Origin and controlling factors of groundwater chemistry and quality in the Zhiluo aquifer system of northern Ordos Basin, China

Located in an arid and semi-arid region, the northern Ordos Basin is one of the key coal mining areas of China. The Zhiluo (J2z) sandstone aquifers, which overlay the Yan’an (J2y) coal seam layer, are a major source of drinking water. However, the hydrochemical evolution and possible impacts of coal mining on pollution of J2z groundwater over recent decades, and the water inrush hazard it presents, are not well understood. In this study, we investigate the groundwater’s origins and the factors controlling its quality and hydrochemical characteristics in these areas. In the northern and southern areas, J2z groundwater hydrochemistry shows distinct hydrochemical features, being Ca-HCO3 type with lower total dissolved solids (TDS) in the north, and Na + K-SO4 type with higher TDS in the south. Different water–rock interactions and coal mining activities may be the causes of such differences. We suggest that greater attention should be paid to preventing water inrush events during mining, as J2z groundwater has a good hydraulic connection with shallow groundwater in the northern area. Water treatment is also needed, as J2z groundwater in the southern area exceeds the safe limits for drinking and irrigation. In conclusion, the main factors determining local groundwater hydrochemistry and quality are mining activity, ground temperature, groundwater flow velocity, and groundwater circulation.