Groundwater occurrence condition and hydrochemical characteristics：A case study of Anlong county map sheet in Guizhou Province

The development of fissures and karst caves in soluble rock strata in karst mountainous areas leads to a complex condition for groundwater occurrence, and affects the ability of water to dissolve solid substances. To have a better understanding of the relationship between groundwater occurrence and hyrochemical field in karst area, a case study was conducted in Anlong county of Guizhou Province, based on the data derived from the 1/50，000 hydrogeological mapping of Anlong country map sheet. The area is located in the transition zone between the Yangtze land mass and Youjiang orogenic belt, with a fairly complex condition in geological structure, strata and lithology. The strata of the whole area are mainly those the middle to lower Triassic, with occasional exposures of the Quaternary clay, sub-clay and the Permian clastic sedimentary rocks, which belong to typical special karst hydrogeological and geomorphic areas. Through exploration well water test and groundwater hole pumping test, we obtained the data of aquifer flow rate, drawdown, water chemical index values, then established the relationship between groundwater occurrence and the the hydrochemical characteristics, in order to explore the influence of the resource occurrence conditions on hydrochemical characteristics of the aquifer studied. Results show that the aquifers in the study area can be divided into four types, i.e. pure carbonate rock, chert-rich fractured carbonate rock, fractured clastic rock and loose primary aquifers. Hydrochemical type of the aquifers is dominated by HCO-Ca?Mg type. Except for the negative correlation between the depth of the exploration and production wells in the chert-rich carbonate rock aquifer and the concentration of Mg2+and HCO-3 ions in water (R2=-0.77/-0.74), the correlations between other ion concentrations and hydrogeological parameters (unit water inflow, drawdown) are not significant, and the coefficient R2 is less than 0.3. The total dissolved solids (TDS) and total hardness (TH) contents of the chert-rich carbonate rock aquifers and primary aquifers are lower than that of the pure carbonate rock aquifer by more than 5 mg?L-1, respectively. Furthermore, the total salinity and total hardness both with high values are concentrated in areas of the pure carbonate rock aquifers, while the low value areas are distributed in primary aquifers.