Land-use controls on sources and fate of nitrate in shallow groundwater of an agricultural area revealed by multiple environmental tracers

Abstract

Sources and transformation processes of nitrate in groundwater from shallow aquifers were investigated in an agricultural area in the mid-western part of South Korea using a multi-tracer approach including δ 2H and δ 18O values of water, δ 15N and δ 18O values of nitrate, Cl/Br ratios and chlorofluorocarbons (CFCs). The study area was comprised of four land-use types with natural areas at higher altitudes, upland areas with fruit orchards, paddy fields and residential areas at lower elevations. The isotopic composition of water was suitable for distinguishing groundwater that had infiltrated in the higher elevation natural areas with lower δ 2H and δ 18O values from groundwater underneath paddy fields that was characterized by elevated δ 2H and δ 18O values due to evaporation. δ 18O–H 2O values and Cl − concentrations indicated that groundwater and contaminant sources were derived from three land-use types: natural areas, residential areas and paddy fields. Groundwater age determination based on CFCs showed that nitrate contamination of groundwater is primarily controlled by historic nitrogen loadings at least in areas with higher nitrate contamination. Nitrate sources were identified using the stable isotope composition of nitrate and Cl/Br ratios. Higher δ 15N–NO 3 − values and Cl/Br ratios of 300 to 800 in residential areas indicated that waste water and septic effluents were major nitrate sources whereas lower δ 15N–NO 3 − values and Cl/Br ratios of 100 to 700 in upland areas suggested that synthetic fertilizers constituted a major source of nitrate contamination of aquifers. With only few exceptions in the natural area, contributions of atmospheric nitrate were insignificant due to the resetting of δ 18O–NO 3 − values via immobilization and re-mineralization of nitrate in the soil zone. In groundwater underneath paddy fields, 30% of samples had δ 18O–NO 3 − values at least 2‰ higher than expected for nitrate formed by chemolithoautotrophic nitrification; these samples were also characterized by low DO and NO 3–N concentrations and elevated Cl and Mn concentrations indicating anthropogenic contamination and denitrification in the aquifer. These conditions were observed primarily in aquifers on floodplains. Statistical comparison between land-use groups revealed that Cl/Br ratios were more diagnostic for the impact of different land-use types on groundwater quality than stable isotope compositions of nitrate. This indicates that the former is an additional efficient tracer for the effect of land use on groundwater quality in agricultural areas. We conclude that the combination of groundwater age dating together with the use of chemical and isotopic parameters is a highly effective but yet underutilized approach for elucidating the sources and the fate of nitrate in aquifers in Asia.