Environmental isotopic and hydrochemical study of groundwater in the Ejina Basin, northwest China

The study investigates the groundwater evolution and its residence time in the Ejina Basin, northwest China according to isotope and hydrochemical analyses results. The groundwater chemistry is mainly controlled by the dissolution of halite, Glauber's salt, gypsum, dolomite and calcite, also influenced by other processes such as evaporation, ion exchange, and deposition. Based on tritium content in atmospheric precipitation and by adopting a model with exponential time distribution function, the mean residence time of the unconfined aquifer groundwater with fairly high tritium activities (21-49 TU) is evaluated. The results show that these groundwaters have low residence time (5-120 years) and are renewable. In contrast, the deep confined groundwaters are tritium-free and radiocarbon values range from 18.3 to 26.7 pmc. According to the most commonly used C-14 correction models, the radiocarbon groundwater ages were calculated which yield ages of approximately 4,087-9,364 years BP. Isotopic signatures indicate formation of deep confined groundwaters in a colder and wetter climate during the late Pleistocene and Holocene. It is suggested that long-term, rational water usage guide should be set up for the Heihe River Basin as a whole to permit a considerable discharge to the Ejina Basin.