The Environmental Protection Agency defines an underground storage tank as “any combination of tanks, including underground pipes connected thereto, that is used to contain an accumulation of regulated substances and the volume of which is 10 percent or more beneath the surface of the ground.”

In the definition of an underground tank, the EPA goes on to identify certain tanks that are not covered: septic tanks, flow-through process tanks and basement tanks.

tank installed below grade, but which is not in contact with soil or backfill, is not considered an underground storage tank for regulatory purposes.