Capping of contaminated sediments with cleaner sediments is a technique that has been used by the US Army Corps of Engineers, New England Division (NED) since 1979, to avoid or minimize the impacts of contaminated sediments disposed at open water sites. A case study of contaminated sediments from a project on the Thames River, capped at a disposal site offshore of New London, Connecticut, illustrates the application of this technique. Several steps, both regulatory and operational, must be accomplished to ensure proper employment of this technique.
First, once it is determined through the permit evaluation process that material to be dredged from a project is not suitable for unconfined open water disposal, the quantity of uncontaminated dredged material needed to achieve a desired cap thickness of 50 to 100 cm must be identified. This quantity may be determined by the use of a computer model - the DAMOS (Disposal Area Monitoring System) Capping Model - which simulates the disposal events and mound formation.
Next, the applicant/project proponent must submit a capping plan to NED, which includes provisions for obtaining the necessary quantity of cap material and a schedule for dredging and disposal of both contaminated and cap materials. Upon approval of this plan by NED, the contaminated material may be dredged and disposed at a taut-wire moored buoy located at a specified set of coordinates. The use of such a buoy is critical to the success of capping, since it aids in limiting the distribution of the contaminated material on the seafloor. Post-disposal bathymetric and sediment-profile camera surveys of the contaminated material are conducted to delineate the areal extent of the mound formed during disposal. Several sets of coordinates are then chosen by NED for disposal of the cap material, with the aim of covering all contaminated sediments.
The operational success of the capping technique is measured by adequate areal coverage and thickness of the cap over the contaminated material. Thus, following disposal (and sometimes during disposal) of cap material, additional surveys are conducted for this purpose.