Fingerprints of micropollutants under baseflow and event discharge conditions: analysing gradients along two urban streams

This study presents the results from a monitoring campaign at two urban streams with multiple monitoring sites along an increased urban gradient, carried out in April to August 2022. The aim of the monitoring campaign was to identify pressures and sources of organic micropollutants using large-volume solid phase extraction. The campaign was using different sampling modes to distinguish between baseflow and evens. Samples were analysed for 735 compounds; the measured concentrations were aggregated for compound classes and related to discharge data. A source analysis showed different emission patterns for compound classes, using the correlation between concentration and discharges during baseflow. The number of detected compounds followed an increasing trend along the streams during baseflow and increased during events. The comparison between baseline samples and event samples showed that events cause a significant increase in the number of detected chemicals as well as in their concentration and relative loads. The event samples also yield higher ecological concerns based on a comparison of measured with effect concentrations (Toxic unit approach). We did not find correlations with different event characteristics (i.e. peak discharge, event duration, etc.). A high variability of compounds among the catchments and sampling sites within the catchments was evident.