Effect of DOM transformation on As enrichment from an Alpine river basin in the Western Tibetan Plateau

Abstract

The geogenic enrichment of arsenic (As) extensively occurred in the riverine systems from the Qinghai-Tibetan Plateau under active geothermal discharge and chemical weathering conditions, while little is known about how dissolved organic matter (DOM) transformation regulates the aquatic As concentrations. The present study revealed that the DOM components from the Singe Tsangpo River (STR) basin primarily consisted of protein-like components (81.30 % ± 6.48 %), with the microbially-endogenous production being a predominant source under the control of temperature and glacier-runoff recharge along the river flow path. Notably, the chemical weathering processes have significantly facilitated the enhancement of humic-like components in the river water. Besides, the groundwater DOM characteristics were predominantly influenced by the mobilization of sedimentary organic matter and the introduction of allochthonous DOM resulting from surface-water recharge. Interestingly, humic-like components facilitated As enrichment through complexation and competitive adsorption effects in both surface water and groundwater under oxidizing conditions, which was supported by the significant positive correlations between As and humic-like component (R² = 0.31/0.65, P < 0.05/0.01) and the concurrent mobilization of As and humic-like components from sediment incubation experiments. Moreover, the Structural Equation Modeling analysis revealed a stronger contribution of humic-like components to the As enrichment in the groundwater compared with surface water, possibly due to the relatively stronger microbial activity and enhanced mobilization of humic-like components in alluvial aquifers. The present study thus provided new insights into the transformation of DOM and its important role in facilitating As enrichment in the aquatic environment from alpine river basins.