From sources to rivers: Regional water quality modeling as a tool to disentangle pharmaceutical pollution from urban and rural sources

Despite considerable management efforts, surface waters are still polluted by unintended chemical mixtures affecting aquatic life and other societally relevant water functions. One systematic option to improve on the causes of this condition is prospective modelling of emissions, fate and risks of unintended mixtures on a regional scale, accounting for different land uses and the characteristics of the hydrological system. Such a model would allow to identify key emission sources and guide protective interventions to effectively improve the water quality. In the present study, we developed a multi-chemical model, focusing on combined human and veterinary pharmaceuticals emissions from urban and rural sources. The model integrates data on use and location-specific emission patterns of chemicals with the hydrological characteristics of the river basin. We selected a densely inhabited and heavily farmed area in the Netherlands as a case study, i.e., the Eem River basin. River water concentrations of six pharmaceuticals were predicted and compared to measured concentrations. Two of the studied pharmaceuticals are mainly used in urban settings (i.e.,, fipronil and carbamazepine) and four are used in both urban and rural settings (i.e.,, dexamethasone, permethrin, sulfamethoxazole and trimethoprim). Comparison of predicted to measured concentrations shows that our model yielded spatio-temporal exposure patterns resembling those of the measurements. The results imply that preliminary environmental quality standards for fipronil and permethrin will be exceeded during the Summer and Spring seasons. The model allowed identification of dominant sources, which supports prioritisation of measures to improve protection of water quality. For the four multi-sourced compounds, urban sources dominated over rural sources. We recommend that our approach is broadly implemented, complementing the current generic, single-chemical prospective assessments and the retrospective water quality assessments, as the approach assists water managers with the opportunity to prognostically evaluate which emission sources and chemicals contribute most to water quality problems and in which season to act to reduce emission at selected sources.