Identification of nitrate and sulfate sources in groundwater using dual stable isotope approaches for an agricultural area with different land use (Chuncheon, mid-eastern Korea)

The identification of sources and behavior of agricultural contaminants is important to control and manage groundwater quality of aquifer systems in rural areas. In this study, hydrogeochemistry of major constituents and stable isotope ratios of NO3− and SO42− in groundwater were determined to identify contamination sources and transformation processes occurring in soils and shallow groundwater of Yupori (Chuncheon district), a rural area in Korea with intense agricultural activities. The western gently sloped part of the study area, used mainly for vegetable cropping, was characterized by locally recharged Ca-Cl(NO3 + SO4) type groundwater with elevated NO3− concentrations (>10 mg L−1 NO3−-N). Low δ34S values of sulfate and δ15N values of nitrate between 6 and 10‰ indicated that a mixture of chemical fertilizers and manure is responsible for groundwater contamination with agro-chemicals. In the steeper eastern part of the study area, in which fruit orchards are the predominant land use, the groundwater type was Ca-HCO3− water with lower NO3− (<3 mg L−1 NO3−-N) and sulfate concentrations. Elevated δ15N values of NO3− indicated that manure was the predominant NO3− source, but lower fertilizer application rates were the predominant reason for the lesser extent of groundwater contamination. This study demonstrates that a thorough evaluation of hydrodynamic and hydrochemical parameters in concert with multiple stable isotope ratios including dual isotopes of NO3− (N, O) and SO42− (S, O) constitutes an effective approach for identifying sources and transformation processes of NO3− and SO42− in shallow groundwater systems underneath agricultural areas with different land use.