Leaking underground fuel storage tanks (USTs) can cause fires or explosions and/or contaminate groundwater. More than 50% of the population in the U.S. depends upon groundwater for domestic use. Petroleum products, including gasoline, are highly mobile as they float on a sloped or flowing groundwater surface. Flammable liquids and/or vapors seeping into basements or other subterranean spaces can create explosive conditions, inhalation exposure hazards, or both. Thus, leaking USTs have been and are a major threat to the public health and safety and to the environment.