Using hydrochemical and isotopic data to determine sources of recharge and groundwater evolution in an arid region: a case study in the upper–middle reaches of the Shule River basin, northwestern China

Understanding sources of recharge and mechanisms for hydrogeochemical evolution of groundwater in the Shule River basin, an arid inland river basin in northwestern China, is essential for successful water resource management. Sources of water and associated groundwater recharge processes were investigated using hydrogeological, hydrogeochemical, and isotopic methods. The study area was divided into three parts: Changma, the Shule valley, and the Beishan area. The hydrogeochemical and isotopic analysis results show that groundwater in Changma mainly originated from precipitation in the Qilian Mountains. Lateral flow from Changma is the primary recharge mechanism, while direct recharge by infiltration of precipitation can be neglected in the Shule valley. Minerals within the aquifer material (e.g., halite, calcite, dolomite, and gypsum) dissolve into water that recharges the groundwater system. Therefore, strong linear relationships were found between Na⁺ and Cl⁻ and between Ca²⁺+Mg²⁺ and SO₄ ²⁻+HCO₃ ⁻, with stoichiometry ratios of approximately 1:1 in both cases. Groundwater quality in the region is generally good, with low salinity and hardness. Local precipitation is the only source of recharge for groundwater in the Beishan area. Low groundwater velocity and dry climatic conditions suggest that dissolution of minerals from the aquifer material controls hydrogeochemical evolution. Water rarely travels from the Beishan area to the Shule valley; therefore, almost all the groundwater in the Shule valley originates from Changma. Global climate change and loss of glaciers in the Qilian Mountains may lead to a reduction of recharge; therefore, the groundwater resources of the Shule valley must be managed to sustain water supply.