Invisible to the human eye, yet ubiquitous... The occurrence and the potential adverse effects of micropollutants in the environment are uncontested. Due to incomplete removal by conventional wastewater treatment technologies, the aquatic environment is contaminated with pharmaceuticals and other micropollutants (biocides, pesticides). This thesis discusses the occurrence and photolytic fate of micropollutants in the Vidy Bay of Lake Geneva, Switzerland and the resulting environmental risk. The spatio-temporal variations in the concentrations of 39 micropollutants were investigated during a 10-month sampling campaign, at different locations around a wastewater outfall in Vidy Bay. A pronounced wastewater plume was observed from April to October, leading to locally elevated pharmaceutical concentrations compared to the surrounding water column. The plume depth followed the thermal lake stratification, which moved to lower 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 wastewater outfall, but concentrations typically decreased with increasing distance from the 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 in the vicinity of the plume. On the other hand, from November to January, when uniform temperature prevailed throughout the water column, the plume surfaced or was not detected due to enhanced mixing of the water column. 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. To overcome the limitations of punctual sampling, a more complete picture of the distribution of wastewater-derived micropollutants in Vidy Bay was obtained by augmenting field measurements with a validated model, which incorporates the most important physical and chemical attenuation processes. Given the importance of photolysis in the fate of many organic pollutants, we proposed a model which combines the current hydrodynamics via 3D particle tracking, and the depth-dependent photodegradation of micropollutants. Moreover, the total ecotoxicological risk associated with the predicted concentrations of all micropollutants concentrations was determined. Direct discharge of effluent wastewater into the Bay resulted in a limited zone, in which micropollutant concentrations were sufficiently high that commonly applied water quality criteria were exceeded. The resulting ecotoxicoloigcal risk could mainly be attributed to only three among the 24 wastewater-derived compounds investigated, namely three antibiotics. As expected, photodegradation was an important removal mechanism for many compounds and thus, along with dilution, contributed to a reduction in the ecotoxicological risk over time and distance from the WW outfall especially for surface plumes in winter. In contrast, during thermal stratification, the importance of photodegradation processes in attenuating the plume extent is greatly reduced, due to light screening by the water column. Analysis of various scenarios showed that wind conditions and solar irradiation play an essential role in determining hydrodynamic currents and mixing processes, as well as photodegradation kinetics and thermal stratification. The presence of human metabolites of pharmaceuticals and their phototransformation products may also contribute to the ecotoxicological risk, as they may be more persistent than the parent compound and retain biological activity. Hence, the occurrence of various human metabolites of pharmaceuticals in Vidy Bay was also investigated. On average, the detected concentrations of human metabolites were similar to, or lower than, their associated parent compound. Moreover, the metabolites targeted in this study were determined to be more susceptible to environmental degradation processes than their parent. Nevertheless, their presence in the aquatic environment may still lead to an increase of the ecotoxicological risk. In continuation, the propensity of selected human metabolites to undergo photodegradation and resulting photodegradation products was studied in detail. In particular, we determined the direct photolysis kinetics and identified the major photoproducts of four human metabolites of the antibiotic sulfamethoxazole, which is one of the substances that exceed water quality criteria in Vidy Bay. The majority of the investigated metabolites were found to be more photostable than the parent compound under environmentally relevant conditions, in contrast to observed field data, indicating that other transformation processes such as indirect photolysis processes and biodegradation may also contribute to the total degradation in the investigated environment. In addition, identification of the major phototransformation products of several human metabolites showed that one was retransformed back to the parent compound via a photoreduction. As such, human metabolites could represent a thus overlooked environmental source of pharmaceuticals in the environment. This work has revealed new insights regarding the environmental impact of direct discharge of effluent wastewater into lakes. Moreover, it allowed for the identification of important processes and parameters affecting the fate of micropollutants in the aquatic environment, as well as of the problematic compounds which should be considered priority substances for wastewater treatment plant optimization and further monitoring campaigns. Applying a precautionary principle, the results of the present study underline the importance of reducing or preventing the release of micropollutants to the environment.