Trace elements in streambed sediments of floodplains: consequences for water management measures

The occurrence of trace metals was studied at the interface between groundwater and the drainage system of a large floodplain in NE Germany, the Oderbruch region. Depending on the predominant hydraulic connectivity between groundwater and the drainage channels, the geochemical environment creates a high variability in the accumulation of Fe, Mn, Cd, Zn, Cu, and As. The mobility of the trace metals depends on spatial variable redox transition zones which act as geochemical barriers between the anaerobe aquifer and the oxic surface waters. In drainage ditches with high exchange flow between groundwater and surface water the transition zone is small and unstable with a low retention potential for trace metals. Decreasing hydraulic gradients and respectively decreasing base flow cause the change for an extensive transition zone with increasing trace metal accumulation in the channel sediments. The accumulation is mainly controlled by oxidation and degassing of CO2. In the streambed sediments of channels which periodically run dry an effective chemical barrier can be observed. This Fe dominated oxic horizon controls the accumulation of Mn > Cu > As > Zn > Cd, which are mainly associated with fresh, amorphous Fe oxyhydroxides. The chemical barriers can be instable and reversible. Therefore, water management decisions are discussed which stabilize the barriers by controlling aquifer-channel exchange rates, channel oxygen content and surface water levels.