Isotopic and microbiological signatures of pyrite-driven denitrification in a sandy aquifer

Abstract

Denitrification driven by pyrite oxidation can play a major role in the removal of nitrate from groundwater systems. As yet, limited information is available on the interactions between the micro-organisms and aqueous and mineral phases in aquifers where pyrite oxidation is occurring. In this study, we examine the groundwater and sediment composition along a well-characterized redox gradient in a heavily nitrate-polluted pyritic sandy aquifer in Oostrum (the Netherlands) to identify the sequence of steps involved in denitrification coupled to pyrite oxidation. Multi-isotope analyses (δ15N-NO3−, δ18O-NO3−, δ34S-SO42−, δ18O-SO42− and δ34Spyrite) confirm that pyrite is the main electron donor for denitrification at this location. Enrichment factors derived from the observed changes in nitrate isotopic composition range from −2.0 to −10.9‰ for ε15N and from −2.0 to −9.1‰ for ε18O. The isotopic data indicate that pyrite oxidation accounts for approximately 70% of the sulfate present in the zone of denitrification. Solid-phase analyses confirm the presence of pyrite- and organic matter-rich clay lenses in the subsurface at Oostrum. In addition, sulfur XANES and iron XAS results suggest the presence of a series of intermediate sulfur species (elemental sulfur and SO32−) that may be produced during denitrification. Consistent with geochemical analysis, 16S rRNA gene sequencing revealed the presence of bacteria capable of sulfide oxidation coupled to nitrate reduction and that are tolerant to high aqueous metal concentrations.