Sources and biogeochemical behavior of nitrate and sulfate in an alluvial aquifer: Hydrochemical and stable isotope approaches

Highlights: → The hydrochemical and N-O isotope study to evaluate the source(s) and biogeochemical behavior of nitrate and sulfate in shallow alluvial aquifer. → Nitrate originated from manure and fertilizers is attenuated by denitrification in the lower oxic and sub-oxic groundwater. → The δ34Ssulfate values (up to 64.1 per mille) indicate that sulfate from fertilizers is attenuated by BSR in the sub-oxic groundwater. → Combined, isotope and hydrochemical data are effective to discriminate different sources for the high sulfate and low nitrate waters. - Abstract: Based on hydrochemical and environmental isotope data (δ15N and δ18O of NO3-, and δ34S of SO42-) of depth-specific groundwater samples from multi-level samplers, the source(s) and biogeochemical behavior of NO3- and SO42- in a shallow (<25 m below ground level) sandy alluvial aquifer underneath a riverside agricultural area in South Korea were evaluated. The groundwater in the study area was characterized by a large variability in the concentrations of NO3- (0.02 to ∼35 mg/L NO3-N) and SO42- (0.14 to ∼130 mg/L). A distinct vertical redox zoning was observed sub-dividing an oxic groundwater at shallow depths (<8-10 m below ground surface) from sub-oxic groundwater at greater depths. The δ15N and δ18O values indicated that elevated NO3- concentrations in the oxic groundwater are due to manure-derived NO3- and nitrification of urea- and ammonia-containing fertilizers used on agricultural fields. Chemical and isotopic data also revealed that groundwater NO3- concentrations significantly decrease due to denitrification in the lower oxic and sub-oxic groundwater. The δ34Ssulfate values of the oxic groundwater ranged from -14.4 per mille to +2.4 per mille. The relationship between δ34Ssulfate values and SO42- concentrations with depth showed that increasing SO42- concentrations were caused by S-bearing fertilizers, not pyrite oxidation. Bacterial (dissimilatory) SO42- reduction occurred locally in the sub-oxic groundwater, as indicated by increasing δ34Ssulfate values (up to 64.1 per mille) with concomitant decreases of SO42-concentrations. This study shows that isotope data are very effective for discriminating different sources for the waters with high SO42- and low NO3- concentrations in the lower oxic zone. It is also suggested that the use of N- and S-containing fertilizers should be better controlled to limit nitrate and SO42- contamination of shallow groundwater.