Background <p>Sulfonylurea and other herbicides inhibiting the enzyme acetolactate synthase (ALS) are mobile compounds that may leach to groundwater, as indicated by computer simulations of predicted environmental concentrations (PEC_gw) for authorization in Europe. However, monitoring data for these substances are scarce. In this study, 19 active substances and 12 metabolites were analysed in 177 groundwater samples from 112 sites in Switzerland which are part of the Swiss national groundwater monitoring. Measured concentrations were compared with PEC_gw values from the authorization and with scaled PEC_gw values that take herbicide use into account. Because field-specific use data were not available, herbicide use was estimated based on annual sales volumes for Switzerland, the cultivated areas of crops with authorized uses of the substances, the proportion of these crops in groundwater catchment areas, and mean application rates.</p> Results <p>Most herbicides and metabolites were detected only sporadically or not at all in the investigated groundwater samples. However, the maize herbicide nicosulfuron and its metabolite ASDM were frequently found, in 20% and 56% of the samples, respectively, in concentrations of up to 7.8 and 48&#xa0;ng/L. Concentrations of nicosulfuron-ASDM were significantly higher in regions with arable crops as main land use than in regions where meadows, pastures, urban areas, or forest predominated. For 18 wells, the contributing recharge area had been attributed and a significant correlation was found between the proportion of land cultivated with maize and the concentration of nicosulfuron-ASDM. Furthermore, concentrations of nicosulfuron-ASDM correlated with those of other maize herbicide metabolites (metolachlor-ESA and terbuthylazine-LM6). Measured concentrations of nicosulfuron and nicosulfuron-ASDM were more than an order of magnitude lower than PEC_gw values from the authorization and also below PEC_gw values scaled for the estimated use of nicosulfuron in contributing recharge areas.</p> Conclusions <p>Measured concentrations in groundwater of all currently authorized ALS inhibitor herbicides, including some of their metabolites, were consistently below the European (and Swiss) parametric drinking water limit of 100&#xa0;ng/L even though PEC_gw for some of these compounds would suggest otherwise. When scaled to the estimated use in contributing recharge areas, these PEC_gw describe actual concentrations much better, but still tend to be conservative.</p>