Abandonment of an Underground Storage Tank

In-place abandonment by filling the underground storage tank with an inert solid is a very good method for taking a buried storage tank out of service. But it should only be chosen if certain conditions are met.

When can an underground storage tank be abandoned in-place?

An underground storage tank should only be abandoned in-place if the soils underlying the tank have been verified to be clean, or, in some cases where the underlying soils are contaminated, if the State determines that because of site limitations, abandonment is more appropriate than removal of the underground storage tank.

Remember, home heating oil tanks are unregulated so the tank itself is not the issue. What matters is whether or not the underlying soils and/or the groundwater have been contaminated by leakage from the tank (for more information see our State Regulations page).

Legal Liability Remains

Soil samples from the native (undisturbed) soils beneath the underground storage tank should be obtained and then confirmed by a North Carolina Licensed Geologist or Engineer to be clean (free of contamination) prior to tank abandonment. If the soils are not clean, then even if the tank is subsequently (or has already been) abandoned, the legal liability for clean-up of the contaminated soils remains. A Cedar Rock site inspection usually confirms the presence or absence of soil contamination immediately. Here is an example of an improper tank abandonment and what you do not want to happen to you!

Filling tank with flowable fill. Click to play.

Video of tank being filled with cement slurry
Abandonment costs significantly less than removal, typically around $2,200. If in-place abandonment is a viable option, it is extremely important that the tank be properly cleaned and a special, flowable-fill cement slurry be used to fill it. This prevents the buried tank from causing future soil contamination and prevents subsidence resulting from tank collapse. A report, including photographic documentation, should be generated as proof the abandonment was properly performed.

Occasionally, a tank cannot be removed because of where it is located. If the soils are contaminated the State may approve abandoning the tank in place. In these cases environmental assessment is required by the State before the State will allow closure of the incident.

Proper procedure for in-place abandonment of an underground storage tank

The slide show shows stages of several Cedar Rock tank abandonments.

  1. Most importantly, the tank owner must have documentation with the seal of a NC Licensed Geologist or Engineer stating that the underlying soils have not been contaminated by a release from the tank.
  2. A hole is cut at one end of the tank, to allow access to the interior for cleaning and filling activities. Any water, sludge or oil are pumped out of the tank prior to pressure washing the interior.
  3. Flowable fill, a sandy cement slurry that will completely fill the tank without leaving air pockets, is poured into the tank. Cedar Rock highly recommends that sand or foam (see explanation below) not be used.
  4. The metal flap is folded down and the landscape restored. As the before and after photos show, no one would suspect that a tank had been taken out of service.

Why an underground storage tank should NOT be filled with foam, sand or water

The following photos illustrate some reasons why the correct method of tank abandonment should be used. The left photo shows how the mass of soft foam has shrunk leaving approximately 60% of the tank void. When the metal rusts away the ground will eventually subside.

UST filled with foam showing shrinkage UST filled with sand slurry showing voids from incomplete filling

The photo to the right shows a UST filled with a sand slurry. Clearly, the contractor believed that the tank had been filled but as a sand slurry does not flow sufficiently a void nearly always remains within the tank.

The left photo, below, shows how soft the foam is that is sometimes used to fill an tank. The hand auger was simply pushed through to the bottom of the tank. Cedar Rock was asked, in this case, to confirm that a previous abandonment had been performed adequately. This is the risk with a poorly performed and documented tank abandonment. A buyer may still require the tank to be checked and if there is still contamination the owner is up for the expense again.

UST filled with foam UST filled with foam not cleaned of oil

The right photo shows another tank filled with foam. The fact that the tank was not properly cleaned of its contents is obvious by the discoloration of the foam and the presence of free oil in the tank.

It is essential that tank abandonment be performed correctly to avoid further problems and expense in the future.