Accumulation of Fine Sediments and Particle-bound Pollutants in a German Urbanised Stream

In urbanised areas, the surface runoff generated by heavy rainfall events mobilise particles carrying contaminants such as heavy metals and polycyclic aromatic hydrocarbons. Those particle-bound pollutants (PBPs) are likely to reach streams through combined sewer overflows or stormwater discharges. Hence, stormwater runoff from urban impervious surfaces affects hydrological and sedimentological conditions of urban streams. Therefore, it is necessary to assess sediment sources, pathways and storage in urbanised catchments to improve sediment management and receiving water quality.

This study aimed at characterising the impacts of urban wet weather discharges (UWWDs) along a stream bed. Thus, the intrusion of fine sediments and the concentration of heavy metals was evaluated along a downstream urbanisation gradient. Our study area is a small catchment (Lockwitzbach, 84 km²) located in Dresden, Germany. It has a main stream length of 29 km and its land use is dominated by non-irrigated arable land (40%), pastures (21%) and urban areas (14%). The urbanised area is clustered towards downstream. In this study we focused on the last 7 km within the city of Dresden, where 9 combined sewer overflows and 19 storm water outlets are located. The urban catchment was subdivided into 9 sewersheds considering the characteristics of the urban drainage network.

Between March and October 2021, sediment samples were collected along the stream bed in 7 points, before and after heavy rainfall events. A total of 75 sediment samples were characterised considering 9 elements concentration (i.e., Al, B, Cd, Co, Cr, Cu, Pb, Sr and Zn) of the fine sediment fraction (<63µm), total solids and volatile solids. Additionally, suspended sediment samples were taken upstream and downstream the urban area. In those two sampling points, high-resolution discharge and turbidity data were continuously monitored. Fine sediment loads were calculated in order to compute a mass balance of the urban catchment. This allowed to understand the dynamic transport mechanisms of fine sediments and relevant PBPs in the urban stream, considering complex runoff and discharge processes.

Furthermore, identification of main sources of sediments was carried out using finger-printing analysis. K-means clustering allowed to group the stream bed sediment samples into two distinct types: 1) “relatively clean sediment” and 2) “sediment affected by UWWDs”. The urban discharges increase the element concentration in the fine sediment fraction along the first 6 km of the stream. This suggests an accumulation of contaminants towards the urban gradient. However, results showed a high attenuation capacity of the urban stream, since after receiving 27 UWWDs, the elements concentration of the sediment collected in the last 1 km is statistically similar to the fine sediment collected upstream the urban area (cluster type 1).

Fine sediments export was calculated for each sewershed. Areas with UWWDs carrying high sediments and PBPs loads were distinguished. Likewise, potential hotspots of intrusion of fine sediments in the stream were clearly determined. Those hotspots could be potential locations to control fine sediments and PBPs. The findings will help prioritising and locating possible strategies to improve river water and sediment quality.