Hydrogeochemical and isotope evidence of groundwater evolution and recharge in Minqin Basin, Northwest China

A hydrochemical investigation was conducted in the Minqin Basin to identify the groundwater evolution and recharge in the aquifer. The mBr/Cl ratio is strongly depleted (average 0.000451) compared with sea water (0.0035), indicating an evaporite origin. The ionic ration plot, saturation index (SI), and chloro alkaline indices (CAI) suggest that the dissolution of halite, the glauberite, gypsum, dolomite and calcite determine Na+, Cl-, Ca2+, Mg2+, , and chemistry, but other processes, such as Na+ exchange for Ca2+ and Mg2+, and calcite precipitation also contribute to the water composition. The (delta)18O and (delta)2H in precipitation near the study area are linearly correlated, similar to that for the world meteoric water line (WMWL), with an equation of (delta)2H = 7.49(delta)18O + 5.11 (r^2 = 0.97). According to radiocarbon residence time estimates, the deep groundwater is approximately 40 ka old, and was recharged during a period when the climate was wetter and colder. The radiocarbon content of shallow groundwater shows a clear evolution along the groundwater flow path. From the beginning of the groundwater flow path to ~31 km the radiocarbon values are >73.6 pmc, whereas beyond this point the values are <42.9 pmc. Based on radiocarbon content, the shallow groundwater is older than 1 ka, and represents palaeowaters mixed with a limited quality of modern recharge. The rain-fed groundwater direct recharge was estimated by chloride mass balance (CMB) method to range from 1.55 to 1.64 mm yr^-1, with a mean value of 1.6 mm yr^-1. This value represents about 1.5% of local rainfall. The direct recharge volumes is about 0.666 × 10^8 m^3 yr^-1. Indirect recharge volumes by the surface water is about 0.945 × 10^8 m^3 yr^-1. The total natural recharge in the Minqin Basin is 1.6 × 10^8 m^3 yr^-1, whereas the groundwater abstraction has reached 11.6 × 10^8 m^3 yr^-1, far exceeding the groundwater natural recharge.