Coastal management and water protection must provide acceptable water quality for shellfishing and tourist activities. To determine the impact of sewage on microbial water quality, engineers need to know the laws of bacterial reduction. In this study, experiments were performed with Escherichia coli in the laboratory and in situ (Atlantic and Mediterranean coast) using diffusion chambers at several depths. T90 was calculated from culturable bacterial counts over time. Establishing an abacus integrating the depth of water impacted by sewage and turbidity enabled us to calculate T90 from daily sunlight illuminations. Results obtained by cultures minimise health risks and yet viability and cellular integrity of E. coli remain, even under intense sunlight stress. When direct viable count is used to calculate a VT90, the results are dramatically higher and show how necessary – and difficult – it is to develop a concept which takes account of better understanding of sanitary risks in impact studies.

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