Worldwide, arsenic contamination of groundwater supplies threatens both human health and the environment. Contamination sources vary from geogenic to anthropogenic, the result of mining or other industrial activities. Standard remediation strategies that are robust and sustainable are needed to address these problems, given the prevalence and severity of this contaminated water. A promising strategy is the creation of environmental conditions unfavorable to metal mobility through the use of permeable reactive barriers (PRBs). To treat arsenic, this technology can be implemented to cause in-situ induced microbial sulfate reduction. These systems are often supplemented with the addition of zero-valent iron (ZVI). The altered biogeochemical conditions result in the formation of iron sulfides and other minerals that incorporate arsenic or provide a surface for adsorption. Laboratory studies have demonstrated the effectiveness of this technique, but field-scale studies are limited. Furthermore, the exact mechanisms of arsenic immobilization have not been explicitly demonstrated, leaving uncertainty as to the long-term stability of arsenic sequestration in the remediation treatment.  Here we present the results of laboratory investigations of a large-scale field application of this remediation strategy in a wetland area near Tacoma, WA, that contains elevated levels of arsenic and iron from a nearby slag-containing landfill. Sequential chemical extractions performed on sediment collected from the PRB highlight the pools of solid-phase arsenic present in the system. Furthermore, to expand on these operationally defined associations, sediment samples were analyzed by microscale x-ray absorption (μXAS) to elucidate the solid-phase speciation and local coordination environment. These experiments provide critical information on the mechanisms responsible for arsenic immobilization — information needed to assess the technology’s longevity and sensitivity to changes in groundwater chemistry.