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

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 ∼35mg/L NO3N) and SO42- (0.14 to ∼130mg/L). A distinct vertical redox zoning was observed sub-dividing an oxic groundwater at shallow depths (<8-10m 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 δ³⁴S_sulfate values of the oxic groundwater ranged from -14.4‰ to +2.4‰. The relationship between δ³⁴S_sulfate 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 δ³⁴S_sulfate values (up to 64.1‰) 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.