Pollutant patterns resulting from rain events in a large river: lessons learned for future monitoring

Abstract Background With climate change the frequency and intensity of heavy rain events is expected to increase. These events can severely affect river pollution with organic micropollutants (OMPs)—implying adjustments in monitoring efforts. However, data on event-based pollution is still sparse for large rivers. This study aimed to provide an enhanced understanding of event-based emission and pollutant processes in a large river to support future monitoring. Here, daily water samples during rain events and monthly water and suspended particulate matter (SPM) samples were collected over several months in the Moselle River. Samples were analysed by target and non-target screening (NTS). Results During a heavy rain and flood event in July 2021, three main pollution patterns were identified in NTS data by cluster analysis reflecting (A) wastewater and immediate surface runoff emissions, (B) increasing groundwater emissions following peak discharge and (C) flood-related emissions. The identity and sources of flood-related features were largely unclear due to few annotations by our spectral library. Structure elucidation of unknown flood-related features let to unambiguous identification of three natural compounds—partially with potential environmental and (eco)toxicological relevance. The importance of the July event was further highlighted by high concentrations and loads of dissolved and particle-bound OMPs—above all pesticides and transformation products—compared to monthly averages. Two main pollution patterns, i.e., wastewater (A) and groundwater (B), were transferable to smaller rain events. While the two pollutant patterns were not directly relatable to precipitation and discharge, pattern-representative OMPs (e.g., benzotriazole and metolachlor ESA) indicated changes in predominant emissions pathways clearly. Conclusions Based on NTS and cluster analysis, the study provided comprehensive pollution data and revealed event-based emission processes in a large, regulated river. It was demonstrated that chemical water quality of large rivers can be considerably impaired by heavy rain events—with effects lasting for several weeks. The study highlights the importance to amend future (routine) monitoring by (i) flow-proportional water sampling and higher temporal resolution in SPM sampling to account for effects of high discharge and SPM events, (ii) incorporating structure elucidation efforts of natural compounds and (iii) using pattern-representative OMPs as indicators for pathway contributions.