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

Groundwater nitrate (NO) contamination has emerged as a critical global environmental issue, posing serious human health risks. This study systematically investigated the hydrochemical processes, sources of NO pollution, the impact of land use on NO 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 NO (δN-NO and δO-NO). The groundwater samples were characterized by neutral to slightly alkaline nature, and were dominated by the Ca-HCO 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 NO concentration. A total of 19.35% of the samples exceeded the Chinese guideline limit of 20 mg/L for NO. Isotopic evidence suggested that primary sources of NO in groundwater include NH-based fertilizer, soil organic nitrogen, sewage, and manure. Spatial distribution maps indicated that the spatial distribution of NO concentration correlated strongly with land use types. Elevated NO levels were observed in areas dominated by agriculture and artificial surfaces, while lower concentrations were associated with grass-covered ridge areas. The unabsorbed NH 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 NO pollution. The nitrogen fixation capacity of the grassland ecosystem led to a relatively low NO concentration in the ridge region. Despite elevated NO 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 NO source identification and hydrochemical processes across varying land-use types.