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

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 SO42- in the aquifer. The SO42- originates from river water infiltration and from oxidative dissolution of FeS2 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 (delta) 34S-SO4 values suggest continuous but slow (t1/2=50 years) sulfate reduction from the beginning of inflow onwards with (delta)34S-SO4 values ranging from +1.8 to +44.7‰ versus CDT and an enrichment factor of -33‰. A zone with a strong sulfate depletion ((delta)34S-SO4 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 SO42plume with concentrations exceeding the original river water content indicates FeS2 oxidation by O2 and/or NO3- within the alluvial loam. The lowered (delta)34S-SO4 value reflects the input of the isotopically lighter SO42from the sulfide.