The Science

Groundwater contamination by arsenic from natural and anthropogenic sources, though a worldwide concern, is primarily monitored in areas with elevated sediment arsenic concentrations. Sharp redox transitions over space and time are also common, particularly in alluvial aquifers, and can influence the molecular speciation of arsenic as well as arsenic release or retention. However, the impact of redox and sediment interfaces on arsenic release and groundwater quality remains largely unexplored, especially where sediment arsenic concentrations are low. In this study, the research team set up a laboratory column experiment with natural, low-arsenic sediments from the sandy aquifer and an organic-rich, sulfidic, clay deposit of an alluvial floodplain at the Riverton, Wyo., U.S. Department of Energy (DOE) Office of Legacy Management site. Through a combination of aqueous- and solid-phase arsenic, sulfur, and iron speciation analyses (including X-ray absorption spectroscopy), the team showed that substantial release of arsenic to the groundwater may occur where there is a consistent supply of aqueous sulfide, but iron reduction promotes iron-sulfide precipitation. High groundwater concentrations of arsenic in this experiment were coincident with the occurrence of thiolated arsenic species (making up £40% of aqueous arsenic), suggesting that elevated groundwater concentrations are caused by conditions promoting arsenic thiolation, which in this study translated into aqueous concentration ratios of sulfide:arsenic > 100 and sulfide:iron < 1.

The Impact

In this study, reducing conditions were exported from the small, sulfidic, organic-rich, clay lenses into the initially oxic aquifer sand. This promoted toxic levels of arsenic release to groundwater in the aquifer sand, effectively recruiting a much larger volume of aquifer to function as reducing arsenic-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 results 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 Southeast Asia). In this study, however, the team used natural floodplain sediments with arsenic concentrations below the global average (1.6 milligrams per kilogram of sediment) and examined the influence on arsenic concentrations in groundwater by the presence of fine-grained, organic-rich sediment lenses in groundwater aquifer sand. 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 iron reduction that in turn leads to iron-sulfide precipitation and elemental sulfur formation. The elemental sulfur then reacts with arsenic to form thiolated arsenic species, which appear to have a higher solubility and mobility than other arsenic species. Thus, the combination of high-sulfate groundwater and heterogeneous sediment composition (e.g., fine-grained, organic-rich/coarse interfaces) can locally promote severely elevated arsenic concentrations, even when sediment arsenic 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 arsenic and possibly other contaminants.

Principal Investigator

John Bargar
SLAC National Accelerator Laboratory
bargar@slac.stanford.edu

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

Funding was provided by the Subsurface Biogeochemical Research (SBR) program of the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science, to the SLAC National Accelerator Laboratory Scientific Focus Area under contract DE-AC02-76SF00515. Use of the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC is supported by the Office of Basic Energy Sciences, within the DOE Office of Science.

References