growing areas. The introduction of daily fluxes data directly linked with meteorological data and anthropogenic inputs into a hydrodynamic model represents a new interesting approach. The results indicated the rapid and large fluctuations in E. coli fluxes from the watershed (reaching 3 orders of magnitude in less than 24 hours) associated with rainfall events. Response time in terms of seawater quality degradation ranged from one to two days after a rainfall event. The effect of rainfall and agricultural practices was found to be significant compared with tidal effects on shellfish and seawater quality. After additional validation, these models could be efficient tools to test different management scenarios and evaluate their impact on coastal water quality.

Last Modified: 10/1/2014
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