In-Situ Capping (ISC) is a potentially economical and effective approach for remediation of contaminated sediment. ISC is a non-removal remediation technique which includes the placement of a subaqueous covering or cap of clean isolating material over an in-situ deposit of contaminated sediment. Many sites have been remediated by in-situ capping operations across the nation and world. Envirocon has conducted successful ISC projects and offers the following as a brief overview of this remediation approach.

Caps for in-situ sediment remediation are constructed of clean sediments, sand, gravel, or may involve a more complex design with geotextiles, liners, and multiple layers. In-situ capping provides several primary functions: physical isolation of the contaminated sediment from the benthic environment; stabilization of contaminated sediments, preventing re-suspension and transport to other sites; and, reduction of the flux of dissolved contaminants into the water column. The design of the cap and also the characterization of the surrounding areas are of equal importance and drive the feasibility of the entire project.

Cap materials must be identified and assessed prior to the project as they typically represent the largest cost to the project. Granular materials used include quarry sand, naturally occurring sediments, or soil materials. Fine grained materials and sandy materials can be effective in the construction of an in-situ cap. Fine-grained materials have been shown to act as better chemical barriers than sand caps. Also, increased levels of organic matter in sands increase the retardation of hydrophobic organic contaminants through the cap and encourage degradation of contaminant. The use of granular activated carbon and other mitigating materials sandwiched within geotextile layers has been used successfully as a barrier to organic transport.

The construction of the cap will directly affect the ability of the in-situ cap to perform as designed. It is important to note that many contaminated sediment sites exhibit exceedingly soft sediments that are easily disturbed, dislocated, or destabilized by uneven placement, and have insufficient load bearing capacity to support some cap materials. The design must address the thickness of cap materials to be placed per lift to ensure system stability. Monitoring of system stability during and after cap placement is typically a crucial aspect of the project to ensure any global or rotational failure does not occur.

Granular cap material is handled and placed in a number of ways. Materials that have been mechanically dredged and soils excavated from an upland site or quarry have relatively little free water, and are handled mechanically in a "dry" state until released into the water over the ISC site. These mechanical methods rely on the gravity settling of cap materials in the water column, and may be depth-limited in their application. Granular cap materials can also be entrained in water slurry, and carried to the cap site where they are discharged into the water column at the surface or at depth. These hydraulic methods offer the potential for a more precise placement, although the energy required for slurry transport may require dissipation to prevent resuspension of contaminated sediments.

There are two basic ways to construct an in-situ cap:

• Land-based placement: this involves using equipment near the shore or working in narrow channels. The cap is constructed with standard construction equipment such as backhoes, clamshells, dumped from trucks, and/or spread with bulldozers. The major limitation of this method is the reach of the equipment.
• Pipeline or barge placement: this involves placing the in-situ cap with a barge and/or a pipeline. Using different types of equipment to place the cap components on the ocean bed or lake bed. This is typically the desired method when working in deep areas or off the shore.

The placement operational methods include the following: Direct Mechanical Placement; Surface Discharge Using Conventional Dredging Equipment; Spreading by Barge Movement; Hydraulic Washing of Coarse Sand; Pipeline with Baffle Plate or Sand Box; Submerged Diffuser; Sand Spreader Barge; Gravity-fed Downpipe (Tremie); and Hopper Dredge Pump-down.

The ability to keep barges and work vessels in position may require considerable effort at sites subject to currents, waves, and tidal movements. Where granular cap material is placed by surface discharge, barge spreading, or hydraulic washing, vessels can be positioned by tug boats or other support vessels. Spuds may not be appropriate for use during cap placement, as the spuds might penetrate and damage the cap. Cables attached to large "deadman" anchors deployed outside the cap footprint have been used to position work barges for ISC construction.

Envirocon has successfully completed a number of projects placing in-situ subaqueous caps by both mechanical and hydraulic means. Hydraulic placement operations included the St. Louis River Interlake/Duluth Tar (SLRIDT) site in Duluth, MN and the Indiana Steel and Wire project in Muncie, IN. Both projects included the use of a Spreader Barge to hydraulically place granular material in a lift thickness ranging from 3, 6, and 12 inches. Also, Envirocon placed a carbon mat as an organic barrier on the SLRIDT project. Please see the link to Envirocon's SLRIDT project description.

Acknowledgements:

Michael R. Palermo and Steve Maynord
U.S. Army Engineer Waterways Experiment Station
Vicksburg, Mississippi

Jan Miller
U.S. Army Engineer Division, Great Lakes and Ohio River
Chicago, Illinois

Danny D. Reible
Louisiana State University
Baton Rouge, Louisiana