Hydro-geochemical processes of the deep Ordovician groundwater in a coal mining area, Xuzhou, China

The Ordovician aquifer is a typical limestone aquifer associated with Pangzhuang coal mine in North China. The groundwater properties in this area have been investigated, as the aquifer is a major drinking-water source for local people. The hydro-chemical type of the water is Ca-Na-HCO3. The bicarbonate concentration (290-428 mg/L) is relatively higher than that of other major anions (SO42-, Cl- and CO32-). Three reasons for the high bicarbonate concentration are proposed. Firstly, from geochemical calculations and correlation analysis, this study finds that the high-bicarbonate water results from dissolution of calcite and dolomite. Secondly, the high bicarbonate content is the result of influence from the regional hydrogeology. The study area is located in the recharge and runoff zone of the Ordovician aquifer, which outcrops in the southeastern hills, and the hydraulic connection between the upper and lower aquifers is confirmed in terms of faults and fissures developed from mining. Finally, high values of partial pressure of CO2 indicate an open carbonate system. Additionally, SO42- is confirmed as arising from the dissolution of gypsum, even though gypsum is not the major mineral in the aquifer. Inverse geochemical modeling of three cross-section lines provides additional evidence for the geochemical evolution. It also implies that more calcite is dissolved than dolomite. The coupled approach, using multiple evidence from both hydrogeological and hydro-geochemical insights, as well as geochemical modeling, is able to illustrate the groundwater hydro-geochemistry from a more intrinsic perspective.