Mechanisms of groundwater arsenic variations induced by extraction in the western Hetao Basin, Inner Mongolia, China

Abstract

High arsenic (As) groundwater has posed a serious threat to human health. Due to the complex spatial-temporal distributions of dissolved As, the influence of groundwater extraction on As enrichment has attracted great attention. To ascertain spatial-temporal variations in dissolved As and driving mechanisms impacted by extraction, we monitored groundwater levels and chemicals in different aquifers in the western Hetao basin, China. Results revealed that groundwater levels decreased during irrigation season in both shallow (<40 m below land surface (bls)) and deep (>42 m bls) aquifers, and the decreases in deep aquifer were more evident. During irrigation, the estimated proportion of recharge from shallow groundwater to affected deep groundwater (around 45 m bls) increased from 21% to 53% in the distal of the alluvial fan and sharply from 53% to 86% in the flat plain. In the distal of the alluvial fan, the vertical recharge of shallow groundwater with low As concentrations (<10 μg/L) directly decreased dissolved As concentrations in deep groundwater, and the introduced oxygen-containing groundwater promoted As fixation. In the flat plain, As concentrations of affected deep groundwater increased by around 80% during irrigation, which may result from the direct introduction of dissolved As, labile organic matter and high Na/Ca0.5 molar ratio from shallow groundwater. The latter two promoted As release by enhanced dissimilatory reduction of Fe oxyhydroxides and As desorption, respectively. The two-year monitoring data of conservative tracers and dissolved As concentrations in irrigation wells also validated the hypothesis. This study highlights the different variation patterns of deep groundwater As concentration impacted by extraction in different hydrogeological units, and provides an insight into mechanisms of As variations impacted by anthropogenic activity.