Free draining column leaching experiment and mechanism of water-rock interaction in main aquifer of Xinzhi coal mine

The water quality parameters of the K2 limestone aquifer of the Carboniferous Taiyuan Formation and the O2 f limestone aquifer of the Ordovician Fengfeng Formation overlap each other in the Xinzhi coal mine,China. Therefore, the two water sources cannot be effectively discriminated by the conventional discriminating methods. In order to solve the problem of water source discrimination and reveal the water-rock interaction mechanism of the four main aquifers in the Xinzhi coal mine,this paper conducts a free draining column leaching experiment on the core samples of four main aquifers in Xinzhi coal mine. The results show that ① the core samples contain sulfate of non-mineral phase,and the four main aquifer formations also contain sulfate and gypsum minerals,which cause the leachate and limestone aq- uifer water samples to be rich in sulfate ions;② the equivalent percentages of SO24- and Ca2+ ions in the K8 ,K3 ,K2 and O2 core leaching filtrates exceed more than 74% and 40% ,respectively,and the corresponding water chemical types of all leaching filtrates are SO4 -Ca type;③ the order of anion content in all leaching filtrates is:SO24- >HCO- >Cl- ,the order of cation content in the K8 ,K3 ,K2 and O2 f core leaching filtrates are Ca>Mg>K>Na,Ca>Na>Mg>K,Ca>Mg>Na>K and Ca>Mg>K>Na,respectively;④ the contents of Mo,Sb,U and Sr in K2 limestone core samples and leaching filtrate are higher than those in Ordovician O2 f limestone core samples and leaching filtrate,but the distribution of Fe ion content is opposite. These characteristics can be as a reference factor for distinguishing K2 and O2 f limestone aqui- fers. The water chemistry characteristics of the aquifer in Xinzhi Coal Mine are controlled by lithology,burial condi- tions,groundwater recharge and hydrodynamic conditions. Therefore,the actual water quality of the aquifer is more complicated and changeable than the leachate. The water chemical types of K8 sandstone aquifer and K3 limestone aq- uifer are HCO3 -Na and SO4 -Na,respectively,which are different from the corresponding leachate water chemical types. However,the water chemistry types of the K2 and O2 f limestone aquifers are generally the same as the corre- sponding leaching filtrates. The main water-rock interaction in the Permian K8 sandstone aquifer is mainly dissolved plagioclase,and a small amount of sulfate dissolution reaction may exist in some areas. The water-rock interactions in the K3 limestone aquifer of the Taiyuan Formation are mainly the dissolution of calcite and dolomite minerals,as well as the dissolution of some sulfates and sodium salts. The main water-rock interactions in the K2 limestone aquifer of the Taiyuan Formation and the O2 f limestone aquifer of the Ordovician peak group are the dissolution of calcite,dolomite and sulphate,and the desulfurization occurring in local area.