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Millions of people rely on active pharmaceutical ingredients (APIs) to prevent and cure a wide variety of illnesses in humans and animals, which has led to a steadily increasing consumption of APIs across the globe and concurrent releases of APIs into the environment. APIs in the environment can have a detrimental impact on wildlife, particularly aquatic wildlife. Therefore, it is essential to assess the potential adverse effects to aquatic ecosystems. The European Water Framework Directive sets out that risk assessment should be performed at the catchment level, crossing borders where needed. The present study defines ecological risk profiles for surface water concentrations of eight APIs (carbamazepine, ciprofloxacin, cyclophosphamide, diclofenac, erythromycin, 17α-ethinylestradiol, metformin and metoprolol) in the Vecht River, a transboundary river that crosses several German and Dutch regions. Ultimately, three main goals were achieved: 1) the geo-referenced estimation of API concentrations in surface water using the GREAT-ER model, 2) the derivation of new predicted no-This article is protected by copyright. All rights reserved. AcceptedArticle effect concentrations (PNECs) for seven of the studied APIs of which three were lower than previously derived values, and 3) the creation of detailed spatially explicit ecological risk profiles of APIs under two distinct water flow scenarios. Under average flow conditions, carbamazepine, diclofenac and 17α-ethinylestradiol were systematically estimated to surpass safe ecological concentration thresholds in at least 68 % of the catchment's water volume. This mounts up to 98% under dry-summer conditions.Graphical abstract. Spatially explicit environmental pharmaceutical risk profiles in the Vecht River basin under two climate condition scenarios.

Anthropogenically influenced transboundary catchment areas require an appropriately adapted exposure modelling. In such catchments, water management decisions strongly influence and override natural river hydrology. We adapted the existing exposure assessment model GREAT-ER to better represent artificially overprinted hydrological conditions in the simulations. Changes in flow directions and emission routes depending on boundary conditions can be taken into account by the adopted approach. The approach was applied in a case study for the drug metformin in the cross-border catchment of Vecht (Germany/Netherlands). In the Dutch part, pumps to maintain necessary water levels and minimum flow rates during dry periods lead to a reversal of the (natural) flow directions and as a consequence to additional pollutant input from the Lower Rhine/Ijssel along with a spatial redistribution of emissions in the catchment area. The model results for the pharmaceutical product metformin show plausible concentration patterns that are consistent with both monitoring results and literature findings at mean discharges and the effects of the changed hydrology in times of low natural discharges, namely an increase in polluted river sections under dry conditions due to the pumping activities. The adapted methodology allows for realistic application of the GREAT-ER model in anthropogenically modified catchments. The approach can be used in similar catchments worldwide for more realistic aquatic exposure assessment.