Dynamics of potentially toxic elements in small rivers during high-flow events.

This study investigates the concentration dynamics of potentially toxic elements (PTE) during flow events to improve process understanding of the transport dynamics and derive conclusions for monitoring and management. The case study is a medium-sized lowland river catchment (387 km) with mainly agricultural and urban land use. Three-to-four events were sampled at three monitoring sites in sub-catchments with different characteristics, with one-to-six samples taken per event. Total and dissolved concentrations of 29 major and trace elements were analysed, and concentrations in suspended particulate matter (SPM) derived by calculation. This was complemented by 10 riverbed sediment sample and 10 soil composite samples collected through a land-use stratified sampling design, ensuring high representativeness of soil across the entire catchment. The results confirm that PTE transport is primarily driven by the transport of SPM, but the PTE content in SPM exhibits distinct and systematic dynamics. Higher content of road traffic related elements (e.g. Sb) at earlier stages of the event indicates street runoff inputs, which has important implications for selecting effective emissions mitigation measures. Variability in both SPM transport and its quality has major implications for designing fit-for-purpose monitoring, for example for the accurate estimation of river loads or at the identification of contamination sources. The investigation of the dissolved fraction of the PTE concentrations shows significant differences between some events and therefore offers high potential to discriminate between different event types. Furthermore, we were able to identify the duration of the pre-event low-flow period as a possible driver for differences in events. The contamination level found in the catchment is low to moderate, with no exceedance of environmental quality standards. The calculation of enrichment factors identifies elements impacted by anthropogenic emissions, indicating important pollution sources and pathways. This is key to effective management.