Hydrologic changes induced by groundwater abstraction lead to arsenic mobilization in shallow aquifers

Intensive groundwater abstraction leads to hydrologic changes of groundwater. Nevertheless, the effects of hydrologic change on groundwater arsenic (As) mobilization remain controversial. Here, we investigated fluctuations in water levels and their effects on As mobilization in the shallow aquifer of the Hetao Basin. Results showed that large groundwater level fluctuations and high horizontal hydraulic gradients occurred in irrigation seasons. In the groundwater near the wetland with higher surface water levels than groundwater levels, biological index values of dissolved organic matter (DOM) ranged from 0.54 to 0.72, and a positive correlation between delta 18O values and dissolved organic carbon (DOC) was observed, indicating that groundwater DOM was mainly sourced from surface water. The degradation of allochthone labile DOM drove the reductive dissolution of As-bearing Fe(III) oxides to Fe(II). Both DOC and humification indices of DOM exhibited positive correlations with horizontal hydraulic gradients downstream of the study area, implying that the humified organic matter flushed from aquifer sediments contributed to groundwater DOM. The humified DOM controlled by hydraulic conditions participated in the redox reactions mainly by shuttling electrons to As-bearing Fe(III) oxides. These findings highlight distinct roles of hydrologic changes induced by groundwater abstraction in As mobilization.