I undertake Health & Safety on filling stations for my living.
Petrol tanks are not permitted to be left empty when a filling station closes. This does not apply to diesel tanks. In addition to the Health and Safety at Work Act this is controlled by the Public Health Act. The operation of the site and closure is regulated by the fire brigade or Trading Standards Department of your local council. The land contamination is dealt with by Environmental Health, or if the site is over a water extraction (source protection) zone then the Environment Agency will be involved. It's hard to hide a petrol station so not many escape being made safe properly.
When sites initially close it is normal for the tanks to be water filled for up to three years or made safe permanently straight away. Making safe involves filling with resin generated foam such as
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a lean concrete mix (which must be vibrated to remove voids) or removal and cutting up after de-gassing.
Tanks cannot generally be washed out or steam cleaned as the hydrocarbons will leach out of the tank shell and create an explosive atmosphere again. Water alone has been shown to be insufficient in removing petroleum residues from the tank shell.
Anyone competent can make tanks safe (apart from de-gassing or foam filling which require specialist kit) this operation is overseen by the fire brigade or TSD after a method statement and risk assessment has been produced. This is not really surprising when you consider that most filling station are in populated area and huge amounts of petrol vapour can be is released during this operation. The regulator overseeing the work makes no charge for their attendance or approving the method statements, nor have I ever heard of them being bunged! There are also no Building Regs fees or planning consent required for making tanks safe.
AFAIK the NHBC will not issue their 10 year warranty to houses built over old petrol tanks even if they have been made safe to current standards.
What is strange is that the Public Health Act puts the responsibility of making safe with the occupier of the site and not the owner. Many tenants are caught out when they close a petrol station and keep open a car sales or workshop and find they are liable for remediation costs.
If you think a closed site has not been made safe, ask the local fire brigade or TSD. Contact details are here
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