Metal accumulation in sediments and amphipods downstream of combined sewer overflows

The aim of this study was to investigate the concentrations of the metals Ag, Cd, Cu, Mn, Mo, Pb, Pt and Zn in sediments and amphipods along a 100m transect downstream of three different combined sewer overflows (CSOs). Moreover, the study was implemented to better understand the biological availability of metals downstream of CSOs as well as to identify a potential risk of CSOs for benthic or pelagic biota. Samples were taken at downstream sampling sites, which were 1, 5, 10, 20, 50 and 100m away from the outlets of the CSOs. An upstream-located sampling site each was used as reference for the respective transect. Additionally, sediments in two retention zones, located between the CSOs and the receiving creeks were analyzed. All downstream located creek sediments showed a similar metal pattern. Metal concentrations were found to be highest within the first 20m of the creek sediments. Elevated metal concentrations were also detected in the sediments of the retention zones. Metal accumulation in the amphipods was different at all three locations, but highest levels were always found downstream of the sediment accumulation hot spots. This might indicate that the metals were remobilized and became available for the amphipods further downstream of the CSOs. Although the general contamination pattern with respect to each reference site was similar, the degree of contamination was different at each location as demonstrated by anthropogenic enrichment factors. The discharge frequency as well as anthropogenic influences from upper reaches appear to be the most important factors for high metal concentrations in the sediments. Accordingly, in one case high background concentrations in combination with the CSO lead to an exceedance of the predicted effect level of Zn for aquatic organisms. Therefore, sediment contamination should be included in risk assessment when constructing CSOs to protect aquatic life.