Temporal trends and spatial variations of total mercury in sediments of the legacy-contaminated river Elbe (Germany)

River systems often retain the imprint of historical mercury (Hg) contamination over long time scales since decreasing trends after reduced Hg inputs are impeded by legacy storage in sediments. Here, we report temporal and spatial variations of total mercury (THg) concentrations in sediments of the Elbe River, Germany. Temporal THg trends (1996-2019) were investigated at five long-term monitoring sites in harbors and groyne fields. In contrast to well-documented declines of THg in suspended particulate matter (SPM) in recent decades, significant trends of decreasing THg in sediments were only found for two harbors, while the other sites exhibited no decreasing trends. Spatial distribution patterns of THg in sediments (0.11-10.6 mg kg^-1) showed increases in areas of the Middle Elbe downstream of the confluences of the tributary rivers Mulde and Saale. Similar spatial patterns of increasing concentrations were observed in floodplain soils (0.04-4.20 mg kg^-1) and SPM (0.37-1.84 mg kg^-1). Regardless of the spatial origin or sample type (sediments, floodplain soils, SPM), the binding forms of Hg studied by sequential extraction showed a clear dominance of strongly bound Hg extractable with 12 M HNO₃. While this seems to indicate a low mobility of Hg, uptake into aquatic food webs may occur following methylation. Our results demonstrate that the reduction of Hg releases into the Elbe River basin has so far only been able to improve sediment quality in some parts. Deposits of Hg-containing sediments will therefore remain a future problem with implications for the continued exceedance of environmental quality standards and the management of waterways.