Spatiotemporal contributions of hydrological components to streamflow and nitrogen load in Gyeongancheon Watershed, South Korea

This study examined the spatiotemporal dynamics of nitrogen (N) sources and transport in the Gyeongancheon Stream mid-watershed. Stable isotopes of nitrate (δ 15 N-NO 3 and δ 18 O-NO 3 ) and boron (δ 11 B), combined with a Bayesian statistical mixing model, indicate that manure/sewage and wastewater treatment plant (WWTP) effluent were the dominant N sources. However, due to isotopic overlap and complex mixing, precise source apportionment between WWTPs and groundwater pathways was difficult. To overcome this limitation, hydrograph separation using the recursive digital filter method was combined with water chemistry data to estimate the contributions of WWTP effluent, groundwater baseflow, and direct runoff to streamflow and N load under hydrological conditions. During the dry period, WWTP effluent dominated streamflow and N load in the main stream. Notably, during the growing season, the N load declined owing to biological uptake and streambed infiltration under losing stream conditions. In contrast, during the wet period, groundwater baseflow and direct runoff dominated both streamflow and N load following rainfall events. Under mid-range conditions, groundwater baseflow prevailed in the upper and middle reaches of the main stream, whereas the lower reach was almost entirely sustained by WWTP effluent, with substantial N load losses attributable to prolonged losing stream conditions. These findings underscore the critical role of hydrological variability and groundwater–surface water interactions in governing N dynamics in mixed-source watersheds, and demonstrate that isotope-based source apportionment alone is insufficient when end-member signatures overlap. Nutrient management strategies must be spatiotemporally differentiated to effectively mitigate N transport impacts under a changing climate.