Persistence of trace metal contamination in a fluvial system: Lahn River, central Germany

Abstract

To monitor short-term changes in trace metal concentrations (Cu, Pb, Zn) at geomorphically sensitive floodplain locations and examine the persistence of contaminants in the fluvial environment, near-channel sites along the Lahn River, central Germany, that were sampled in 2007 were revisited in 2019. Thirty-three cores were extracted from sites <5 m from the active channel along a 75-km reach of the river. The median concentrations of Cu and Zn in 2019 were statistically unchanged from concentrations in 2007, but median concentrations of Pb in 2019 were statistically higher than in 2007. Concentration of all three trace metals remain above the background concentrations, but only Zn concentrations exceed the 2ϭ background upper limit. On an individual sample site basis, the Cu concentration increased at 48 % of the sites and the Zn concentration increased at 45 % of the sites between 2007 and 2019. Pb concentration increased at 70 % of the sites. Concentrations of trace metals generally increased with distance downstream; the lowermost reach of the Lahn River continues to have the highest concentration of trace metals. Between 2007 and 2019, the concentrations (mean and median) of all three metals increased in the uppermost and lowermost sections of the study area whereas in the middle section the mean and median concentrations of Cu and Zn and the median concentration of Pb decreased (the mean concentration of Pb increased in the middle reach). Changes in metal concentrations between 2007 and 2019 do not appear to be caused by factors such as changes in substrate of samples, variable sedimentation rates and flood history, or the percentage of clay and organic matter content. Elevated concentrations of trace metals persist in near-channel sediments and will continue to pose a risk of future downstream heavy metal pollution in the study reach, especially along the lowermost reach of the Lahn River where the most contaminated sediments are located. Trace metal concentrations in the study reach appear to have become “uncoupled” from overall decreases in metal releases to the environment.