Is the Groundwater in the Hunshandake Desert (Northern China) of Fossil or Meteoric Water Origin? Isotopic and Hydrogeochemical Evidence

To gain an insight into the origin of groundwater in the Hunshandake Desert (HSDK), stable and radioactive isotopes and the major ion hydrochemistry of groundwater, as well as other natural waters, were investigated in this desert. The results showed that the groundwaters in the HSDK are freshwater (total dissolved solid (TDS) < 700 mg/L) and are depleted in delta H-2 and delta O-18 when compared with the modern precipitation. The major water types are the Ca-HCO3 and Ca/Mg-SO4 waters. No Cl-type and Na-type waters occurred in the study area. The ionic and depleted stable isotopic signals in groundwater, as well as the high values of tritium contents (5-25 TU), indicate that the groundwaters studied here are young but not of fossil and meteoric water origin, i.e., out of control by the modern and palaeo-direct recharge. A clear difference in the isotopic signals are observed between the groundwaters in the north and south parts of the study area, but the signals are similar between the groundwaters in the north HSDK catchment and its neighboring catchment, the Dali Basin. The topographical elevation decreases from the south (1396 m a.s.l.) to the north (1317 m a.s.l.) and the Dali (1226 m a.s.l.). Groundwaters in the north are characterized by lower chloride and TDS concentrations, higher tritium contents, higher deuterium excess, and more depleted values of delta H-2 and delta O-18 than those in the south. The spatial distribution pattern of these environmental parameters indicates a discrepancy between the hydraulic gradient of groundwater and the isotopic and hydrochemical gradients of groundwater in the HSDK, suggesting different recharge sources between the two parts in the desert. A combined analysis using the isotopic and physiochemical data of natural waters collected from the Dali Basin and the surrounding mountains was performed to investigate this problem. It indicates that groundwaters in the HSDK Desert are recharged from remote mountain areas (about 150-200 km to the east and southeast) but not from the north neighboring catchment.