Groundwater recharge and evolution in the Wuwei Basin, northwestern China

Understanding the mechanisms and processes of groundwater recharge and evolution is critical for sustainable water resources management to meet human and agriculture needs under climate change, because groundwater is the primary water source in semiarid and arid regions, where the surface water resources are usually highly unstable and scarce. However, few studies investigated the recharge and evolution processes of groundwater combining with isotopic geochemistry and radiocarbon data, especially focused on the interactions among precipitation, surface water, groundwater, and rock. This study examined the recharge and evolution processes of groundwater in the Wuwei Basin based on stable isotopes, chemical indicators, and radiocarbon data. Our results showed that the Na⁺ (sodium ion) and K⁺ (potassium ion) concentrations of the groundwater were controlled by the dissolution of sylvite and halite origin from sediments, whereas the increase of Na⁺ and Cl⁻ (chloride ion) concentrations were not in accordance with a ratio of 1:1, indicating that the Na⁺ and K⁺ concentrations in groundwater were barely affected by the dissolution of halite and sylvite. Meanwhile, we also found that bicarbonate ion (HCO₃⁻) was the dominant ion with a decreased ratio in the groundwater. The SO₄²⁻/Cl⁻ (sulfate ion/chloride ion) ratio decreased with the sample profile from Southwest to Northeast due mainly to the increases of Cl⁻ concentration. The Ca²⁺/Cl⁻ (calcium ion/chloride ion) ratio decreased with the enhancement of Cl⁻ in the hydrodynamic sluggish belt. In addition, the δ¹⁸O (oxygen isotope) and δ²H (hydrogen isotope) values of groundwater gradually increased from Southwest to Northeast along the flow path. The heavy isotopic values were more strongly depleted than the isotopic values of precipitation in the ground water samples, suggesting that the recharge of ground water in the plain region was very limited from precipitation. Moreover, the groundwater in the phreatic aquifer was younger water with ³H (tritium isotope) values from 47 to 71 a.BP (before present), while the groundwater age in the confined aquifer was 1000–5800 BP evidenced by the ¹⁴C (carbon isotope) values between 48 and 88 pmc (percentage modern carbon content). Overall, these results suggested that the unconfined groundwater enriched along the overall groundwater flow path from the southwest to northeast of the Wuwei Basin and the melt water from the Qilian Mountains may determine the water isotopic composition and contributed to the steady long-term runoff of the Shiyang River. Our findings may have important implications for inter-basin water allocation programmes and groundwater management in the Wuwei Basin.