In sediment capping submerged contaminated sediments are covered by stable layers of sediment, gravel, rock, and/or synthetic materials. In certain conditions, the cap reduces contaminant mobility and subsequent interaction between aquatic organisms and the contaminants. Even where contaminated sediments must be excavated, underwater capping may still be preferable to above ground disposal. In this case, sediments may be deposited in either a natural depression or a space formed by dredging clean sediment before capping.
Capping is only appropriate if the following circumstances apply:
Limitations and Concerns
With capping, contaminants remain in place at the site, requiring long-term monitoring and maintenance to ensure that contaminants are not migrating. The integrity of the cap should be investigated regularly. The cap should be designed to provide containment for as long as the contaminated sediment requires management.
Moving contaminated sediment (if required) and placing the first layer of capping material can suspend sediment in the water column.
Strong currents can displace capping materials.
Capping materials reduce the available clearance for water traffic. Thus, at times the use of the waterway may dictate that contaminated sediments be moved from their original site.
The tendency for sediments to flow because of the momentum generated during cap placement should be carefully analyzed to prevent sediment entrainment and contaminant release.
This technology is best used to contain volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs) [including polychlorinated biphenyls (PCBs) and polynuclear aromatic hydrocarbons (PAHs)], pesticides, energetics, and metals. It is used in ocean shoreline situations. It is being studied for some river applications.
Technology Development Status
This is a commercial technology.
Other Resources and DemonstrationsSee https://ert2.navfac.navy.mil/printfriendly.aspx?tool=contaminatedsediment for characterization and remediation techniques for contaminated sediment.