Redox Interfaces Can Produce Toxic Arsenic Levels in Groundwater from Low Arsenic-Abundance Sediments

Kumar, N., Noël,, V., Planer-Friedrich, B., Besold, J., Lezama-Pacheco, J., Bargar, J.R., Brown, G.E., Fendorf, S., Boye, K. (2020) Redox Heterogeneities Promote Thioarsenate Formation and Release into Groundwater from Low Arsenic Sediments. Environmental Science & Technology, Water Research. 54 (2019), 3237-3244;  

Sulfate-rich groundwater promote formation of thioarsenates at fine-coarse sediment interfaces and increases arsenic solubility

The Science
Groundwater contamination by As from natural and anthropogenic sources is a worldwide concern, but is primarily monitored in areas with elevated sediment As concentrations. Sharp redox transitions over space and time are also common, particularly in alluvial aquifers, and can influence the molecular speciation of As as well as As release/retention. However, the impact of redox and sediment interfaces on As release and groundwater quality remains largely unexplored, especially where sediment As-concentrations are low. In this study, we set up a laboratory column experiment with natural, low-As sediments from the sandy aquifer and an organic-rich, sulfidic, clay deposit of an alluvial floodplain at the Riverton, WY, Department of Energy, Office of Legacy Management site. Through a combination of aqueous- and solid-phase As, S, and Fe speciation analyses (including x-ray absorption spectroscopy), we showed that substantial release of As to the groundwater may occur where there is a consistent supply of aqueous sulfide, but Fe-reduction promotes Fe-sulfide precipitation. High groundwater concentrations of As in this experiment were coincident with the occurrence of thiolated arsenic species (making up  40% of aqueous As), suggesting that elevated groundwater concentrations are caused by conditions promoting thiolation of As, which in this study translated into aqueous concentration ratios of sulfide:As>100 and sulfide:Fe

The Impact
In this study, reducing conditions were exported from from the small, sulfidic, organic-rich, clay lenses into the initially oxic aquifer sand and promoted toxic levels of arsenic release to groundwater in the aquifer sand, effectively recruiting a much larger volume of aquifer to function as reducing As-thiolating zones. Although there are multiple factors influencing the outcomes, such as relative spacing and sizes of fine-grained materials, as well as flow and groundwater-pumping rates, these result suggest that there is a considerable risk of underestimating threats from geogenic arsenic (and likely other toxic trace elements) unless relatively small-scale but drastic variation in sediment compositions are taken into consideration when installing groundwater wells.

Summary
Arsenic contamination of groundwater is a globally recognized concern but is most often considered in areas of extensive anthropogenic contamination (e.g. through mining operations) or naturally elevated geogenic concentrations (e.g., in the large river deltas of South and South-East Asia). In this study, however, we used natural floodplain sediments with As-concentrations below the global average (1.6 mg As/kg sediment) and examined the influence on As-concentrations in groundwater by the presence of fine-grained, organic-rich sediment lenses in groundwater aquifer sand. Our results indicate that when sulfate concentrations in the groundwater are high, the export of reducing conditions from fine-grained, sulfidic lenses into aquifer sand, can promote Fe-reduction that in turn leads to Fe-sulfide precipitation and elemental S formation. The elemental S then reacts with As to form thiolated As species, which appear to have a higher solubility and mobility than other As species. Thus, the combination of high-sulfate groundwater and heterogeneous sediment composition (e.g. fine-organic rich/coarse interfaces) can locally promote severely elevated As concentrations, even when sediment As-concentrations are below the global average.
The findings from this study suggest that zones and lenses with differing redox regime and sediment composition that are small enough to be disregarded (or even completely missed) during evaluations for well-installations, could still generate concerning or even toxic concentrations of As, and possibly other contaminants.