Spatio-seasonal variability of source contributions to water quality in a large irrigation drainage lake basin based on the entropy weighted quality index, positive matrix factorization, and isotopic tracers.

Understanding spatial-temporal variations in lake water quality sources is essential for addressing regional eco-environmental challenges. This study presents an integrated framework-combining stable isotope tracing, PMF modeling, and EWQI-to examine spatio-temporal variations in water quality across China's Ulansuhai Lake basin. Integrated analysis of temporal and spatial sample ratios from 198 samples collected in April, July, and October 2023 revealed that 95 % of groundwater, 53 % of river water, and 15 % of lake water exceeded the EWQI drinking-water threshold (EWQI ≥100). Basin-wide water exhibited an alkaline character (mean pH 9.54), and 40 % of samples were classified as slightly saline (TDS 3000-10 000 mg/L). Source apportionment identified four main pollution contributors: evaporite dissolution (32.68 %), silicate dissolution (26.26 %), fluorine source and phosphorus source (23.96 %), and nitrogen source (17.10 %). Spatially, 27 % of eastern groundwater samples showed significant agricultural impacts, whereas western groundwater pollution was dominated by evaporite (50 %) and silicate (28 %) dissolution. Irrigation is a key driver of increased nitrogen pollution, and evaporite and silicate dissolution and fluorine-phosphorus release are characterized by significant seasonal variations. Water-rock interactions contribute to increased salinity and the release of fluorine and phosphorus when the water table drops, and irrigation activities also contribute to nitrogen mobilization and transport, which can increase nitrogen contamination. These findings support sustainable water resource management and agricultural non-point source pollution mitigation arid and semi-arid lake basins.