Landfarming is an ex-situ waste treatment process that is performed in the upper soil zone or in biotreatment cells. Contaminated soils, sediments, or sludges are transported to the landfarming site, mixed into the soil surface and periodically turned over (tilled) to aerate the mixture. Landfarming commonly uses a clay or composite liner to intercept leaching contaminants and prevent groundwater pollution, however, a liner is not a universal requirement.