Elevated levels of dissolved arsenic (∼300μgL−1) have been detected beneath and in groundwater plumes extending away from a closed landfill in southern Maine. This study sought to determine the source of arsenic to the aquifer, the processes responsible for arsenic mobilization, and to evaluate the effectiveness of remediation efforts that have occurred at this site. The As appears to originate in the natural (glacial) aquifer solids, which contain ∼5mgkg−1 As on a dry weight basis. This conclusion is supported by the relatively uniform distribution of As in sediment samples, results of laboratory batch incubation experiments, and comparisons with groundwaters in nearby wetlands, which also have high levels of dissolved As that do not appear to originate within the landfill. The As is mobilized in the subsurface by strongly reducing conditions beneath the landfill and in nearby wetlands. In the aquifer beneath the landfill, the average oxidation–reduction potential (ORP) is −95mV (Eh+105mV), and these reducing conditions were primarily induced by landfill leachate. Remediation efforts at this site have included installation of a low permeability clay cap; groundwater extraction, oxidation, and re-injection; and subsurface oxidation by injection of magnesium peroxide. The natural source of arsenic within the aquifer solids, coupled with widespread reducing conditions, has severely limited the effectiveness of these interventions on groundwater arsenic concentrations.

The behaviors of dissolved arsenic and iron are significantly decoupled in groundwaters beneath a closed landfill in southern Maine and in laboratory experiments using these groundwaters. At this site naturally-occurring arsenic is mobilized from the glacial sediments under reducing conditions induced and promoted by landfill leachate. Differences in arsenic and iron behavior were seen in laboratory titration experiments which progressively oxidize groundwater from this site and in field data for groundwater composition in the area surrounding this former landfill. The persistence of elevated levels of dissolved arsenic in groundwaters where dissolved iron is relatively low provides direct indication of the difficulty of preventing transport of arsenic from this type of site.