Study on Groundwater of the Ejin Basin at the Lower Reaches of the Heihe River Using Isotopes

Ejin Basin with low precipitation but high evaporation, is located in the lower reaches of the Heihe River in northwest China. Most of the basin is desert where the ecosystems are vulnerable. The growth of vegetations in the basin relies mainly on regional groundwater, and the Heihe River is a main recharge source of groundwater. It is significant to understand the formation and circulation of groundwater for planning water conservancy projects and redistributing water resources in the lower reaches. In this paper, the behaviors of groundwater beneath the Ejin Basin are researched by using hydrochemistry and isotope-tracing technology based on considering the regional hydrogeologic conditions. Hydrochemical and environmental isotopic data are analyzed so as to reveal the interaction between surface water and groundwater and the recharge resources of groundwater in the different regions in the basin. The results show that the distribution characteristics of hydrochemistry and isotopes are different from the different locations because of the different structures of aquifers, formation conditions, recharge resources and flow paths of groundwater beneath the basin. The stable isotopic composition of stream water is similar to that of shallow groundwater near the river channel, and the shallow groundwater is very young based on the tritium and CFC data, these reveal that the properties of shallow groundwater are closely related to the stream water, and groundwater is mainly recharged by the Heihe River rising from the Qilian Mountains. The stable isotopic composition of groundwater beneath Gurinai, where the evaporation is violent, is different from that of stream water and shallow groundwater near the river channel, and the tritium and CFC data show that groundwater here is young, which indicate that groundwater beneath Gurinai could be recharged by local rainfall and from the Badain Jaran Desert. The stable isotopic composition of deep confined groundwater beneath a zone between the Juyan Lake and the China-Mongolia border are very negative, and deep groundwater is old compared with shallow groundwater, these reveal that the renewing capability of groundwater is weak. The research could give some suggestions for the management of water resources in improving environment in the basin. Because of stream water is the main recharge source of groundwater, a considerable discharge of the river must be assured. Some factors have to be considered in planning water conservancy projects, for example, the river realignment should not affect the surface water recharge to groundwater. The renewing capability of deep groundwater is weak, so we have to take into account cautiously in utilizing deep groundwater beneath the basin.