Groundwater–surface water interactions across an arid river basin: spatial patterns revealed by stable isotopes and hydrochemistry

Abstract. A comprehensive understanding of groundwater–surface water (GW–SW) interactions is essential for managing water resources in arid regions, where hydrological processes are highly sensitive to climate variability and human activity. This study investigates spatial variations in GW–SW relationships across the Shule River Basin in Northwest China, based on hydrochemical and stable isotopic analyses of 31 river water and groundwater samples. Isotopic results reveal a clear altitude effect in river water, with δ18O values decreasing at a rate of −0.08 ‰ (100 m)−1, which is lower than the rate observed in the adjacent Qinghai–Tibet Plateau. In the upper reaches, river water is mainly derived from precipitation, glacier meltwater, and groundwater. In the midstream area, river water recharges groundwater at higher elevations, while spring discharge contributes groundwater back to the river at lower altitudes. In the lower reaches, irrigation return flow becomes a key recharge source for shallow groundwater. Hydrochemical results show progressive salinization along the flow path. River water total dissolved solids (TDS) increases from 371.40 mg L−1 upstream to 1072.13 mg L−1 downstream, while groundwater TDS ranges from 506.51 to 1499.65 mg L−1. River water is primarily influenced by silicate and carbonate weathering, whereas groundwater chemistry is governed by mineral dissolution and cation exchange reactions. These findings highlight strong spatial heterogeneity in water quality and GW–SW interactions. A conceptual model of the basin-scale hydrological cycle is proposed based on the above understanding. This model not only provides important insights into typical river–groundwater systems in arid regions of Northwest China but also serves as a valuable reference for analogous studies and the sustainable management of water resources in arid regions worldwide.