My research has focused on controls on arsenic mobility at two Superfund sites: the Vineland Chemical Company in Vineland, New Jersey and the Winthrop landfill in Winthrop, Maine. Although these sites are very different in terms of the source of arsenic (geogenic at Winthrop and anthropogenic at Vineland) and the scale of the remediation efforts (relatively modest at Winthrop and very large at Vineland), many of the mechanisms controlling the mobility of arsenic in the subsurface and therefore controlling the effectiveness of interventions are similar.
We have demonstrated that the arsenic in the groundwater at Winthrop is not anthropogenic and is released from the sediments beneath the closed landfill. Comparison of geochemical data from an upgradient and unaffected region with a region beneath the landfill has helped demonstrate that the trigger for arsenic release is the highly reducing state of the subsurface. These reducing conditions are induced by decay of landfill wastes into organic-rich landfill leachate that percolates into the aquifer below. Observations from a nearby swamp strengthen the conclusion that naturally-occurring arsenic is mobilized by reducing conditions at this site. Arsenic's ability to be mobilized by the reducing conditions typical within sub-landfill aquifers raises the possibility that this process could be occurring at many other landfill sites across the country and around the world.
Arsenic may also be immobilized under reducing conditions, when sulfate is reduced and insoluble iron and arsenic sulfides are formed. We have conducted a series of small-scale incubations of sediment and water under different conditions to induce sulfide mineral formation. In these experiments, addition of acetate promoted the growth of sulfate-reducing bacteria, thereby permitting the conversion of sulfate to sulfide and decreasing the concentrations of dissolved arsenic. Ongoing research is examining which components of the microbial community were responsible for these geochemical changes and which solid phases served as arsenic sinks. This laboratory research is a preliminary step in investigating an innovative remediation technique for removing arsenic (and potentially other redox-sensitive metals) from groundwater.