Identification of spatiotemporal behavior of organic micropollutants in an agricultural and urban lowland river catchment

Mixed land-use, medium-sized lowland rivers are subject to contamination by various input pathways for organic micropollutants. This study provides a unique long-term (2016-2020) and high-resolution investigation of the Münstersche Aa River (Northwestern Germany) focusing on the thorough characterization of input pathways for pesticides, biocides and pharmaceuticals in the catchment with heterogeneous land-use (62 % agricultural and 26 % urbanized). 468 samples were collected using two automated high-frequency samplers and five catchment-wide sampling campaigns providing a comprehensive data set. While the input of pesticides is the major stressor in the agriculturally dominated upper catchment, input of treated wastewater is dominating the water chemistry of the lower catchment, especially during summer low flows (up to 100 % wastewater fraction). Detected pharmaceuticals in the catchment include carbamazepine, diclofenac, metoprolol, phenazone and sulfamethoxazole. Moreover, caffeine as an indicator substance suggests input through non-conventionally treated wastewater. The pesticides chlortoluron (winter) as well as terbuthylazine and 2-methyl-4-chlorophenoxyacetic acid (MCPA) (summer), occur related to their respective application time. Moreover, a belated, accumulated input of terbuthylazine was observed in winter 2019/2020 after an extremely dry period from 2018 to 2019. Urban runoff adds further pesticides such as mecoprop-P while tebuconazole input from both agricultural and urban land-use was identified. Even though detected concentrations are below thresholds for acute toxicity, mixture toxicity could pose risks in the Münstersche Aa. Ongoing climate change is expected to intensify these seasonal patterns in the catchment. Lake Aasee may however function as a sink for OMPs through degradation processes especially during low-flow conditions. This study enhances the understanding of river pollution dynamics in small, mixed land-use catchments and aids targeting the goals of the Water Framework Directive for the Münstersche Aa and other lowland streams with comparable land use.