Hazardous Substance Research Centers/South & Southwest

Choosing the Right Material

Research has shown that the best caps are finely grained, organically rich, and able to bind with contaminant particles. Materials used in capping have varying advantages and disadvantages.

Material requirements are similar for any type of cap: they must provide physical stability along with the ability to chemically and physically isolate contaminants. Capping materials can either be dredged from clean sediment layers or collected on land. The following are the key materials used to form in-situ caps:

Sand

Sandy caps are easy to place on sediment beds and attract the kind of burrowing organisms that do not penetrate deeply. But because sands are coarsely grained and relatively low in organic content, they do not bind well with contaminants in sediment beds and provide less long-term containment of pollutants than other materials.

Silt

Silty capping soils are organically rich and therefore bind strongly with contaminated materials. By keeping pollutants in place, silts give natural degradation processes a chance to detoxify sediment beds. Silts are difficult to place accurately and often return to suspension in water but can be contained effectively with an armoring layer. They are susceptible to colonies of burrowing organisms, which are effective in transporting contaminants from sediments to water.

Multiple Layers

The cap that provides the best attributes is often composed of several materials. The lower layer is a fine-grained silt that allows the cap to bind strongly with the contaminants in the underlying sediment bed. The upper layer is a sandy material that can be placed accurately, has good stability, and resists thec olonization efforts of burrowing animals. The sandy upper layer's ability to hold contaminants in place may be improved by the addition of fly ash or an ionic surfactant.

Armoring

Sometimes, armoring layers such as gravel or riprap give stability to an underlying silt or sand cap, but the placement of large stones is feasible only in rivers and near-shore disposal areas.

Geotextiles

One possible means of reducing the mixing between capping and sediment layers is to place a geotextile fabric before applying the cap. These materials are also used to improve sediment stability during placement operations and to limit the intrusion of bioturbators into the cap. Researchers have not yet determined whether such a capping material is needed or effective. These studies require specialized equipment and a specially equipped barge for placement. They are only practical in shallow waters.