Groundwater replenishment analysis by using natural isotopes in Ejina Basin, Northwestern China

The Ejina Basin underlying complex aquifers is located in the lower reaches of the Heihe River with an arid climate and 40 mm mean annual precipitation. As the balance of the natural ecosystem in the Ejina Basin is fragile and easily upset, it is very important to estimate and rationally use the limited groundwater resources to maintain the balance. Water samples were collected from the Heihe River and wells for chemical and isotopic measurements across the basin. The Piper diagram gives two main types of hydrochemical features. Against the background of the regional geology, combining isotope ¹⁸O, tritium, and chemical analysis with groundwater flows indicated by a shallow groundwater level contour map, different kinds of groundwater sources and ways to replenish groundwater were discovered. North of the study area are artesian wells that are replenished by the mountainous area at the boundary between China and Mongolia. Replenishment for most of the groundwater resources of the Gurinai oasis comes from the Heihe River seepage flow of the highly conductive paleochannel, not from the Badain Jaran Desert as indicated by TDS and tritium analysis. The different groundwater ages which are younger than 35 years were approximately estimated by radioactive isotope tritium (T). By such efforts, groundwater resources can be effectively evaluated with the engineering impact of the Heihe River Project.