Effect of Active Caps On Contaminant Bioavailability – Measurement and Prediction of the Bioavailable Pool of Contaminants in Sediments.

Active capping is a relatively new approach for treating contaminated sediments. It involves applying chemically reactive amendments to the sediment surface. The main role of active caps is to stabilize contaminants in contaminated sediments, lower the bioavailable pool of contaminants, and reduce the release of contaminants to the water column. In addition, downward migration of the amendments used in active caps can neutralize contaminants located deeper in the sediment profile; i.e. in the zone of influence (ZOI).

Metals and organic contaminants are common in many marine and fresh water environments as a result of industrial and military activities. The mobile, soluble forms of contaminants are generally considered toxic. They are bioavailable to gill breathing organisms, easily pass through cell walls, and can bioaccumulate in living organisms. Induced chemical precipitation of these metals can shift toxic metals from the aqueous phase to a solid, precipitated phase which is often less bioavailable. This approach can be achieved through application of sequestering agents such as rock phosphates, organoclays, zeolites, clay minerals, and biopolymers (e.g., chitosan) in active capping technology. Active capping holds great potential for a more permanent solution that avoids residual risks resulting from contaminant migration through the cap or breaching of the cap. In addition to identifying superior active capping agents, research is needed to optimize application techniques, application rates, and amendment combinations that maximize sequestration of contaminants. A selected set of active capping treatment technologies has been demonstrated at a few sites, including a field demonstration at the Savannah River Site, Aiken, SC. This demonstration has provided useful information on the effects of sequestering agents on contaminant immobilization, bioavailability, and toxicity.