In Situ Sequestration of Arsenic in Groundwater: Manipulating Geochemical Conditions to Remediate Sites (Invited)

Dealing with arsenic in groundwater can be a challenge because of its geochemical nature as a metalloid and the fact that arsenic can be present in groundwater from natural sources (e.g., rocks and minerals) or from past or current uses of arsenic-containing compounds (e.g., pesticides, wood-treating compounds). Both recent publicity regarding naturally occurring arsenic in Asian groundwater and regulatory pressure have stimulated the development of cost-effective methods to mitigate arsenic in groundwater. Because of potentially lower capital and operating costs, in situ methods can be attractive alternatives to costly pump-and-treat systems for smaller-scale operations. Design of appropriate in situ remediation methods should consider the source of the arsenic. Releases of arsenic from arsenic minerals can result from changes in oxidation-reduction potential (ORP) or pH changes. Displacement of arsenic sorbed onto iron oxides can occur through either reductive dissolution of the oxide or through competitive sorption of another ionic species, such as phosphate or carbonate. In situ methods for remediation of arsenic in groundwater include natural attenuation, ORP adjustment, and pH adjustment. The appropriate in situ approach for a site can depend on the source of the arsenic, background groundwater chemistry, site mineralogy, and other factors. The behavior of arsenic in groundwater is described in terms of ORP, pH, and sorption to iron oxides. Data from several sites are used to illustrate the conditions discussed, and case studies showing the use of ORP adjustment, pH adjustment, and natural attenuation to remediate arsenic in groundwater are presented. These case studies include projects ranging from bench-scale testing, to pilot scale demonstrations and full-scale remedial operations.