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

Program Links

Final Progress Reports

Year:   2009 

Dr. Peggy O’Day and her collaborators, Dr. Dimitri Vlassopoulos and Dr. Brad Bessinger, carried out reactive transport simulations of contaminated sediments and model reactive cap systems to examine the evolution of aqueous and solid phases, and to predict the fate of arsenic and mercury in these systems. In this part of the project, her research group used kinetic and thermodynamic models developed for sulfate-cement amendments in the previous project year. The team conducted these simulations over a wide range of environmental conditions (aerobic/anaerobic, freshwater/marine) and timescales. The simulations indicate that sequestration mechanisms and their effectiveness at limiting contaminant availability vary spatially and temporally within the sediment cap system on timescales of interest (years to decades). Model simulations predict that transient waves of contaminant-sequestering neoformed minerals migrate through the sediment cap within the first few decades before a steady state is reached. The transient patterns are different for arsenic and mercury and depend on environmental conditions.

The team investigated sulfate-cement amendments for mercury immobilization in sediments in laboratory experiments designed from the results of geochemical modeling simulations. They conducted batch experiments by adding Portland cement (PCV) and ferrous sulfate (FeSO₄) to different substrates (quartz, quartz and clay minerals, and natural sediments) to react for 1, 7, 30 and 90 days. Researchers characterized products by bulk extraction and spectroscopic and microscopic methods to determine the mechanism of immobilization. After 1 day of reaction, 80% of Hg was retained in the solid phase and just 5% was present in the exchangeable fraction. The team found similar results after 7, 30 and 90 days. These extraction results indicate that FeSO4 and PCV are effective for mercury sequestration. Spectroscopic characterizations are in progress to identify the sequestration mechanism.

Program leaders also took advantage of an opportunity to engage in a new partnership with the EPA at a Superfund site. The 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 site identified for study is a seasonal wetland area adjacent to the former mine site. The site contains elevated levels of mercury and other metals in surface sediments but is less contaminated than the mine or lake sediments. The wetland is partly owned by a local Native American tribe, the Elem Indian Colony of the Elem Pomo Tribe. Tribe residents living adjacent to the wetland are concerned about exposure and transformation to methyl mercury. Their environmental director has allowed researchers to access the wetland area for sediment and water collection.