Agricultural Irrigation as a Dominant Driver of Surface Water–Groundwater Interactions and Hydrochemical Evolution in Arid Northwest China

In the arid plains of Northwest China, intensive agricultural activities and extreme climatic conditions have profoundly reshaped regional hydrological cycles. This study integrates hydrochemical analysis with multi-isotope tracing, including stable water isotopes (δ²H and δ¹⁸O) and nitrate isotopes (δ¹⁵N and δ¹⁸O), to quantify the complex interactions between surface water (SW) and groundwater (GW). Our findings demonstrate that large-scale agricultural irrigation serves as the primary physical driver enhancing the intensity and frequency of SW–GW exchanges. Isotopic signatures reveal that persistent irrigation return flows have strengthened the hydraulic connectivity between surface water bodies and shallow aquifers. This physical interaction further triggers significant biogeochemical responses. The irrigation-induced leaching of soil salts, coupled with intensive evaporation, is identified as the core factor governing groundwater salinization and quality deterioration, with Total Dissolved Solids (TDS) values ranging from 516 to 2684 mg/L. Hydrochemical modeling confirms that anthropogenic intervention, specifically cropland expansion and groundwater overexploitation, has superseded natural rock-water interactions in controlling the hydrochemical facies. To maintain water–ecology–agriculture security in arid regions, we propose that optimizing irrigation quotas and enhancing floodwater utilization are essential for sustainable groundwater management. This research provides critical insights into the mechanism of water quality evolution under the dual pressure of climate change and human interference.