Urban stormwater discharges during wet weather has always been a sensitive issue to deal with. By inducing combined sewer overflows, substantial impacts on receiving water are commonly feared. Aware of this problematic situation, the VSA already created the STORM guideline to cope with this issue in small rivers. However, such a guideline is still lacking for large rivers and lakes. This project is a first step towards this direction and tries to prospect the necessity of such instructions for lakes. For this purpose, the present master thesis studies the impact of urban stormwater discharged via the Flon river into the Vidy Bay of Lake Geneva, next to the city of Lausanne.
A Sediment Quality Triad, meaning a combination of three assessment methods (chemistry, ecotoxicology and in situ benthic communities), was implemented to assess the impact of stormwater discharges on the sediments in this part of the lake. On a large sampling grid composed of 15 sites, heavy metals were measured and ostracod bioassays were performed with the sediment samples collected at these sites. Subsequently, on the six sites located in the central transect of this sampling grid, further experiments were led to understand the incidence of stormwater discharges on sediment quality while moving away from the Flon river’s effluent. In addition to heavy metals, PAHs and PCBs were measured at these sites. A battery of three extra ecotoxicological bioassays was also carried out on five of these six sites. These tests were implemented with the following organisms: chironomids, macrophytes and nematodes. Finally, two kind of in situ communities were studied in these five sites: microbial communities in order to perform a pollution-induced community tolerance (PICT) and oligochaete communities in order to implement the oligochaete index of lake bioindication (IOBL).
The implementation of these experiments showed that the contamination induced by urban stormwater discharges is limited to the proximity of the combined sewer overflow. A set of pollutants mainly composed of copper, zinc, PCBs and PAHs was identified within this contaminated area. Although no toxicity was observed on the sites affected by this contamination, in situ community studies revealed substantial shifts towards resistant species in both oligochaete and microbial community structures.
Another contamination area, observed in remote sites and mainly composed of cobalt, chromium, iron, manganese and nickel, remains not well understood. The laboratory toxicity tests detected some toxicity in benthic invertebrates but the benthic communities studied in situ showed no apparent alteration. Modeling tools are required to get a better understanding of the source of this contamination.