How to Decommission an Oil Tank
You may wonder how to decommission an oil tank. The process of oil tank decommissioning depends on the site conditions. Let’s have a look.
The most common situation:
Where there is only one oil tank found in the oil tank search, the tank is in the yard, and the tank top is less than 5′ below the ground surface, and the soil samples are not too high. – This is a straightforward oil tank decommission.
Tank decommissioning process
At least 2 workers will be present by law. They will dig a hole directly over the center of the tank is possible. A 2′ X 3′ hole is big enough for most tanks. If the tank is deep the hole will need to be bigger to allow the shovel to work properly.
Once they reach the tank they will cut off the top to allow access into the tank. The technicians will then empty the tank of all liquid inside which almost always includes a layer of watery sludge. They can then clean the tank and will probably use either a vacuum truck and pressure washer or bucket and scraping tool.
The next step is to collect soil samples and inspect the tank. Workers put on Tyvek suits and respirators and after they establish a safe atmosphere inside the tank they ensure adequate airflow and enter it. If previous soil samples shows no contamination then there is no need for new samples unless they see holes inside the tank. If the workers see holes inside then they collect samples from under the area with holes.
The day of decommissioning after the cleaning the workers cover and fence-off the hole that accesses the tank; to secure the hole from pets and children. They place the samples on ice until they go to the lab for overnight testing via a private courier. The soil sample results take one business day to get back.
If the tank has not leaked workers will come back and backfill the tank with an approved material. Usually perlite, gravel, or concrete.
If the tests show hydrocarbon contamination then the DEQ will want more information before issuing a certification. Assessing the extent of the contamination is the next step.
When the soil samples are high:
With a non-detect result the tank needs no more sampling. This is a result below 50 ppm (parts per million). 50 ppm is also nearly the resolution limit of diesel oil detection.
If some contamination is present but it can be shown to not be a large spill then the tank can stay in place. If contamination is high the DEQ will want to remove the tank and the affected soil under it. When it is too dangerous to remove soil then a provider can do further testing to see if a health risk is present. They can add a vapor reduction system if contaminated air is present but removal of contaminated soil is not possible.
When soil samples show oil contamination over at 10,000 parts per million (PPM) then the DEQ wants further tests. If this soil volume is over 65 cubic yards then the soil needs to be removed or additional testing needs to be performed to prove there are no health risks. This can include sub-slab vapor sampling and water samples taken from multiple locations.
In Washington if the PPM is over 2,000 tank removal is the way to certification.
A ‘Risk Based Analysis’ is made on a contaminated site. This is a method of ‘boxing off’ the dirty soil with clean soil and finding out the maximum physical volume of contaminated soil. This involved multiple soil sampling bores in the yard and possibly in the neighbor’s yard.
When there is Poor access to the tank
There can be tanks inside homes if there are additions. Tanks can be underground beneath decks, driveways, walkways, garages, pavers and gardens.
For an additional fee workers will remove & replace deck boards and cut & replace holes in concrete or asphalt.
When there is no access to the tank
How to decommission an oil tank if you can’t get to it?
If the tank is more than 5′ down OSHA will not allow hand digging to the tank without shoring and shoring cannot fit in a hole that small.
A sinkhole or tank collapse may threaten the structural integrity of a home if the house is on top of a tank.
In situations where access is impossible, a tank service provider may perform a triple rinse. When they can only obtain one soil sample they can use mathematical modeling to figure out a ‘rate of reduction’.
With enough time, and tests, and money, almost any tank can get a DEQ certification of oil tank decommissioning.
When decommissioning is over
After filling the tank the packed soil should be left in a mount, this will enable it to settle. If the soil if put back without a mound there will be a depression later after it rains a few times.