Evidence for denitrification regulated by pyrite oxidation in a heterogeneous porous groundwater system

Denitrification is an important natural attenuation process that has been observed in many fissured and porous aquifers. However, an important factor limiting denitrification in aquatic systems is the microbial availability of electron donors. Pyrite as the most abundant sulfide mineral in nature represents one of the potential electron sources for denitrifiers to reduce nitrate, but the reaction mechanisms coupling denitrification processes to pyrite oxidation are still questionable. We utilized hydrochemical data and stable isotopes of nitrate and sulfate in groundwater, isotope ratios of sulfur compounds in aquifer sediments and tritium based groundwater dating for assessing denitrification processes in a pyrite-bearing porous groundwater system. The oxic part of the aquifer with mean water transit times of approximately 60 years was characterized by nitrate concentrations of around 15 mg/l and δ15N values were similar to those typical for nitrification. In contrast, in the anoxic part with mean water transit times of up to 100 years, low nitrate concentrations accompanied by elevated δ15N values were observed. Furthermore, isotope data of groundwater sulfate and sulfur compounds in the aquifer sediment suggest that pyrite oxidation is the dominant source of sulfate in the aquifer. The trend of increasing δ15N values and decreasing nitrate concentrations in concert with depleted δ34S values of groundwater sulfate similar to δ34S values of pyrite, FeS2, suggests that denitrification is coupled to pyrite oxidation, particularly when water mean transit time is elevated.