Sewage impact on shellfish microbial contamination.

Coastal areas are frequently contaminated by microorganisms of human origin, due to high population density and low seawater renewal. To evaluate the impact of wastewater input on shellfish quality, a study was conducted in Brittany (France) over a period of 20 months. A hydrodynamic model was used to simulate wastewater impact on microbial water quality. To validate the model, wastewater from the three main sewage treatment plants and shellfish from three sites were sampled monthly. Bacterial indicators (E. coli), F-RNA phages were searched for by culture and noroviruses by RT-PCR and hybridisation. These microorganisms were detected in the three effluents and clams, with no marked seasonal variation. The microbial concentrations in the two oyster beds, distant from the effluent outfall, were low, and only three of the samples were positive for norovirus. For simulation, the winter wastewater inputs of E. coli and phages were calculated and an estimation for norovirus flux was made from the epidemic situation in the population. The microbial behaviour was included in the model by a decay-rate factor. Results from the model calculations were found to be very similar to E. coli and phage concentrations observed in shellfish. For noroviruses, the model indicated that shellfish distant from the wastewater input were under the detection limit of the RT-PCR method. This study demonstrated the use of modelisation to interpret norovirus contamination in various areas.