Groundwater origin, flow regime and geochemical evolution in arid endorheic watersheds: a case study from the Qaidam Basin, Northwest China

Groundwater origin, flow and geochemical evolution in the Golmud River watershed of the Qaidam Basin was assessed using hydrogeochemical, isotopic and numerical approaches. Results show groundwater in the basin originates from precipitation and melt water in the mountainous areas of the Tibetan Plateau. Modern water was found in the alluvial fan and shallow aquifers of the loess plain. Deep confined groundwater was recharged by paleo-water during the late Pleistocene and Holocene under a cold climate. Groundwater in the low-lying depression of the central basin is composed of paleo-brines migrated from the western part of the basin due to tectonic uplift in the geological past. Groundwater chemistry is controlled by water-rock interaction and evaporation-salt precipitation, and varies from fresh to brine with the water types evolving from HCO 3 ·Cl-Ca·Mg·Na to Cl-Na, Cl-K-Na and Cl-Mg type waters along the flow path. The groundwater flow pattern is closely related to stratigraphic control and lithological distribution. Three hierarchical groundwater flow systems, namely local, intermediate and regional, were identified using numerical modelling. The quantity of water discharge from these three systems accounts for 82.69 %, 14.26 % and 3.05 %, respectively, of the total groundwater quantity of the watershed. This study can enhance the understanding of groundwater origin, circulation and evolution in the Qaidam Basin as well as other arid endorheic watersheds in northwest China and elsewhere worldwide.