Hydrochemical characteristics, groundwater nitrate sources and potential health risks in a typical alluvial plain of northwest China

Abstract

Due to anthropogenic pressures and the unpredictable impacts of climate change, groundwater quality issue has emerged as a critical concern. This study was carried out to investigate the evolving geochemical profile, identify the nitrate sources and transformation mechanisms, and assess the associated health hazards of phreatic groundwater in the Hua District, China. To achieve this goal, 65 and 37 phreatic water samples were collected in in October 2018 and March 2023, respectively, and analyzed for main water quality parameters and stable isotopes (δ¹⁸O-H₂O, δ²H-H₂O, δ¹⁵N-NO₃^-, and δ¹⁸O-NO₃^-). Piper diagram and ionic ratios were used for understanding the hydrochemical types and its evolution, stable isotopes were employed for tracing nitrate source, and human health risk assessment was also conducted to quantify the human health risk caused by exposure to groundwater NO₃⁻. The results showed that main ion composition of groundwater hydrochemistry is controlled by the rock weathering and dissolution of carbonates and sulfate minerals. The chemical types of groundwater present an evolution trend from SO₄•Cl-Ca•Mg to HCO₃-Ca•Mg. Severe nitrate pollution was detected particularly in the central and southern regions, with peak nitrate concentrations reaching 442 and 271 mg/L in 2018 and 2023, respectively. Nitrate sources include sewage, soil organic nitrogen, and fertilizers. A shift from single (M&S) to multiple nitrate sources in groundwater was observed, with nitrification as the primary transformation process. The non-carcinogenic risks for children and adults declined from 0.245–3.455 to 0.039–2.586 and from 0.236–2.392 to 0.031–1.469, respectively, from 2018 to 2023. Additionally, these risky regions are shifting northward. Hence, individuals in the north-central part should heighten their vigilance towards groundwater quality to reduce the threats of nitrate pollution to human health.