Hydrogeochemical Characteristics and Controlling Mechanisms of Groundwater in the Xinzhou Basin: Implications for Human Health Risk

As a strategically important groundwater source in Shanxi Province, the Xinzhou Basin contends with localized fluoride and nitrate exceedances whose hydrogeochemical controls remain insufficiently characterized. This work compares the unconfined and confined aquifers of the basin on the basis of 40 samples collected in July 2025, combining hydrogeochemical diagramming, multivariate statistics and the USEPA non-carcinogenic risk model. Both systems are mildly alkaline, with pH means of 7.96 and 7.95. Ca2+ and Mg2+ dominate the cations and HCO3− the anions, and the two groups share the same facies spectrum: 27 of the 40 samples are HCO3-Ca type and seven are Na-HCO3 type, while no sample reaches the Cl-Na corner. Water–rock interaction is the overarching control. Combined carbonate and silicate weathering supplies most of the solute, and evaporite dissolution is restricted to three samples in which sulphate exceeds bicarbonate. Calcite and dolomite are at or above saturation in 39 and 38 samples, whereas gypsum and fluorite remain undersaturated throughout (SI −4.17 to −0.32 and −2.64 to −0.12) and continue to dissolve. The seven Na-HCO3 samples carry the lowest Ca2+ fractions and the most negative chloro-alkaline indices, which identifies forward cation exchange as the process that removes Ca2+ and sustains fluorite dissolution. Nitrate has a mixed provenance, combining synthetic fertilizer, manure and domestic effluent, and soil organic nitrogen, and its imprint is stronger in the unconfined aquifer. Nitrate carries the larger hazard burden (HI up to 4.34, against 2.35 for fluoride), and unconfined groundwater records higher HI values than its confined counterpart, with ingestion as the dominant pathway. The two contaminants peak in different places: fluoride risk in the southern Xinfu–Dingxiang sector, and nitrate risk in Yuanping in the central basin. These results provide an evidence base for groundwater quality management, fluorosis mitigation and drinking-water security planning in the Xinzhou Basin.