关键词: 董志塬, 地下水, 水化学, 稳定同位素

Abstract: The Dongzhiyuan region is a very particular area in China，which has the thickest loess deposition landforms reaching up to approximately 300 m. The water resource in this area is very limited owing to the low precipitation and its especial hydrogeological condition. Recently，much attention has been focused on the groundwater system，however，little studies have been performed to this groundwater aquifer. In this study，the groundwater evolution and its recharge are examined using a combination of chemical indicators and stable isotopes. The groundwater types are studied by the Piper diagram. The water-rock reaction of groundwater is determined by the relationship between the different ions and the Phreeqc model. Finally，the possible recharge of the groundwater is confined by the stable isotopic data. The study shows that the groundwater is quite depleted with the stable isotope，the value of δ18O is between -9.2‰～and -13.16‰，which is lower than the value of the modern rainfall，indicating the groundwater may form in a cold environment in ancient time. In addition，the calculated ¹⁴C age ranges from 3.17 to 24.32 ka，showing the groundwater is quite old. The groundwater type is HCO₃-Ca and HCO₃-Ca-Mg with a very low TDS of 270 mg/L，however，the salty of the rivers is very high with the value of 1 533 mg/L. The concentration of Na⁺，K⁺，Mg²⁺，NO-3 is below 1 mg/L . The mean value of m K/Na is 0.014，which is lower than that of the precipitation，indicating the loss of K absorbed by the plant. mNa/Cl between 4.54 to 46.07 but shows decrease along the flow path，which illustrates the dissolution of Glauber’s salt，however，the Na may exchange into the rock when the groundwater salinity increases. The plots of HCO^-₃+SO^2-₄ vs Ca²⁺+Mg²⁺ in groundwater lies below the 1∶1 stoichiometry line，there is a deficiency in （Ca²⁺+Mg²⁺） relative to （HCO^-₃+SO^2-₄），this must be balanced by Na+ origins from the Glauber’s salt. The relationship between the Na+-Cl- and SO^2-₄-（Ca²⁺+Mg²⁺） is very good，indicating the cation exchange is very important in the groundwater. Another significant characteristic of the groundwater is that all of the samples are unsaturated with the gypsum. The plot of SO^2-₄ and Ca²⁺ shows a positive correlation，indicating the gypsum has participated in the groundwater evolution. The SI value of the CaCO³ is above 0，indicating the precipitation of the calcite. The relationship between major ions and the results from the inverse modeling of the groundwater show that dissolution and precipitation of the carbonate，silicate and the gypsum controls the groundwater geochemistry，in addition，cation exchange and common-ion effects are also very important to the groundwater evolution. This paper carried out a systemic study of the groundwater，giving the overall view for the groundwater type，evolution and the recharge，the results have important implications for the groundwater management in the Dongzhiyuan region.

Key words: Dongzhiyuan, groundwater, chemical evolution, stable isotope