Geochemical evolution of the Niangziguan karst water system under the impact of human activities

As the largest karst spring in north China, the Niangziguan spring is the most important source of water for Yangquan city in Shanxi province. Affected by the water-rock interaction and recharge by the leakage of coal mining waste water and surface water, the hydro-chemical type of the karst water turns gradually from the HCO3-SO4 or HCO3 type into the SO4, SO4-HCO3 or SO4-HCO3-Cl one from recharge area to discharge area. In the centralized discharge area of the spring group, good quality HCO3-SO4 or SO4-HCO3 type water is formed by the mixing of groundwater from regional and local water system. During the transport of groundwater, over-saturation of calcite and dolomite resulted in precipitation and recrystalization. The dissolution of gypsum prevails except for areas where the groundwater is affected by coal mining and gypsum precipitation may occur. In the karst aquifer, groundwater quality is controlled at the beginning by calcite (dolomite) dissolution and then by gypsum dissolution once calcite (dolomite) dissolution is restrained. Gypsum in groundwater may precipitate and the carbonate dissolution capacity of groundwater is enhanced with leakage of coal mine drainage. Iron hydroxide in the groundwater precipitates with the evolution of water-rock interaction and heavy metals in the groundwater are removed via co-precipitation and adsorption by iron hydroxide.