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

• Published in: Applied geochemistry Vol. 25; no. 1; pp. 105 - 115
• Main Authors: Pauwels, Hélène, Ayraud-Vergnaud, Virginie, Aquilina, Luc, Molénat, Jérôme
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 2010; Elsevier
• Subjects: Aquifers; Atmospherics; Earth Sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Environmental Sciences; Exact sciences and technology; Fingerprints; Fractionation; Geochemistry; Global Changes; Groundwater; Isotopes; Pollution, environment geology; Sciences of the Universe; Soil (material); Wetlands; Brittany; France; Europe; Western Europe; fractionation; regolith; salinity; iron sulfides; fertilizers; nitrogen; mixing; ground water; oxidation; sulfates; nitrates; pyrite; denitrification; atmospheric deposits; assimilation; drainage basins; wetlands; aquifers; correlation; isotopes; reduction; soils; sulfides; BASSIN AQUIFERE
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2009.11.001

Stable SO 4 isotopes ( δ 34S -SO4 and δ 18O -SO4), and more occasionally δ 15N -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 SO 4 and NO 3 in the different aquifer compartments. Nitrate-contaminated groundwater occurs in the regolith; δ 34S fingerprints the origin of SO 4, such as atmospheric deposition and fertilizers, and δ 18O -SO4 provides evidence of the cycling of S within soil. The correlation between the δ 18O -SO4 of sulfates and the δ 15N -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 SO 4 isotopes. During autotrophic denitrification, both δ 34S -SO4 and δ 18O -SO4 decrease due to the oxidation of pyrite and the incorporation of O from the NO 3 molecule in the newly formed SO 4. Within wetlands, fractionation occurs of O isotopes on SO 4 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. δ 34S -SO4 and δ 18O -SO4 data fingerprint the presence of a NO 3-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.