What are caps?

• They must prevent the vertical movement of water through the contaminated soil
• They must provide efficient draining of surface water from the site to prevent the occurrence of standing water
• They must be easily maintained
• They must be resistant to damage caused by the settling and consolidation of soils and other adverse conditions (heat, cold, UV, radiation, etc.)

What are caps used for?

Capping is required when contaminated soils and/or materials are to be left in place at a site. It can be used when:

• the underground contamination is so extensive that it prohibits excavation and removal

• the chemicals present in the contaminated soils do not migrate (to and significant degree) if water does permeate the soil

• the removal of contaminated soils from the site would pose a greater threat to human health and the environment than simply leaving then in place.

Capping is often used in combination with groundwater extraction (removal) or containment technologies to reduce and, if possible, prevent contaminant migration. Groundwater monitoring wells are often used in the area where a cap has been installed to detect any migration of the wastes that may unexpectedly occur. Capping is also associated with surface water controls such as ditches, dikes, and berms. These structures are used to receive rainwater drainage that flows from the cap.

What do caps look like?

The primary purpose of a cap is to minimize contact between rain or surface water and the contaminated soil. The only type of cap that is generally accepted is the multi-layer cap. This type of cap generally has four layers: vegetation, drainage, water-resistant, and foundation. The vegetation layer prevents erosion of the surface soils of the cap. The drainage layer channels rainwater away from the cap and keeps water from collection on the water-resistant layer which covers the waste.

The foundation layer is composed of soil materials that are structurally capable of supporting the weight of the finished cap. The foundation material should be spread over the wastes in six-inch increments and compacted. Structural stability tests should be run on each increment to assure uniformity. The water-resistant layer is placed in six-inch increments and compacted with a bulldozer or other heavy equipment. The thickness of the water-resistant layer should be at least two feet, but should be increased if settling is expected in the underlying contaminated soils. A synthetic liner should be placed and sealed according to the manufacturers specifications. The liner should be at least 20 mils thick. (One mil is equal to one-thousandth of an inch). A thicker liner should be used if more than a few inches of settling is expected.

The drainage layer is also placed in six-inch increments and should be at least one foot thick. If the drainage layer is placed directly over the liner, the soil material must be free of sharp objects that could puncture the liner. Filter fabric should be placed above the drainage layer to prevent the soil from clogging the drainage pores. The pore size of this layer should be large enough to allow for proper drainage, but small enough to prevent the soil from moving into the drainage layer.

The vegetation layer should be at least two feet thick to accommodate root penetration. It should be spread evenly and not overly compacted. The vegetation should be non-woody plants, preferably grasses, which require low maintenance and do not have deep roots.

Figure 1: Cap Diagram

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