Profiles of geochemical and isotopic signatures from the Helan Mountains to the eastern Tengger Desert, northwestern China

We used environmental tracers to provide insights into groundwater evolution and flow processes on the eastern margin of China's Tengger Desert and determine recharge sources and recharge timing along the flow paths. The chemical composition was generally Na+ enriched, with no dominant anions. The chemistry is strongly influenced by evaporation and subsequent dissolution of minerals during recharge in the rainy season. Other processes, including cation exchange and weathering, also contribute to the water composition. The median groundwater delta O-18 value was around -9.1 parts per thousand, and most groundwater in the basin was depleted in heavy isotopes. delta C-13 in deep groundwater ranged from -4.73 to -9.56 parts per thousand, indicating that carbonate mixing is common but that isolated carbonate still exists. Radiocarbon values in groundwater ranged from 5.4 to 63.5 pmc. We estimated a residence time of 17.9 and 19.6 kyr at two desert sites, revealing that some replenishment of desert aquifers occurred in the late Pleistocene and early Holocene, when some of the upper reaches of rivers were characterized as modern water. (C) 2012 Elsevier Ltd. All rights reserved.