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

ABSTRACT 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 0.40), 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.