The importance of selective transport and sedimentation in trend monitoring of metals in sediments. An example from the Glomma estuary, east Norway

Monitoring data have been utilised to examine how well sedimentsreflect changes in industrial discharges of metals over time.One aim was to assess the role of selective transport andsedimentation in regulating the spatial distribution of themetals in the sediments, and how this could affect trendmonitoring. The results showed a significant decrease insediment concentrations of zinc, lead copper and mercury overtime in accordance with the reductions of the industrialdischarge. Copper seemed to favour terrestrial organic matterfor adsorption, which primarily settles out in the inner part ofthe estuary. Zinc and lead followed the fine fraction togetherwith organic carbon and settled out in the outer part of theestuary. Mercury showed the same, though there was nocorrelation to organic carbon. The innermost part of the estuaryhad a patchy distribution of zinc, copper and mercury, but notlead due to its diffuse supply. When reductions in sedimentconcentrations are large, selectivity in transport andsedimentation has little influence on the possibility ofdetecting significant trends from one year to another. When thereductions are less, it becomes increasingly important to focuson sampling in the likely main deposition areas for eachspecific metal.