Contamination of groundwater and soils with volatile organic compounds (VOCs) is a problem at many industrial sites. Soil contamination by VOCs is a source for continuous air pollution and is also a source for groundwater pollution. The level of volatile organic compounds allowed in discharged wastewater or drinking water is lowered every few years. Examples of contaminants are petroleum hydrocarbons, methyl tert-butyl ether, chlorinated hydrocarbons such as trichloroethylene and carbon tetrachloride. Many of the VOCs are potential carcinogens.