Kumar, and 8 co-authors, reported in 2020 that groundwater contamination by As from natural and anthropogenic sources is a worldwide concern. Redox heterogeneities over space and time are common and can influence the molecular-level speciation of As, and thus, As release/retention but are largely unexplored. Here, we present results from a dual-domain column experiment, with natural organic-rich, fine-grained, and sulfidic sediments embedded as lenses (referred to as “reducing lenses”) within natural aquifer sand.
The authors demonstrate that redox interfaces in sulfur-rich, alkaline aquifers may release concerning levels of As, even when sediment As concentration is low (<2 mg/kg), due to the formation of mobile thioarsenates at aqueous sulfide/Fe molar ratios <1. In the authors' experiments, this behavior occurred in the aquifer sand between reducing lenses and was attributed to the spreading of sulfidic conditions and subsequent Fe reductive dissolution. In contrast, inside reducing lenses (and some locations in the aquifer) the aqueous sulfide/Fe molar ratios exceeded 1 and aqueous sulfide/As molar ratios exceeded 100, which partitioned As(III)−S to the solid phase (associated with organics or as realgar (As4S4)). These results highlight the importance of thioarsenates in natural sediments and indicate that redox interfaces and sediment heterogeneities could locally degrade groundwater quality, even in aquifers with unconcerning solid-phase As concentrations.
The authors concluded that the presence of S-complexed As species in both aqueous (thioarsenates) and solid (adsorbed to organics and FeS) phases in this experiment adds to the increasing evidence for the importance of these species in sulfidic environments. Further, the fact that thioarsenates were most abundant in samples with high aqueous As concentrations suggests that thiolation promotes As mobilization, as has been proposed by others. Thus, as previously found for bioamended sediments, the authors' results indicate that aquifers with low (<2 mg/kg) sediment concentrations of As may be at risk for groundwater As concentrations exceeding the drinking water standards (10 μg/L) if the formation of thiolated As species is favorable, i.e., under dominant Fe-reducing conditions with a low, but consistent, supply of sulfide and near-neutral pH.
Importantly, the influence of sulfate-reducing zones on groundwater chemistry extended downgradient from the fine-grained, reducing lenses at distances up to four times the diameter of the lens itself. Although the authors only used one size and shape of reduced lenses, this suggests that relatively small layers or lenses of fine-grained, reducing material can have an outsized influence on otherwise oxic aquifers. Unfortunately, borehole samples and geology-based assessments that are performed prior to well installations are typically conducted on a scale where the presence of fine-grained, reducing lenses (i.e., redox heterogeneities) can easily be missed (or their presence is disregarded).
The authors also concluded that future studies should investigate the relevance of dimensions and spacing of redox heterogeneities induced by fine-grained, reduced lenses on their influence on groundwater chemistry. Such data, and the data from this study, provide guidance for developing predictive groundwater quality models for aquifers exhibiting heterogeneous sediment composition.l
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