Hydrochemical characteristics, controlling mechanisms, and nitrate health risk of shallow groundwater in the Longzi Lake Basin, middle reaches of the Huaihe River, China

In agriculturally intensive regions, groundwater quality is increasingly challenged by nitrate contamination, posing risks to drinking water safety. This study systematically investigates the hydrochemical characteristics, water quality status, and nitrate-related health risks of shallow groundwater in the Longzi Lake Basin, located in the middle reaches of the Huaihe River, China. Forty groundwater samples collected in 2025 were evaluated using an integrated framework combining hydrogeochemical analysis, GIS-based spatial assessment, Water Quality Index (WQI), and hazard quotient (HQ) modeling. Groundwater is generally neutral to slightly alkaline, with HCO₃–Ca·Mg as the dominant facies. Hydrochemical evolution is mainly driven by carbonate weathering, with additional contributions from silicate dissolution and cation exchange processes. Although most major ions comply with drinking-water standards, nitrate contamination is widespread, with 45% of samples exceeding the WHO guideline (50 mg/L) and 7.5% classified as poor quality based on WQI. End-member mixing analysis reveals that nitrate enrichment originates from mixed anthropogenic sources, including agricultural input, livestock manure, and domestic sewage. Health risk assessment reveals clear age-dependent differences. Mean HQ values are 2.08 for infants and 1.74 for children, both exceeding the acceptable threshold, whereas adults show lower mean HQ values of 1.26 for females and 0.99 for males. High-risk areas are concentrated in the southwestern basin. The results indicate that while natural water–rock interaction governs baseline hydrochemistry, anthropogenic activities drive nitrate enrichment, highlighting the need for targeted groundwater management in agricultural areas.