Nitrate sources and migration in rural karst aquifers: a case study in Pingyin karst catchment, north China

Due to the dramatic rise in global population and intensified agricultural activities, groundwater environments are increasingly threatened by nitrate pollution. To elucidate the sources and transformation processes of nitrate contamination in rural karst groundwater, this study selected the Pingyin karst catchment in Shandong Province, northern China, as the research area. This study employed a combination of isotopic analyses (δ¹⁵N-NO₃, δ¹⁸O-NO₃, δ²H-H₂O, and δ¹⁸O-H₂O) and hydrochemical data to investigate karst groundwater, pore groundwater, and Yellow River water. In the study area, nitrate concentration in karst groundwater samples ranged from 11 to 294 mg/L, with 80 % of samples exceeding the WHO safe drinking water limit of 50 mg/L for NO₃. Stable isotopic results (δ²H-H₂O, δ¹⁸O-H₂O) show meteoric precipitation is the main source of groundwater recharge. The ¹⁸O-Cl relationship bubble diagram revealed that nitrate contamination primarily originated from anthropogenic sources. Furthermore, both hydrochemical (e.g. TIN vs. Cl, NO₃/Cl vs. Cl) and isotopic (δ¹⁵N-NO₃ vs. δ¹⁸O-NO₃) evidence indicate that untreated human and livestock wastewater is the primary contributor to elevated nitrate levels in groundwater. Isotopic analysis also revealed that nitrification is the dominant biogeochemical process occurring in groundwater. The MixSIAR model further estimated source contributions as follows: manure and sewage (49.92 %), soil nitrate (39.43 %), chemical fertilizers (9.19 %), and atmospheric deposition (1.46 %). Finally, based on these findings and considering the environmental conditions of the study area, environmental protection strategies to prevent nitrate pollution were proposed. These findings serve as a valuable reference for promoting the sustainable use and pollution control of karst groundwater in rural areas.