The composition and distribution of chemicals and isotopes in precipitation in the Shiyang River system, northwestern China

SummaryWe intensively investigated the composition and distribution of several major ions and stable isotopes (δ2H, δ18O) in precipitation in 2008 and 2009 at three sites in the Shiyang River Basin of northwestern China. The goal was to provide basic data that would help us to understand the geochemical evolution and recharge sources of groundwater in the basin’s Quaternary aquifer. The δ2H and δ18O values for 75 precipitation samples ranged from +35.7‰ to −170.9‰ and from +4.6‰ to −23.3‰, respectively. The relationship between δ2H and δ18O defined a well-constrained local meteoric line, which was nearly identical to the meteoric water line for northern China. The evaporation process in this dry region of northwestern China obviously altered the original relationship between rainfall δ2H and δ18O, resulting in d-excess values < 8‰, as has been previously reported in many arid regions. The relationship between local temperature and precipitation δ18O was statistically significant based on monthly average δ18O values and air temperatures. Mean annual concentrations of SO42-, NO3-, Cl−, NH4+, Ca2+, Mg2+, Na+, and K+ in mountain and desert areas were lower than those in most of China’s cities. The majority of the rainfall samples had a Cl− concentration of 1.5–2.5 mg L−1, and the excess of Na+ over Cl−, combined with a strong excess of non-marine SO42- and the overall precipitation chemistry, indicates that some solutes were contributed from terrestrial sources during the air mass trajectory over land. These values will provide reliable rainfall input information that can be used in future groundwater recharge calculations in the study area.

► We investigated the chemicals and stable isotopes in precipitation of an inland river Basin.
► The evaporation process altered the original relationship between δ2H and δ18O and resulted in d-excess values < 8‰.
► The relationship between precipitation δ18O and local temperature was statistically significant.
► The majority of the rainfall samples had a Cl− concentration between 1.5 and 2.5 mg/L.
► The Cl− and isotopes provide reliable rainfall input information for groundwater recharge calculations.