Hydrochemical and isotopic characteristics of groundwater in the Yanqi Basin of Xinjiang province, northwest China

In this study, hydrochemical and isotope investigations were conducted in the Yanqi Basin to determine the chemical composition, and to gain insight into the groundwater recharge process in the Yanqi Basin. It mainly used hydrochemistry, environmental isotopes, and a series of comprehensive data interpretation, e.g., statistics, ionic ratios, and Piper diagram to obtain a better understanding of the functioning of the system. The following hydrochemical processes were identified as the main factors controlling the water quality of the groundwater system: weathering of silicate minerals, dissolution, ion exchange, and to a lesser extent, evaporation, which seemed to be more pronounced down gradient of the flow system. As groundwater flows from the recharge to discharge areas, chemical patterns evolve in the order of Ca2+-HCO3 (-), Ca2+/Mg2+-HCO3 (-) to Ca2+-Mg2+-Cl--SO4 (2-), Na+-K+-Cl--SO4 (2-) and Na+-Cl- according to lithology. The environmental isotope (delta O-18, delta H-2, H-3) measurements further revealed that precipitation was the main recharge source for the groundwater system; some local values indicated high levels of evaporation. Tritium and CFC analysis were used to estimate the ages of the different groundwater; the tritium values of the groundwater samples varied from 2.82 to 29.7 TU. The age of the groundwater at depths of < 120 m is about 30-50 years. CFC values obtained for six samples to determine groundwater age; the age of the groundwater is about 20-50 years.