Recharge sources, flow regime, and hydrogeochemical evolution of saline groundwater in an arid inland basin, northwestern China

Uneven water resources distribution and saline groundwater have become urgent concerns in inland arid areas because they pose difficulties for managing water resources. Policies and decisions depend on understanding the recharge sources, flow patterns, and hydrogeochemical evolution of groundwater systems. A comprehensive approach of hydrogeochemistry and environmental isotopes (δD, δ¹⁸O, ³H, ¹⁴C) was used to assess the groundwater recharge sources and hydrogeochemical evolution, and the Aksu River Basin was taken as the study area, which is located in the northwestern Tarim Basin, NW China. Results indicate that groundwater was originated from precipitation and meltwater in mountainous areas of Tianshan Mountain. Modern groundwater was found in the mountain front area and shallow groundwater locality near the surface area, while the deep confined groundwater was recharged by the paleo precipitation during the last glacial period. In the lowest discharge area, groundwater was recharged by the lateral flow from both the south desert and north Mountain areas. Along the flow path, groundwater evolves from freshwater to brine water and saline water, with a shift in hydrochemistry type from Na·Ca·Mg-HCO₃·SO₄ and Na·Ca·Mg-HCO₃·SO₄·Cl to Na-Cl. Mineral dissolution dominates the groundwater chemistry in the alluvial fan. The groundwater in the flow-through area is dominated by mineral (halite, gypsum) dissolution and cation exchange due to the longer residence times, leading to an increase in solute inputs along the flow paths and thus the evolving trend from freshwater to brackish water and finally saline water. On the contrary, infiltration of surface water decreases the salinity of groundwater partly. In the discharge area, the mineral dissolution and reverse cation exchange are the primary geochemical process controlling groundwater chemistry. This study could provide essential information for groundwater resource management.