Hydrogeochemical characteristics and controlling factors of confined water in Yinchuan

The strong evaporation have made the unconfined water have poor quality in Yinchuan plain, the middle temperate arid region. Confined water is the main water source for the mining of groundwater in Yinchuan City. Spatial analysis of ArcMap, conventional graphical methods, stable isotopic were used to analyze the distribution characteristics of confined water chemistry, and the difference among different aquifers. Then the controlling factors of confined water chemistry were discussed. The results show that the hydrochemical characteristics of unconfined water are significantly different with the two confined water in horizontal and vertical direction. The first and the second confined water have similar changing trends in the horizontal direction, but certain difference exist between them in the vertical direction. In the horizontal direction，the confined water near the west side recharge area has small TDS with the main anion of HCO-3, and the main cation of Mg2+ and Ca2+. The TDS increase eastwards. Around the Yinchuan fault the chief anion are Cl- and SO2-4, and the chief cation is Na+. The unconfined water, however, has no obvious zoning characteristics in the horizontal direction. In vertical direction, owing to the Yinchuan fault and the head difference between the two layers of confined water, the second confined water may be mixed by deeper layer groundwater, and the first confined water may be mixed by the deeper layer groundwater and unconfined water simultaneously. These make some second layer confined water samples have higher TDS, more Na+and Cl-, and less Ca2+ and HCO-3 than the first layer confined water. And some first layer confined water samples have higher TDS than the second layer confined water, and individual samples higher than the unconfined water. These shows that the development of Yinchuan fault throughout the entire confined and unconfined aquifer is significant to the groundwater quality. If excessive exploitation is conducted continually to the confined water, the confined water lever will continued to fall, which will further stimulate the interfusion of the unconfined and deeper layer water eventually lead to the deterioration of the confined water quality. Therefore controlling the excessive exploitation of confined water is significant to sustainable utilization of confined water resources in Yinchuan.