Groundwater-fed oasis in arid Northwest China: Insights into hydrological and hydrochemical processes

As an inland groundwater-fed oasis in northwest China, the Dunhuang West Lake Wetland (DWLW) has been threatened by decreasing groundwater recharge over the past several decades. Understanding recharging processes for DWLW is a key step for better protection of the oasis, but poorly studied. To this end, we carried out a comprehensive water chemistry and isotope sampling and analysis, by taking the oasis and its water sources as a complete system to reveal the oasis water recharging mechanism and flow paths. Water samples, including mountain snowmelt water, river water, groundwater, spring and lake water, were collected within and around the DWLW, and their environmental isotopes (δ¹⁸O and δD) and water chemistry were analysed. Results showed that the stable isotope values exhibited strong spatial variations, ranging from −90.69‰ to −42.45‰ for δD and from −13.81‰ to −2.90‰ for δ¹⁸O. The isotope values were extremely depleted for snowmelt water, followed by river water, groundwater, spring water, and were most enriched in lake water. The difference in isotopic values from the different water bodies indicates that seasonal rivers primarily originate from the snowmelt water in the adjacent mountains, discharge into the aquifer through infiltration, then groundwater rises up by forms of springs to the lake oases, the river and groundwater interacted frequently, causing significant changes in solute in different positions. The ion concentrations accumulated continuously along the flow path, the total dissolved solid content varied from 416 mg/L in river water to 20360 mg/L in lake water, and the dominant anion facies changed systematically from ${HCO}_3^{-}$ to ${SO}_4^{2-}$ then ${Cl}^{-}$. The ionic ration plot, chlor-alkali index and saturation index demonstrated that dissolution of minerals plays a major role in the change in chemical composition, and other processes such as carbonate precipitation associated with cation exchange reaction also influence the chemical composition. Our results are helpful for understanding the hydrological and hydrochemical processes in arid groundwater-fed oases, and can be also useful for rational management and assessment of regional-scale groundwater resources in oases.