The evolution of hydrogeochemical characteristics of a typical piedmont karst groundwater system in a coal-mining area, Northern China

Karst groundwater represents important natural resources and is the main target groundwater of drainage in the coal-mine fields of Northern China. It is easily influenced by external environment. However, the evolutionary processes and those factors impacting the Ordovician carbonatite groundwater are not well understood. To investigate the geochemical evolution of Ordovician carbonatite groundwater, a typical piedmont karst water system located in Taihang Mountain, Henan Province, China was selected for study. A series of groundwater samples were collected and analyzed. Isotope and chemical data were used to analyze the hydrochemical evolution of Ordovician groundwater in the research area. The results showed that the main ions of Ordovician groundwater are HCO₃⁻, SO₄²⁻, Ca²⁺, and Mg²⁺, and the predominant water chemical type is HCO₃–Ca·Mg, with some HCO₃–Mg·Ca and HCO₃·SO₄–Ca·Mg. The isotopic characteristics indicated that the groundwater is mainly recharged by precipitation. Groundwater geochemistry changed after rainfall infiltrated the aquifer. The changes to the Ordovician groundwater were caused by the reaction of water and rock, and the effect of evaporation is relatively small. The main source of Ca²⁺, Mg²⁺, SO₄²⁻, and HCO₃⁻ is the dissolution of calcite, dolomite, and gypsum. Na⁺ and Cl⁻ come from the dissolution of halite. The groundwater flow field also is an important factor in the control of groundwater evolution. Coal-mine production alters the groundwater flow field, and leads to the changes of ion content in areas around the coal mine. The evolution processes of the main ion contents in groundwater have a significant relationship with the groundwater flow. TDS and Cl⁻ increased with the seepage of groundwater, and the high values are around the stagnant and discharge area of groundwater. The Ca²⁺ content in groundwater decreases first and then increases to the maximum at the drainage area of coal mine along the groundwater flow field affecting the effect of CO₂. SO₄²⁻ contents increased continuously and significantly, that is effect by the mining operation, over a long period.