Spatial and temporal presence of a wastewater-derived micropollutant plume in Lake Geneva

This study discusses the occurrence and environmental risk associated with a micropollutant plume originating from the direct discharge of treated wastewater into the Vidy Bay of Lake Geneva, Switzerland. The temporal variations and spatial extent of the plume, and its effect on the presence of 39 pharmaceuticals and other micropollutants in the Vidy Bay were assessed over a 10 month period. A pronounced plume was observed from April to October, leading to locally elevated (up to 70-fold) pharmaceutical concentrations compared to the surrounding water column. For three of the measured substances, these plume-associated concentrations were sufficiently high to pose an ecotoxicological risk. The plume depth followed the thermal lake stratification, which moved to deeper depths over the course of the warm seasons. Pharmaceutical hotspots associated with the plume were detected as far as 1.5 km downstream of the effluent wastewater outfall, but concentrations typically decreased with increasing distance from the wastewater outfall as a result of dilution and photodegradation. From November to January, when uniform temperature prevailed throughout the water column, no micropollutant plumes were detected. In contrast to pharmaceuticals, most pesticides showed homogeneous concentrations throughout the Vidy Bay during the whole study period, indicating that the effluent wastewater was not their dominant source. A strong linear correlation between electrical conductivity and concentrations of wastewater-derived micropollutants was identified. This relation will allow future estimates of wastewater-derived micropollutant concentrations via simple conductivity measurements.