In situ thermal treatment methods move or “mobilize” harmful chemicals in soil and groundwater using heat. The chemicals move through soil and groundwater toward wells where they are collected and piped to the ground surface to be treated using other cleanup methods. Some chemicals are destroyed underground during the heating process. Thermal treatment is described as “in situ” because the heat is applied underground directly to the contaminated area. The method is particularly useful for chemicals that do not dissolve readily in groundwater. Examples include solvents, petroleum and the wood preservative creosote.