Probabilistic source apportionment and quantification of nitrate contamination in a karst aquifer system: Revealed by stable isotopic and hydrochemical proxies

Karst basins are characterized by high permeability and complex surface-subsurface hydrological connectivity which intensifies nitrogen migration. As a typical karst basin located in southern Guilin, the Lipu River basin faces environmental pressures from intensive agriculture and industrial activities; however, the contributions of different nitrate sources and the dominant nitrogen cycling processes have not been systematically characterized. In this study, NO ₃ ⁻ stable isotopes (δ ¹⁵ N-NO ₃ ⁻ and δ ¹⁸ O-NO ₃ ⁻ , hydrochemical parameters, and a Bayesian isotope mixing model were integrated to identify nitrate sources and elucidate nitrogen cycling in a karst river basin in Lipu County, southwestern China. The results show that total nitrogen concentrations in river water were significantly higher in winter (7.09 ± 6.58 mg/L) than in summer (2.95 ± 1.48 mg/L). Concentrations of NO ₃ ⁻ in river water during summer were significantly lower than those in winter and were also lower than groundwater concentrations in both seasons, indicating a strong dilution effect during the rainy season. Among different land-use types, groundwater NO ₃ ⁻ concentrations in industrial areas were significantly higher than those in agricultural and residential areas. These spatiotemporal patterns suggest that nitrogen pollution control should prioritize elevated winter nitrogen loads in river water, as well as nitrate contamination in groundwater associated with tributaries and industrial zones. Isotopic signatures combined with modeling indicate that manure and sewage contributed 56.7 ± 8.2% of total nitrogen inputs. This integrated isotopic and hydrochemical approach elucidates nitrogen distribution and source contributions in karst basins, providing scientific support for targeted mitigation strategies to reduce nitrogen-related environmental and health risks.