The fate of nitrogen and sulfur in hard-rock aquifers as shown by sulfate-isotope tracing

摘要

Stable SO4 isotopes (delta S-34(-SO4) and delta O-18(-SO4)). and more occasionally delta N-15(-NO3) were studied in groundwater from seven hard-rock aquifer catchments. The sites are located in Brittany (France) and all are characterized by intensive agricultural activity. The purpose of the study was to investigate the potential use of these isotopes for highlighting the fate of both SO4 and NO3 in the different aquifer compartments. Nitrate-contaminated groundwater occurs in the regolith; delta S-34 fingerprints the origin of SO4, such as atmospheric deposition and fertilizers, and delta O-18(-SO4) provides evidence of the cycling of S within soil. The correlation between the delta O-18(-SO4) Of Sulfates and the delta N-15(-NO3) of nitrates suggests that S and N were both cycled in soil before being leached to groundwater. Autotrophic and heterotrophic denitrification was noted in fissured aquifers and in wetlands, respectively, the two processes being distinguished on the basis of stable SO4 isotopes. During autotrophic denitrification, both delta S-34(-SO4) and delta O-18(-SO4) decrease due to the oxidation of pyrite and the incorporation of O from the NO3 molecule in the newly formed SO4. Within wetlands, fractionation occurs of O isotopes on SO4 in favour of lighter isotopes, probably through reductive assimilation processes. Fractionation of S isotopes is negligible as the redox conditions are not sufficiently reductive for dissimilatory reduction. delta S-34(-SO4) and delta O-18(-SO4) data fingerprint the presence of a NO3-free brackish groundwater in the deepest parts of the aquifer. Through mixing with present-day denitrified groundwater, this brackish groundwater can contribute to significantly increase the salinity of pumped water from the fissured aquifer.