Hydrogeochemical evolution of Ordovician limestone groundwater in Yanzhou,North China

Major‐ion compositions of groundwater are employed in this study of the water–rock interactions and hydrogeochemical evolution within a carbonate aquifer system. The groundwater samples were collected from boreholes or underground tunnels in the Ordovician limestone of Yanzhou Coalfield where catastrophic groundwater inflows can be hazardous to mining and impact use of the groundwater as a water supply. The concentration of total dissolved solid (TDS) ranged from 961 to 3555 mg/l and indicates moderately to highly mineralized water. The main water‐type of the middle Ordovician limestone groundwater is Ca‐Mg‐SO₄, with SO₄^2‐ ranging from 537 to 2297 mg/l, and average values of Ca²⁺ and Mg²⁺ of 455.7 and 116.6 mg/l, respectively. The water samples were supersaturated with respect to calcite and dolomite and undersaturated or saturated with respect to gypsum. Along the general flow direction, deduced from increases of TDS and Cl^‐, the main water–rock interactions that caused hydrogeochemical evolution of the groundwater within the aquifer were the dissolution of gypsum, the precipitation of calcite, the dissolution or precipitation of dolomite, and ion exchange. Ion exchange is the major cause for the lower mole concentration of Ca²⁺ than that of SO₄^2‐. The groundwater level of Ordovician aquifer is much higher than that of C‐P coal‐bearing aquifers, so the potential flow direction is upward, and the pyrite in coal is not a possible source of sulfate; additional data on the stable sulfur and oxygen isotopic composition of the sulfate may be helpful to identify its origin. Although ion exchange probably accounts for the higher mole concentration of Na⁺ than that of Cl^‐, the dissolution of aluminosilicate cannot be ruled out. The data evaluation methods and results of this study could be useful in other areas to understand flow paths in aquifers and to provide information needed to identify the origin of groundwater. Copyright © 2012 John Wiley & Sons, Ltd.