Redox processes in the Oderbruch polder groundwater flow system in Germany

Abstract

Large scale redox processes were investigated in a river recharged aquifer in the Oderbruch polder alongside the river Oder in north-eastern Germany. Major hydraulic and hydrochemical processes were identified qualitatively. As a result of intensive drainage activities in the past 250 a, the groundwater level within the polder is situated below the river water level and a levee prevents flooding of the lowland. As a consequence, river water permanently infiltrates into the shallow confined aquifer. A sequence of redox reactions, driven by organic matter degradation, can be observed during infiltration of oxic river water into the groundwater. Up to 3 km from the river, reduction processes from O 2 respiration to SO 2− 4 reduction dominate the groundwater chemistry. While reduction of Fe- and Mn(hydr)oxides is the source of the high amounts of dissolved Fe 2+ and Mn 2+, carbonate dissolution/precipitation reactions control the actual groundwater concentration of Mn 2+. The first order rate constant for SO 2− 4 reduction was found to be −0.0169 a −1. Fe 2+ is released into the groundwater at a rate of 0.0033 mmol l −1 a −1. The groundwater chemistry is strongly linked to the hydraulic conditions. Near the river, the groundwater is confined and recharged by bank-filtration only. In contrast, in the central polder the groundwater is unconfined and percolation of rainwater through the dried loam is possible because of texture changes such as shrinkage fissures. Geogenic pyrite present within the alluvial loam is oxidised and large amounts of SO 2− 4 are released into the groundwater.