Groundwater recharge environments and hydrogeochemical evolution in the Jiuquan Basin, Northwest China

The groundwater recharge environments and hydrogeochemical characteristics in the Quaternary aquifer of Jiuquan Basin was investigated using a combination of chemical indicators, stable isotopes, and radiocarbon dating. The d-excess values of winter precipitation and surface water revealed that the meltwater from snow and ice played the dominant role in the basin's surface water supply. The unconfined groundwater showed gradual enrichment of heavy isotopes along the flow path, but δ18O and δ2H values were similar to those of surface water, suggesting recent recharge as a result of rapid seepage along rivers combined with the effects of high evaporation. The 14C (pmc) values of unconfined groundwater was between 71.5% and 90.9%, and since 80% modern carbon probably represents the upper limit of initial 14C activity, this suggests that the groundwater is relatively young. The confined groundwater was depleted in heavy isotopes; coupled with low 14C values (∼20–53%), indicating that the groundwater was mainly recharged as palaeowater during the late Pleistocene and Holocene epochs under a cold climate. The surface water and most groundwater samples were fresh rather than saline, with TDS <490 and <1000 mg L−1, respectively. The chemistry of unconfined groundwater changed from HCO3-dominated to no dominant ions and then to SO42-dominated moving along the flow path from the Jiuquan-Jiayuguan Basin to the Jinta Basin, and the confined water was SO42-dominated. The results have important implications for groundwater management in the Basin, where a high proportion of the water being used is in effect being mined (i.e., extracted faster than its replacement rate); thus, significant changes are urgently needed in the regional water-use strategy.