Artificial low-calorie sweeteners are consumed in considerable quantities with food and beverages. After ingestion, some sweeteners pass through the human metabolism largely unaffected, are quantitatively excreted via urine and feces, and thus reach the environment associated with domestic wastewater. Here, we document the widespread occurrence of four sweeteners in the aquatic environment and show that one of these compounds, acesulfame, meets all of the criteria of an ideal marker for the detection of domestic wastewater in natural waters, particularly groundwater. Acesulfame was consistently detected in untreated and treated wastewater (12-46 microg/L), in most surface waters, in 65% of the investigated groundwater samples, and even in several tap water samples (up to 2.6 microg/L) from Switzerland. The sweetener was not eliminated in wastewater treatment plants (WWTPs) and was quite persistent in surface waters, where concentrations increased with population in the catchment area and decreased with water throughflow. The highest concentrations in groundwater, up to 4.7 microg/L, were observed in areas with significant infiltration of river water, where the infiltrating water received considerable discharges from WWTPs. Given the currently achieved detection limit of approximately 0.01 microg/L, it is possible to trace the presence of > or = 0.05% wastewater in groundwater.