Hydrochemical Characteristics, Controlling Factors, and High Nitrate Hazards of Shallow Groundwater in an Urban Area of Southwestern China.

Groundwater nitrate (NO₃^-) contamination has emerged as a critical global environmental issue, posing serious human health risks. This study systematically investigated the hydrochemical processes, sources of NO₃^- pollution, the impact of land use on NO₃^- pollution, and drinking water safety in an urban area of southwestern China. Thirty-one groundwater samples were collected and analyzed for major hydrochemical parameters and dual isotopic composition of NO₃^- (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-). The groundwater samples were characterized by neutral to slightly alkaline nature, and were dominated by the Ca-HCO₃ type. Hydrochemical analysis revealed that water-rock interactions, including carbonate dissolution, silicate weathering, and cation exchange, were the primary natural processes controlling hydrochemistry. Additionally, anthropogenic influences have significantly altered NO₃^- concentration. A total of 19.35% of the samples exceeded the Chinese guideline limit of 20 mg/L for NO₃^-. Isotopic evidence suggested that primary sources of NO₃^- in groundwater include NH₄⁺-based fertilizer, soil organic nitrogen, sewage, and manure. Spatial distribution maps indicated that the spatial distribution of NO₃^- concentration correlated strongly with land use types. Elevated NO₃^- levels were observed in areas dominated by agriculture and artificial surfaces, while lower concentrations were associated with grass-covered ridge areas. The unabsorbed NH₄⁺ from nitrogen fertilizer entered groundwater along with precipitation and irrigation water infiltration. The direct discharge of domestic sewage and improper disposal of livestock manure contributed substantially to NO₃^- pollution. The nitrogen fixation capacity of the grassland ecosystem led to a relatively low NO₃^- concentration in the ridge region. Despite elevated NO₃^- and F^- concentrations, the entropy weighted water quality index (EWQI) indicated that all groundwater samples were suitable for drinking. This study provides valuable insights into NO₃^- source identification and hydrochemical processes across varying land-use types.