Sulfide oxidation and sulfate reduction in a shallow groundwater system (Oderbruch Aquifer, Germany)

Abstract

Detailed groundwater monitoring was carried out over a period of two years in an anoxic, river recharged aquifer of the Oderbruch polder, north-eastern Germany. Isotope data from wells located in a 5 km transect along the flow direction was used to determine sources and sinks of SO 4 2− in the aquifer. The SO 4 2− originates from river water infiltration and from oxidative dissolution of FeS 2 within the alluvial loam covering the aquifer sands. A change of confined hydraulic conditions near the river to unconfined conditions in the central polder effects the hydrochemistry of the aquifer. The confined areas are dominated by sulfate reduction. Increasing δ 34S −SO 4 values suggest continuous but slow ( t 1/2=50 years) sulfate reduction from the beginning of inflow onwards with δ 34S −SO 4 values ranging from +1.8 to +44.7‰ versus CDT and an enrichment factor of −33‰. A zone with a strong sulfate depletion ( δ 34S −SO 4 of up to +85.7‰) exists in a shallow microenvironment rich in solid-phase organic carbon between river and levee. In the unconfined areas of the central polder, a SO 4 2− plume with concentrations exceeding the original river water content indicates FeS 2 oxidation by O 2 and/or NO 3 − within the alluvial loam. The lowered δ 34S −SO 4 value reflects the input of the isotopically lighter SO 4 2− from the sulfide.