Spatial Variations in Groundwater-Surface Water Interactions at the Basin Scale of an Arid Region: Insights from Stable Isotopes and Hydrochemistry

Abstract. A comprehensive understanding of groundwater-surface water interaction patterns is crucial, particularly in arid regions of Central Asia, where typical river-groundwater systems are prevalent. In this study, 31 river water and groundwater samples were collected from the Shule River Basin (SRB) in Northwest China and analyzed for hydrochemical and stable isotopic characteristics to elucidate spatial variations in groundwater-surface water interactions. A notable finding is the significant negative correlation between the δ¹⁸O of river water and elevation, with a vertical lapse rate of 0.08 ‰/100 m, which is markedly lower than that observed in the adjacent Qinghai-Tibet Plateau. Isotopic analysis indicates that groundwater recharges river water in the upper reaches, while river water recharges groundwater in the lower reaches, highlighting a basin-scale transformation in their relationship. Hydrochemical analysis reveals that river water has an average pH of 8.36 and a mean TDS of 649.93 mg/L, while groundwater shows an average pH of 7.65 and a mean TDS of 759.13 mg/L. Both river water and groundwater exhibit increasing TDS from upstream to downstream, transitioning from slightly hard to hard water, yet both are suitable for irrigation. The chemical composition of river water is primarily influenced by silicate and carbonate weathering, whereas groundwater chemistry is dominated by mineral dissolution and ion exchange processes. This study provides critical insights into basin-scale hydrological cycles in Central Asia’s arid regions, offering valuable guidance for the sustainable management of groundwater resources in semi-arid environments.