In this study, the spatial extent of a wastewater-influenced water mass originating from a wastewater treatment plant outlet in Vidy Bay (Lake Geneva) was monitored by two manned submersibles from June to August 2011. The main goal was to assess whether micropollutants in the wastewater mass cause an ecotoxicological risk to the aquatic environment, and to determine how far the zone of risk extends beyond the wastewater outlet. Real-time measurements of elevated electrical conductivity were used as a proxy to indicate the presence of wastewater-influenced water. Conductivity was highest in immediate proximity to the wastewater outlet, though if all measurements obtained over the duration of the sampling campaign were integrated, elevated conductivity extended over an area of at least 1 km2 surrounding the outlet. Additionally, water samples were collected within and outside the Vidy Bay, and were analyzed for 39 micropollutants (pharmaceuticals, pesticides, and corrosion inhibitors). Micropollutant concentrations were generally in the low ng/L range, though for some substances > 100 ng/L was measured. The concentrations of most pharmaceuticals, which are primarily wastewater-derived, decreased with decreasing conductivity and with increasing distance from the wastewater outlet. Pesticide concentrations, in contrast, were homogeneous throughout the Vidy Bay and the lake. An ecotoxicological risk assessment based on the cumulative risk exerted by all measured substances indicated that the wastewater caused a zone of potential ecotoxicological risk that extended well into the deep lake and in the direction of a downstream drinking water intake.