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

Abstract: Based on hydrochemical and environmental isotope data (δ15N and δ18O of , and δ34S of ) of depth-specific groundwater samples from multi-level samplers, the source(s) and biogeochemical behavior of and 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 (0.02 to ∼35mg/L ml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si7.gif" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mtext>SO</mml:mtext> </mml:mrow> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>-</mml:mo> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> (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 concentrations in the oxic groundwater are due to manure-derived and nitrification of urea- and ammonia-containing fertilizers used on agricultural fields. Chemical and isotopic data also revealed that groundwater 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‰ to +2.4‰. The relationship between δ34Ssulfate values and concentrations with depth showed that increasing concentrations were caused by S-bearing fertilizers, not pyrite oxidation. Bacterial (dissimilatory) reduction occurred locally in the sub-oxic groundwater, as indicated by increasing δ34Ssulfate values (up to 64.1‰) with concomitant decreases of concentrations. This study shows that isotope data are very effective for discriminating different sources for the waters with high and low 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 contamination of shallow groundwater. [Copyright &y& Elsevier]