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

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.

Acid mine drainage and smelter-derived sources affecting water geochemistry in the upper Nakdong River, South Korea.

Both the smelter and acid mine drainage (AMD) in uppermost streams impact water geochemistry and deteriorate water quality. Efficient water quality management requires identifying the contribution of each source to stream water geochemistry. In this study, we aimed to determine the natural and anthropogenic sources (AMD and smelting) affecting water geochemistry by considering seasonality. Water samples were collected, from May 2020 to April 2021, in a main channel (Nakdong River) and tributaries in a small watershed including mines and smelters. The watershed is characterized by a carbonate-rich area in the upper-middle reaches and silicate-rich area in the middle-lower reaches. On the plots of Ca/Na vs. Mg/Na and 2(Ca + Mg) vs. HCO3 + 2SO4, the water geochemistry was predominantly explained by the carbonate and silicate weathering associated with sulfuric and carbonic acids. According to typical δ15N values for sources, nitrate contribution from soil-N mainly impacted water geochemistry, regardless of seasonality; the contribution from agricultural activity and sewage was negligible. Water geochemistry in the main channel samples was discriminated before and after passing through the smelter. The effects of the smelter were evident in elevated SO4, Zn, and Tl concentrations and in δ66Zn values; this was further supported by the relationships between Cl/HCO3 and SO4/HCO3 and between δ66Zn and Zn. These results were pronounced during winter, when the flush-out effect was absent. Our results suggest that multi-isotopes and chemical composition analyses can trace multiple sources influencing the water geochemistry in watersheds containing AMD and smelters.