There are thousands of metal-contaminated sites on Department of Defense (DoD) lands that are awaiting cleanup and closure. Lead (Pb), arsenic (As), chromium (Cr), and cadmium (Cd) are of particular concern since these metals drive risk-based remedial decisions for soils at DoD sites. Contaminated soils that lack the ability to naturally sequester and decrease toxic metal bioavailability will remain a human health risk in their natural state. In situ chemical manipulation strategies are an attractive alternative to ex situ methods because the redox state and chemical speciation of metals can easily be altered through changes in soil geochemistry. Despite the success of chemical manipulation strategies for immobilizing Pb in various subsurface environments, chemical manipulation strategies have not been investigated to a significant extent on a vast array of DoD soils or priority contaminants. The rates and mechanisms that control enhanced metal sequestration and decreased bioavailability are largely unknown as are the costs associated with risk-reduction due to decreased bioaccessibility of metals in amended soils.

The overall objective of this project was to develop an improved understanding and predictive capability of the enhanced immobilization and decreased bioaccessibility of hazardous metals in soil as a result of novel chemical amendment strategies.

Representative remedial drivers for metal contaminated soils

The technical approach involved a series of multidisciplinary tasks that coupled mechanistic macroscopic-scale metal immobilization studies with microscopic-scale interrogation of solid-phase interfacial processes and numerical modeling. The focus was on field-relevant, laboratory-based metal immobilization studies that targeted Pb, Cd, As(III/V), and Cr(III/VI) in numerous types of soil. A unique experimental design linked contaminant fate and transport monitoring on chemically amended soils with metal bioaccessibility and molecular speciation. Empirical models that relate metal bioaccessibility to soil properties and amendments were formulated and applied to various DoD sites to develop an economic framework for risk-based cost-benefit analysis associated with soil amendment-based remediation scenarios.

Under Environmental Security Technology Certification Program (ESTCP) project ER-200517, work is under way to obtain regulatory acceptance of in vitro methods and the Soil BioAccessibility Tool (SBAT) for assessing toxic metal bioavailability in DoD soils.

An improved fundamental understanding and predictive capability of the processes that control the enhanced long-term immobilization and decreased bioaccessibility of the DoD priority metals in a wide array of soils treated with various organic and inorganic amendment strategies will result in new remedial protocols that maximize sequestration and minimize bioaccessibility at DoD sites over long periods of time. An easy-to-use, spreadsheet-based, computer model will predict the best site-specific treatment scenarios for optimizing soil metal sequestration and decreased bioaccessibility. The knowledge and tools developed from this study provide site managers and risk assessors with a means of evaluating in-situ treatment options and determining the most cost-effective strategy for site management of metal-contaminated soils. (Project Completed – 2007)