A multi-tracer approach to assess land-use impacts on urban streams in silicate-dominated watersheds, South Korea

Assessing the impact of land use on stream water quality remains challenging because of complex interactions between hydrogeochemical processes, hydrological pathways, and anthropogenic inputs. Water samples were collected monthly from two silicate-dominated streams (Daejeon stream (DS) and Gap stream (GS)) in South Korea, comprising forested, agricultural, and urban watersheds, to evaluate land-use related and hydrological influences on stream water chemistry. DS was dominated by the Ca-HCO₃ water type in the upstream forested reaches; Na, Cl, and NO₃ concentrations increased toward the agricultural and urbanized downstream reaches in the DS and GS. As the level of urbanization increased downstream, Cl/SiO₂, B/SiO₂, and Rb/SiO₂ ratios effectively differentiated anthropogenic inputs from chemical weathering. The progressive increase in δ¹⁸O values with decreasing SiO₂ concentrations toward the lower reaches indicated enhanced contributions from surface runoff and drainage-derived waters in urban areas. Combined nitrate, boron, and sulfur isotope signatures in water samples and wastewater treatment plant (WWTP) effluents distinguished natural forested backgrounds from manure- and wastewater-related contamination downstream. During the dry season, high δ⁶⁶Zn values for GS samples collected from the lowest reaches were not readily explained solely by simple mixing with the sampled WWTP and industrial effluents, suggesting a possible additional source or process. Additionally, water samples collected from small brooks had δ⁶⁶Zn values similar to those of GS. These results suggest that integrated elemental and isotopic tracers, including Zn isotopes, provide useful evidence for evaluating land-use-related influences and downstream hydrochemical changes in silicate-dominated watersheds with mixed land-use patterns.