Identification of nitrate sources in tap water sources across South Korea using multiple stable isotopes: Implications for land use and water management.

Stable nitrate isotopes (δN-NO and δO-NO) in conjunction with stable water isotopes (δO-HO and δD-HO) were used to identify nitrogen (N) sources and N-biogeochemical transformation in tap water sources sampled from 11 water purification plants across South Korea. The raw water sources are taken from rivers within the water supply basins, which indicates the quality of tap water is highly dependent on surrounding the land use type. We estimated the proportional contribution of the various N sources (AD: atmospheric deposition; SN: soil nitrogen; CF: chemical fertilizer; M&S: manure/sewage) using Bayesian Mixing Model. As a result, the contribution of N sources exhibited large seasonal and spatial differences, which were related to the type of land use in the water supply basins. Commonly, the M&S and SN were the dominant N source during the dry and wet seasons in almost regions, respectively. However, in the regions with high N loading ratios from urban and industrial sources, the M&S was the dominant N source during both the wet and dry seasons. In addition, the regions were characterized by high NO concentrations due to the decreased dilution effect of precipitation during the dry seasons. In contrast, the SN was the dominant N source in the regions with high N loading ratios from agricultural areas during both the wet and dry seasons. The NO-N concentration during the wet season was significantly higher than those during the dry season in these regions due to the input of non-point sources with high concentrations. Meanwhile, denitrification and nitrification were observed in the watersheds. It is important to understand the isotope fractionation due to N-biogeochemical transformation for considering the potential misinterpretations of the origin and fate NO. Collectively, our findings provide a basis on N source control strategies to ensure tap water quality in complex land use areas.