Multi-Year Hydrochemical Variability and Metal(loid) Risk Across River, Groundwater, and Irrigation Systems of the Yarlung Zangbo River Basin, Tibet

Abstract

Water quality in high-elevation agricultural river valleys is shaped by regional environmental gradients and localized hydrogeochemical conditions, but multi-year assessments often do not clearly separate routine hydrochemical variability from metal(loid) risk. This study evaluates a public multi-year dataset from the agricultural concentration area of the Yarlung Zangbo River and its two tributaries on the Qinghai–Tibet Plateau. The dataset includes 444 river-water, groundwater, and irrigation-water samples collected in 2019, 2020, 2021, 2023, and 2024. We combined water-type-specific standard assessment, normalized exceedance frequencies, spatial visualization, and descriptive correlation analysis with terrain, land-cover, soil, and climate-hydrological predictors. Because sampling coverage and hydrological-period classification differed among years, annual contrasts were interpreted descriptively rather than as fixed-site temporal trends. Most samples were neutral to weakly alkaline, whereas electrical conductivity (EC), total dissolved solids (TDS), and salinity varied more strongly across years and sampling locations. In total, 70 samples exceeded at least one evaluated criterion, but only 12 samples were flagged in the metal(loid) assessment. Eight samples exceeded the 0.05 mg/L As screening threshold, occurring only in 2023 and 2024 (3.25% and 2.92% of samples in those years, respectively), and were concentrated in a localized reach rather than distributed basin-wide. As was positively associated with Mo and Hg, whereas relationships with Fe, Mn, pH, EC, TDS, and salinity were weak or inconsistent; dissolved oxygen was not available for the high-As samples. These results distinguish broad dissolved-solute variability from localized As-dominated risk and show that the dataset supports regional screening and monitoring prioritization, but not definitive redox, speciation, or source-apportionment conclusions.