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Landfill sites: Has the refuse been capped with a clay layer or other impervious material?

Some landfills are capped with a clay layer, polyethylene, or other material to help prevent rainwater from entering the landfill and to entrap methane. It is often difficult to determine whether a landfill has been capped, but the municipal engineering or public works department should be able to answer the question for you.

It is important to determine this because sites with a cap present fewer problems for trees. Gas contamination of the cover soil tends to be localized, not widespread as it is in a landfill without a cap.

Although many caps eventually crack as refuse settles, allowing landfill gas to migrate into the cover soil in these localized areas, tree growth on the rest of the landfill should be better than on a landfill without a cap.