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University of California-Merced

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Superfund Research Program

Sequestration and Immobilization of Metal and Metalloid Contaminants in Sediments

Project Leader: Peggy A. O'Day
Grant Number: R01ES016201
Funding Period: 2007-2010

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In the course of this project, the research team developed new kinetic and thermodynamic model parameters to describe the behavior of arsenic and mercury in surface and subsurface aquatic environments. The researchers performed reactive transport simulations of contaminated sediments and model reactive cap systems to examine the evolution of aqueous and solid phases. This fully quantitative modeling approach enables the simulation of the fate of arsenic and mercury in these systems over a range of environmental conditions (aerobic/anaerobic, freshwater/marine) and over timescales of 50 to 100 years.

The use of sulfate-cement amendments for arsenic and mercury immobilization in sediments was tested through laboratory experiments designed from the results of geochemical modeling simulations. To test arsenic immobilization, field soils were reacted in the laboratory with three treatments: ferrous sulfate (FeSO4), Type V Portland cement (PTC), and a mixture of both (PTC+FeSO4).

The three soil amendment treatments (FeSO4, PTC, and PTC+FeSO4) were effective in removing arsenic from solution in laboratory samples compared with control soils with no amendment. Arsenic uptake by treated soils was fast, with most arsenic removed from solution after one day of reaction and no distinct trends in concentration with aging time.

The research team also collected and characterized field sediments from a contaminated Superfund site. Sulfur Bank Mercury Mine (SBMM) located at Clear Lake (CA) is an abandoned mercury mine in the coast ranges of northern California that operated intermittently from 1865-1957. The SBMM was placed on the National Priorities List in August 1990. The extensive background data associated with this site made it an ideal location to investigate biogeochemical optimization of amendment and capping strategies since mercury occurrence and distribution in different environmental compartments are well defined. Over a one-year seasonal cycle, ephemeral pond waters and surface sediments were studied at a mercury-contaminated wetland (North Wetland) adjacent to the former SBMM. The researchers characterized mercury speciation, sediment association, and factors that influence sediment-water partitioning and methylation in order to determine possible treatment strategies.

Observations from measurements of mercury and methylmercury at the site suggested that mercury methylation is occurring in the subsurface rather than in surface ponds, with possible stabilization and transport of methylmercury by complexation with dissolved organic matter.

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