Environmental investigations are often performed at the same time as routine site investigations, with the same borehole used for sampling pollutants as well as soil. Liners can be constructed of compacted clay in combination with geomembranes such as high-density polyethylene (HDPE) sheet or geomembrane clay liners (GCLs). Once a landfill has been filled to capacity the waste needs to be covered to prevent ingress of water and to stop gases produced by the waste from venting into the atmosphere. The soil beneath the landfill was assumed to consist of a single layer of clay underlain by a more permeable sand layer. An oxidising agent may be added to the soil such as ozone, hydrogen peroxide, or hypochlorates. Chemical reduction is the process in which a reducing agent (that is an electron donor) is added to the soil. If the soil is highly dispersive, it will begin to go into suspension and make the water cloudy.