Hydrochemical assessments and driving forces of groundwater quality and potential health risks of sulfate in a coalfield, northern Ordos Basin, China

Groundwater is the primary water source in coalfields under arid and semiarid climates. However, the problem of excessive concentrations of sulfate, which is a constant component in coalfields, and its potential health risks are often neglected in Northwest, China. To determine the groundwater quality, health threats, and driving forces of sulfate in coal mine groundwater, this study performed hydrochemical and isotopic analyses of 61 groundwater samples from a typical coalfield in northwestern China. We found that phreatic groundwater had lower total dissolved solid (TDS) and freshwater hydrochemical types (mainly Ca-HCO3 and Ca-Na + K-HCO3 types). In contrast, confined groundwater showed saline affinity (Na + K-SO4 type) and high TDS values, and the quality was unacceptable for drinking, with EWQI values larger than 100, which could be attributed to its high SO42- concentration. In addition, confined groundwater was also unsuitable for irrigation with high values of electric conductivity (EC), sodium absorption ratio (SAR), and Na%. Combining with isotopic analysis (δD, δ18Owater, δ34S and δ18Osulfate), the sulfate of confined and phreatic groundwater was controlled by gypsum dissolution and irrigation activities. As for public human health, SO42- poses potential non-carcinogenic risks to various populations, especially children. Therefore, the impact of geogenic and anthropogenic factors should be paid attention to, including the reduction of the use of sulfur-containing fertilizers and discharge of sulfur-containing sewage; and the water treatment should be carried out. Importantly, there is a need to adopt a strategy of water supply from multiple sources to ensure human health.