Multi-isotope tracing reveals nitrate and sulfate sources and fate across contrasting activity zones in a semi-arid basin of northern China

Study region The Dahei River Basin, a semi-arid basin in northern China. Study focus In semi-arid surface water-groundwater systems, nitrate (NO 3 – ) and sulfate (SO 4 2– ) are jointly influenced by natural processes and anthropogenic activities, yet how their sources, transformations, and export vary across contrasting activity zones remains insufficiently understood. Integrating hydrochemistry, multiple stable isotopes, a Bayesian isotope mixing model, and representative-section flux analysis, this study investigated the source compositions, transformation processes, and export differences of NO 3 – and SO 4 2– in the Dahei River Basin. New hydrological insights for the region: Results showed greater SO 4 2– enrichment in high-activity zones, whereas NO 3 – exceedance was more prominent in low-activity zones. In low-activity areas, NO 3 – mainly originated from manure and sewage (35.2%) and soil organic nitrogen (27.7%), whereas high-activity areas showed more mixed NO 3 – sources. For SO 4 2– , sulfide oxidation and manure and sewage were major sources in both zones, contributing 29.8–36.6% and 24.0–24.8%, respectively. Nitrification dominated NO 3 – transformation, whereas SO 4 2– transformation reflected overlapping effects of sulfide oxidation and mineralization, together with contributions from evaporite dissolution. At nested sections in the low and high activity zones, downstream annual cumulative NO 3 – and SO 4 2– fluxes were 3.16 and 8.41 times the upstream values, respectively, reflecting flow accumulation and combined natural and anthropogenic influences. These findings support zone-based source control and tailored water environment management in semi-arid basins.