Prevent groundwater pollution from underground fuel storage tanks

Decommissioning an underground storage tank

How to decommission an underground storage tank temporarily or permanently and the risks to groundwater.

This decommissioning guidance applies to all underground storage tanks, not just those used to store fuel.

Decommissioning includes a range of activities, from completely closing and removing an underground storage tank (UST) facility as a whole, to replacing individual tanks or lengths of pipework. The type of measures you need to follow apply to all types of facilities, but the scale will vary.

The Blue Book and PETEL 65/34 have guidance on best practice for decommissioning a UST.

Decommission unused USTs

You should immediately decommission any USTs that you’re no longer using. Decommissioning includes both:

  • closing and removing a UST system (the tank and any equipment connected to it) as a whole
  • replacing individual tanks or lengths of pipe

You can decommission a UST either permanently or temporarily. If you only temporarily decommission a UST you must make sure that it doesn’t cause pollution and you have plans to permanently decommission it as soon as possible. As part of the decommissioning process you should:

  1. Carry out a full environmental risk assessment.
  2. Sample any surrounding soil and groundwater (you should do this before, during and after decommissioning) - the results from this will be important in your risk assessment.
  3. Remove any residual product from the tank and pipes (this is called ‘bottoming’).
  4. Remove any explosive vapours from the tank and pipes to make them safe, before removing them from the ground.
  5. Remove and then clean tanks, pipes, dispensers and separators.

You should consult the Blue Book for further technical guidance on decommissioning tanks (including health and safety issues).

To avoid pollution you should remove tanks and associated pipework that you’re unlikely to use again. If you leave tanks in place there’s a risk that some product remains in the tanks that you won’t be monitoring or maintaining.

If you can’t make sure of the following you should review your plans to decommission the UST:

  • your methods don’t lead to a loss of fuel to the ground either accidentally or on purpose
  • you don’t leave any equipment underground
  • you don’t leave any product underground (eg in tanks, pipes or drains)
  • you dispose of contaminated tanks and pipework lawfully
  • you comply with relevant waste management requirements by using registered waste carriers and any permits that you may need for waste treatment or storage

Bottoming a tank

Removing the residual product in tanks and pipelines (bottoming) is a high-risk activity. You must dispose of waste at a permitted waste facility and you must make sure you don’t lose any product to the ground.

After you’ve bottomed the tank you need to have it made safe by removing any explosive vapours, eg by filling it with inert gases or water. If you use water, make sure you dispose of it properly afterwards, eg by transferring it to a waste treatment facility or discharging it to a foul sewer with the consent of the local sewerage undertaker. You should use the same methods before removing pipes from the ground.

You must bottom and make safe any USTs and pipework before removing them from the ground.

When excavating the UST you need to prevent surface water from entering the site as it could become contaminated - you’d then have to treat it as hazardous waste and dispose of it accordingly.

Sample soil and groundwater

You should sample soil and groundwater for subsurface contamination before, during and after removing the tank. You’re likely to need a qualified environmental consultant to plan your sampling programme and take the samples for you. You can contact the Environment Agency or Natural Resources Wales (NRW) for advice before taking any samples.

If you find contamination, you’ll need to carry out further investigations (and most likely a risk assessment) to identify how to deal with the problem.

Make the UST safe if you plan to leave it in place

If you can leave fuel or water in a tank on a temporarily decommissioned site you must continue to monitor the tank. Make sure any tank left permanently on a site is also made safe. You can only leave a tank permanently on site if the Environment Agency or NRW are satisfied that there’ll be no risk of soil or groundwater pollution.

If you plan to leave the UST or pipes in place once you’ve made them safe you need to have them made safe - this is known as ‘inerting’.

Fill it with either:

  • a sand and cement slurry
  • hydrophobic foam
  • foamed concrete

If you plan to leave the tank on the site, keep a record of:

  • its capacity
  • the product it contained
  • the method of decommissioning , if any (eg degassing, filling with concrete)
  • the date of decommissioning

If tanks and their pipework are no longer suitable or safe for storing petroleum spirit you should not use them to store diesel (or other hydrocarbons) without checking their integrity.

Remove separators

When disposing of the oil and water separator you should:

  • dispose of all residual liquid and sludge lawfully away from the site
  • seal all inlets and outlets

You should remove oil and water separators and dispose of them lawfully away from the site. If you can’t dispose of them away from the site you should fill them in in a similar way to USTs.

Make sure any drainage systems that remain active after decommissioning does not provide a channel for pollutants to reach groundwater or other surface waters (eg rivers, streams, lakes or wetlands).

Temporary decommissioning

If you temporarily decommission a site you can leave product or water in tanks. If you do then you must continue your monitoring procedures as if the facility remained operational.

If you can’t continue monitoring then you should empty the tanks and make safe.