National-scale investigation of dual nitrate isotopes and chloride ion in South Korea: Nitrate source apportionment for stream water

Abstract

Nitrate sources in surface water have been identified using dual-isotope compositions of nitrate with various tools to efficiently manage the water quality at the local scale. Correlation between Cl and NO3 has also been used to identify NO3. In this study, we assess the reliability of the dual-isotope approach and Cl in terms of nitrate source apportionment. To this end, we collected stream water samples throughout South Korea to estimate nitrate sources in streams and determine whether the land-use pattern was closely related to nitrate sources. The δ15N–NO3 ranging from −1.3 to 14.8‰ showed a spatial distribution that was lower in mountain ranges (<7‰) than plain areas (>8‰). The Cl concentration in this national-scale distribution was also assessed. The relationship between the proportion of Cl and δ15N–NO3 classifies nitrate sources into areas characterized by three land-use patterns: (1) agricultural and business areas, (2) forests in highlands, and (3) lowland forests, of which (1) had proportions of Cl >50%, while (2) and (3) were <50%. The samples in (3) showed δ15N–NO3 values > 6‰, similar to those of (1). Deuterium excess of samples was negatively correlated (R2 = 0.53) with δ15N–NO3, accounting for the fact that δ15N–NO3 reflected land-use patterns. Samples were dominantly affected by agriculture-derived sources and domestic sewage showed NO3/Cl of <0.4 and δ15N–NO3 of >6‰. These results suggest that nitrate source apportionment should be comprehensively evaluated considering the dual-isotope approach, land-use patterns, and Cl proportions.