Waters were sampled monthly from a profile at the wastewater outlet and a reference point in the Bay of Vidy (Lake Geneva) for a year. The samples were analyzed for O-18/O-16 of water, C-13/C-12 of dissolved inorganic carbon (DIC), major ions, and selected micropollutant concentrations. delta O-18 values, combined with the major ion concentrations, allowed discharged waste and storm-drainage water to be traced within the water column. On the basis of delta O-18 values, mole fractions of wastewater (up to 45 %), storm-drainage (up to 16 %), and interflowing Rhone River water (up to 34 %) could be determined. The results suggest that the stormwater fractions do not influence micropollutant concentrations in a measurable way. In contrast, the Rhone River interflow coincides with elevated concentrations of Rhone River-derived micropollutants in some profiles. delta C-13 values of DIC suggest that an increase in micropollutant concentrations at the sediment-water interface could be related to remineralization processes or resuspension.