Evolution of Groundwater Geochemistry in the Minqin Basin: Taking Chloride as an Indicator of Groundwater Recharge
Cl-指示的石羊河流域地下水演化

Groundwater evolution and supply over the arid zone in northwestern China play a prominent role in desertification and storms, economic development, as well as affecting other major environmental problems. In general, it is infeasible to use traditional methods, such as water balance approaches, to address these problems because of rare precipitation and significant evaporation. It has been found that hydrogeochemical theory and chemical indicators can provide a promising way to resolve these problems. The Shiyang River basin is located in northwestern China, which is a particular watershed due to its ecology and serious environmental issues. Unraveling the evolution and recharge of groundwater over this region can provide a general regulation of water resources in large inland areas. The use of chemical tracers is one of the most effective methods to examine groundwater evolution and water sources. Chloride as an indicator is able to reveal groundwater evolution. To investigate the chloride concentration along the runoff path in the Shiyang River basin, and the relationship between chloride and the other ions, we utilized geochemistry technology and comprehensively analyzed the chemical evolution and its supply across arid areas in northwestern China. It is indicated that the chloride and TDS concentration in groundwater of the Shiyang River basin increases from the upper reaches to the lower reaches, ranging from 1000mg/l upstream to 3000mg/l downstream. The high concentration of chloride (or TDS) characteristics of groundwater in the Minqin basin suggests that the basin suffered from significant evaporation for a long time. The long retention of groundwater indicates low flow rates and high salinity, and a mixture of old and modern water. The relationship between chloride and TDS can be formulated as TDS=0.57Cl-+1.79(R2=0.7913), similar as that in the Jiuquan basin. This might be indicative of similar mechanisms of evolution of groundwater in the two areas. The relationship of Br-/Cl-, Na+/Cl-, S042-/Cl-, HCO3-/Cl-, Ca2+/Cl-, Mg2+/Cl- shows that there exists a close relationship between the surface water and groundwater recharge. In the Wuwei basin, groundwater and surface water, as well as its good quality, leaching and evaporation play an important role in the chemistry of groundwater. In the lower reaches of the Minqin basin, groundwater is generally older and the salinity is relatively high. Modern precipitation is not the major recharge to groundwater. The high magnesium implies that dissolution of gypsum and calcite provides extra magnesium. In summary, its evolution mechanisms include: filtration and water-rock interaction (calcite, dolomite, gypsum, halite and so on), cation exchange and adsorption. All these processes occur in runoff and groundwater flow, and are controlled by evaporation.