Suspended sediment and pollutant transport during heavy rain events: A case study of the Moselle river

Extreme weather events pose major challenges for water managers and will likely increase in the future due to climate change. Heavy rain events potentially lead to short-term, but extraordinary changes in the composition of organic micropollutants (OMPs) e.g., via increased surface runoff and the input of untreated wastewater, as well as high inputs of suspended sediment into water bodies.

This study aims to unravel precipitation-related pollution patterns (including OMPs and suspended sediment) in the Moselle River and identify relevant sources and pathways relevant for rain-related emission. We used monitoring data of suspended sediment, which are derived using 15 min turbidity measurements or work-daily water sampling at seven stations starting in 1974. Furthermore, daily composite samples were collected by automatic samplers at two stations located along the German part of the Moselle River since April 2021. The chemical analyses included nontarget screening as well as target screening for selected fungicides.

From April 2021 to November 2021, 35 daily composite samples were selected for chemical analysis. Only one extreme rain event from July 12^th- 14^th, 2021 affected the water quality concerning suspended sediment concentrations and OMP mixture composition dramatically. During the event, 75 mm rainfall within 3 days lead to a flood with a return period of approximately five years. The estimated suspended sediment load of 141,000 tons during this event corresponds to approximately 13 times the long-term mean for the entire month of July and 23 % of the average annual load. A clockwise hysteresis pattern was observed, indicating instream remobilization of sediment and soil erosion in close proximity of the river. Concerning OMPs, three pollution patterns were identified. These patterns represented a) wastewater-related compounds diluted with increasing water level (e.g., pharmaceutical valsartan) as well as direct surface runoff from immediate surroundings of the river (e.g., fungicide fluopicolide), b) compounds introduced via increased groundwater discharge (e.g., pesticide metabolite metolachlor ESA) and c) compounds likely related to surface runoff in the catchment (e.g., herbicide terbuthylazine). While for the latter, the maximum intensity correlated with the maximum discharge and turbidity, the pattern related to groundwater input was characterized by a delay in maximum feature intensity relative to the maximum water level (i.e., kinematic wave effect).

Other, less extreme rain events that occurred since April 2021, did not show such pronounced OMP dynamics and such a strong hydro-sedimentary response in the Moselle river. This study supports our understanding of heavy rain induced OMP and suspended sediment emissions to a large river. With expected higher frequency and intensities of heavy rain events due to climate change, these emissions might gain in relevance in the future.