SWAT-WASP coupled modeling of ammonia nitrogen in rare earth mining watersheds

High concentrations of ammonia nitrogen (NH₄⁺-N) are a dominant water pollutant in ionic rare earth mining basins, threatening aquatic ecosystems and drinking-water safety. To quantify these dynamics, this study developed a coupled SWAT-WASP model for the upper Dongjiang River Basin (UDRB), integrating remote sensing and long-term monitoring data; the model was calibrated and validated with 2016-2018 monthly observations, and quantitative evaluation via Nash-Sutcliffe Efficiency (NSE) and Percent Bias (PBIAS) showed good performance (runoff: NSE = 0.77-0.80; NH₄⁺-N: SWAT NSE = 0.56-0.61, SWAT-WASP NSE = 0.65-0.87), confirming its reliability. 2022 simulations revealed strong NH₄⁺-N spatial heterogeneity, with concentrations >1.8 mg L^-1 near mining zones versus <0.5 mg L^-1 in upstream natural areas; geodetector analysis identified population density combined with industrial-agricultural activity as the top driver of spatial differentiation (<i>q</i> > 0.40), while interactions between precipitation, temperature, and land use further amplified variability. Overall, the SWAT-WASP framework provides a robust tool for evaluating NH₄⁺-N dynamics and supports targeted pollution control and ecological restoration in rare earth mining watersheds.