Noble gas constraints on the fate of arsenic in groundwater

Groundwater contamination of geogenic arsenic (As) remains a global health threat, particularly in south-east Asia. The prominent correlation often observed between high As concentrations and methane (CH${}_4$) stimulated the analysis of the gas dynamics in an As contaminated aquifer, whereby noble and reactive gases were analysed. Results show a progressive depletion of atmospheric gases (Ar, Kr and N${}_2$) alongside highly increasing CH${}_4$, implying that a free gas phase comprised mainly of CH${}_4$ is formed within the aquifer. In contrast, Helium (He) concentrations are high within the CH${}_4$ (gas) producing zone, suggesting longer (groundwater) residence times.

We hypothesized that the observed free (CH${}_4$) gas phase severely detracts local groundwater (flow) and significantly reduces water renewal within the gas producing zone. Results are in-line with this hypothesis, however, a second hypothesis has been developed, which focuses on the potential transport of He from an adjacent aquitard into the (CH${}_4$) gas producing zone. This second hypothesis was formulated as it resolves the particularly high He concentrations observed, and since external solute input from the overlying heterogeneous aquitard cannot be excluded.
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The proposed feedback between the gas phase and hydraulics provides a plausible explanation of the anti-intuitive correlation between high As and CH${}_4$, and the spatially highly patchy distribution of dissolved As concentrations in contaminated aquifers. Furthermore, the increased groundwater residence time would allow for the dissolution of more crystalline As-hosting iron(Fe)-oxide phases in conjunction with the formation of more stable secondary Fe minerals in the hydraulically-slowed (i.e., gas producing) zone; a subject which calls for further investigation.