Hydrogeochemical processes of As enrichment and migration in the Surface’s critical zone of the Shiquan River basin in the western Qinghai–Tibet Plateau

The enrichment of As in the western Qinghai–Tibet Plateau and its surrounding basins has been widely reported. However, the spatial distribution of arsenic content in the Surface’s Critical Zone (SCZ) of the Shiquan River Basin in the western Qinghai–Tibet Plateau is poorly understood, as is the mechanisms through which water–rock interactions and surface water-groundwater interactions influence arsenic enrichment and migration. In this study, the contents of arsenic and other chemical components in surface water, groundwater, sediment, soil, and rock in the Shiquan River Basin were analyzed to investigate the hydrogeochemical process of arsenic migration. The results indicate that the concentration ratios of HCO₃^–/(Na⁺+K⁺), Ca²⁺/(Na⁺+K⁺), Na⁺/(Na⁺+Ca²⁺) and δ¹⁸O in the surface water of the Shiquan River Basin vary along the runoff path. Carbonate and silicate weathering dominate the middle and upper reaches, while evaporite dissolution gradually increases in the lower reaches. Arsenic is enriched in rocks, soil, surface water, and groundwater in the SCZ, with slightly higher concentrations found in the surface water, sediment, and soil in the middle reaches than in the upper and lower reaches. This spatial distribution suggested that the hydrogeochemical processes along the runoff path controlled the distribution of As in the basin. Water–rock interactions and hot spring discharge of arsenic-bearing minerals contributed to its enrichment in the basin, while adsorption by riverbed sediment led to the gradual decay of dissolved arsenic. The underlying aquifer (alluvial-diluvial layer) recharged by surface water is the main source of high arsenic concentration in groundwater. The findings of this study can serve as a reference for studying arsenic in similar high-arsenic basins worldwide.