EXPERIMENTAL STUDY ON WATER-ROCK INTERACTION OF LIMESTONE WITH DIFFERENT CLAY CONTENT CONSTRAINTS ON THE EVOLUTION OF GROUNDWATER COMPOSITION IN LIMESTONE REGIONS

The chemical composition of groundwater is controlled by water-rock interaction. In order to understand the influence of rock composition on water chemical composition during formation of deep groundwater, this study collected three limestone samples for soaking experiment. Three limestone samples named FLL, SLL, and TLL, are from Taoyuan Coal Mine, Huaibei Mining Area. Solute ratio and soaking time are variables designed for soaking experiment. Experiment results show that :I. total dissolved solids (TDS) value of three soaking liquids increases with the soaking time, while pH value decreases. It is because of dissolution of carbonate minerals and sulfide minerals; II. Except for Na+ and Cl- content of soaking liquids, which little change with soaking time, other major cations and anions content all show exponential increase with soaking time. But rate of increase slows down over time. The order of anions content is HCO3->SO42->Cl-, and HCO3- is the dominant anion. This relationship is consistent with the results for the major elements of rocks; III. The overall water chemistry type changes from Na-K (Ca) -SO4 type or Ca-Na-SO4 type to Mg-Na -HCO3 type or Ca-Mg-HCO3 type at the end of soaking. Changes in the type of water chemistry are characterized by limestone dissolution. It is obviously different from Taoyuan Mine's water chemical type. IV. Gibbs diagram and ion correlation analysis show that the main ion composition of three limestone samples soaking liquids are mainly derived from rock dissolution. The early stage is greatly affected by dissolution of evaporite, then the later stage is mainly affected by dissolution of carbonate and sulfide minerals. No obvious cation exchange occurred during the water-rock interaction. The dissolution of silicate occurs during the soak process of FLL and TLL, and dissolution of silicate is weak during the soaking process of SLL.