Hydrogeochemistry of karst groundwater for the environmental and health risk assessment: The case of the suburban area of Chongqing (Southwest China)

Karst groundwater is a vital resource for drinking, living and irrigation purposes in karst agricultural areas of the world. Due to the vulnerability of karst aquifers, surface pollutants are easily transferred to the subsurface and make karst groundwater be deteriorated, thereby restricting the rational exploitation of karst groundwater resource. In view of this, 49 karst groundwater samples were collected from spring (SW) and underground river (URW) sites in the suburban area of Chongqing City and analyzed for various hydrochemical components. Particularly, the karst groundwater quality was comprehensively uncovered by combining characteristics of hydrogeochemical evolution and health risks caused by nitrate and fluoride. The results revealed that the karst groundwater was slightly alkaline in nature and the water types were mainly characterized by Ca-HCO3 accounting for 93.88% of the total samples due to the heavy dissolution of carbonate rock. The relatively high concentrations of Na+, SO42− and NO3− up to 271.88 mg/L, 277.94 mg/L and 56.94 mg/L were over the corresponding maximum acceptable limits for drinking water, which can be predominately attributed to the emissions of industrial park, dissolution of gypsum and pyrite and excessive application of chemical fertilizers. Although agricultural activities were stopped and chemical fertilizers were no longer applied during the sampling period, long-term application of fertilizers have a persistent adverse effect on the karst groundwater NO3−. The pollution index of the karst groundwater (PIG) revealed that the low pollution and potential pollution zones were noticed in the northwestern parts of the study area. With respect of the SW, all the total hazard index (HI) values were below 1 suggesting no significant health risk. On the contrary, HI values of 0.11–1.16 for adults, 0.15–1.61 for children and 0.17–1.83 for infants in the URW indicated significant noncarcinogenic health risks. Particularly, infants and children were more vulnerable to karst groundwater NO3− than adults. Furthermore, the noncarcinogenic health risks of karst groundwater can be mainly attributed to NO3−, confirmed by the higher contribution ratio (66.55%) to the HI values. Therefore, special and targeted measures need to be taken to decrease the NO3− concentration in agricultural area.