Hydrochemical Characteristics and Health Risk Assessment of Karst Underground River Water in Southwest China: A Case Study of the Pingjiang Underground River Basin, Guangxi

• • Groundwater in the Pingjiang Underground River Basin is uniformly HCO3-Ca type, weakly alkaline and slightly hard, with ion concentrations highest in the dry season (e.g., HCO3− > SO4^2− > NO3− > Cl−; Ca2+ > Mg2+ > Na+ > K+). • • Hydrochemistry is primarily controlled by silicate and carbonate weathering, as indicated by saturation indices: calcite and dolomite SI > 0, while gypsum and halite SI < 0 for most samples. • • Agricultural activities significantly impact groundwater quality, elevating NO3− concentrations; in normal and dry seasons, 64% and 79% of adult samples and 64% and 86% of child samples exceeded the natural threshold of 3 mg·L−1, entering low health risk levels (adults: 0.09<RHI<0.30; children: 0.06<RHI<0.20). • • Spatial health risk (RHI) increases from northwest to southeast, with children consistently at higher risk than adults, indicating a need for targeted management in high-risk zones.