Capping is common practice in landfill and can be a simple, effective solution to many contaminated land issues. Capping is, essentially, putting a lid on contaminated material, providing a barrier to the contaminated materials. Capping can be used for many reasons, to prevent access to people and removing the risks posed by contamination, to providing a barrier from the entry of rainfall or snow-melt and the exit of gases, odours, leachate or any other emissions which can be harmful to the environment.
The most important components of a capping system are the barrier (capping) layer and the drainage (however sometimes a gas collection system is required for putrefying materials). The barrier has to be of low-permeability such as a thick layer of dense clay, sometimes coupled with a geo-synthetic membrane, concrete or asphalt also makes an effective barrier layer. The drainage layer, on-top of the barrier, diverts the water away through low permeability soils such as sandy soils and/or gravels. A layer of vegetation, on a clay cap, can provide slope stability and act as an interceptor for some of the rainfall. If concrete or asphalt is used, managing the run off is essential to avoid flooding the surrounding area.
The main benefits of this type of solution are that it can be the most inexpensive method and it effectively blocks the pathway between contaminants and receptors. Capping layers can be transformed into an attractive feature in a short space of time by using the drainage layer to plant vegetation. The capping system design is easily adapted to suit different ground conditions, climates, budgets and natures of contaminants.
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