The presence of pollutants in soils at concentrations above background levels that pose a potential health or ecological risk. Soils can be contaminated by many human actions including the discharge of solids and liquid pollutants at the soil surface; pesticide application; subsurface releases from leaks in buried tanks, pipes, and landfills; and deposition of atmospheric contaminants such as dusts and particles containing lead. Common contaminants include volatile hydrocarbons—such as benzene, toluene, ethylene, and xylene (BTEX compounds)—found in fuels; heavy paraffins and chlorinated organic compounds such as polychlorinated biphenyl (PCB) and pentachlorophenol (PCP); inorganic compounds such as lead, cadmium, arsenic and mercury; and radionuclides such as tritium. Often, soil is contaminated with a mixture of contaminants. The nature of soil, the contaminant's chemical and physical characteristics, and environmental factors such as climate and hydrology interact to determine the accumulation, mobility, toxicity, and overall significance of...