The impact of a sewage treatment plant's effluent on sediment quality in a small bay in Lake Geneva (Switzerland–France). Part 1: Spatial distribution of contaminants and the potential for biological impacts

The purpose of this study was to assess the quality of bottom sediment collected in the Bay of Vidy (Lake Geneva, Switzerland) which receives sewage treatment plant discharges from the city of Lausanne and its surroundings. Thirty-eight sediment samples were taken, forming a dense network. Samples of that portion of the sediment that has accumulated since phosphorus removal (via iron precipitation) was instituted at the plant in 1971 were analysed for heavy metals, nutrients and organic micropollutants and subjected to Microtox toxicity evaluation (organic extract). The distribution patterns of contaminants, as well as principal components analysis, demonstrated that the sewage treatment plant's effluent is the main source of local sediment contamination. This was not supported by the Microtox results. In close proximity to the outlet pipe of the sewage treatment plant, contaminants showed very high median concentrations for virtually all the parameters measured, at levels above those believed to elicit biological responses. A major environmental impact is clearly occurring in this coastal region of the lake and remediation of this situation is believed to be important in order to protect the Lake Geneva ecosystem.