Contaminated site remediation can take many different forms. A relatively cheap way to control cross contamination involving the infiltration of surface water into a contaminated plume/site is to cap the overlying area. The purpose of capping a site is to prevent surface water from infiltrating the plume/site and in doing so prevent the spread of contaminants into the groundwater as well as the cross contamination of subsurface soils.
Another benefit of capping a contaminated area is that it allows landscaping works to occur over the contaminated area and as such improves the aesthetics of the site. Despite not removing or treating identified contamination on site, capping prevents the offsite migration of contaminated soils and opens the land up for limited land use.
A variety of natural and geo-synthetic materials can be used in the capping systems. These may range from single layer systems to multi layer capping systems. Natural materials most often used are clays and geosynthetic materials often consisting of a layer of high swelling sodium bentonite, sandwiched between two geotextiles. While exact installation procedures for geo-synthetic liners may vary from manufacturer to manufacturer, in general, installation consists of rolling out the panels on a prepared subgrade, with adjacent liner panels overlapped a 300 mm. To maintain the integrity of the sealing system, a bead of granular bentonite is applied to the overlap of the adjacent panels. Subsequent to overlap treatment, a minimum 300 mm of soil cover is placed over the liner panel. The covering material is always placed in the same day as the liner panels are deployed to minimize the chance of unconfined hydration, and possible damage to the liner panels.
Capping methods vary dependent upon site conditions, including contaminant chemistry, soil type, climate, land use, site location and budget to name a few. The design specification of the capping material and system should consider all of the above variants.