Identification and regional quantification of hydrochemical processes at the contact zone between anoxic groundwater and surface water in poldered floodplains (Oderbruch polder, Germany)

Under exfiltrating conditions interrupted by dry phases, more than 50 g kg−1 Fe and 0.25 g kg−1 Mn have accumulated in the drainage ditch floor sediments since the construction of the drainage ditches 35 a ago. The results show a very effective fixation of trace metals in the drainage ditch sediments under these conditions. Under permanent exfiltration conditions, the enrichment of Fe and Mn is relatively low. The maximum Fe content was 4 g kg−1 sediment and the Mn content reached only 0.4 g kg−1. This is less than 10% of the mobile Fe2+ and less than 1% of the Mn2+ which migrates from the aquifer into the surface water.