Deciphering spatio-seasonal impacts of agricultural irrigation on water quality evolution and surface water-groundwater interaction in a large irrigation district, northwest China

Abstract

The irrigation area on the south bank of the Yellow River (IASBYR) in Hangjin Banner, Ordos, China is an important strategic base for grain production in northern China. Yet its complex hydrogeological setting and insufficient monitoring data have limited understanding of water quality dynamics and surface water-groundwater interactions (SWIs). This study employed hydrochemical and isotopic analyses (258 samples), entropy-weighted water quality index (EWQI), positive matrix factorization (PMF), and ²²²Rn mass balance modeling (RMBM) to investigate these processes. Results revealed that groundwater and canal water were predominantly brackish to saline, contrasting with the freshwater of the Yellow River. Water quality degradation was controlled by nitrogen/fluoride contamination and water–rock interactions (WRIs; evaporite dissolution and silicate weathering), with 83 % of groundwater, 60 % of canal water, and 13 % of Yellow River water deemed non-potable due to excessive total nitrogen (TN). Spatially, SWIs exhibited distinct zonation: river leakage dominated in the upper reaches, with the flux rate (q_r) of 5.87 × 10^-4m³/(s∙m), while groundwater discharge flux rate (q_g) is 1.45 × 10^-4m³/(s∙m)). Minimal exchange occurred in the middle reaches (q_r: 2.93 × 10^-4m³/(s∙m), q_g: 0.32 × 10^-4m³/(s∙m)), and groundwater discharged into the river in the lower reaches (q_r: −16.06 × 10^-4m³/(s∙m), q_g: 0.64 × 10^-4m³/(s∙m)). Temporally, groundwater discharge prevailed during the spring and summer irrigation periods, while river leakage and groundwater discharge occurred in the autumn irrigation period. SWIs critically governed TN and F^- migration, with excessive nitrogen inputs driving eutrophication of riparian wetlands and elevated fluoride concentrations can be hazardous to human health. The findings offer critical insights for optimizing agricultural water management and ecological conservation in the Yellow River Basin.