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UST | Spill & Overfill Prevention

Underground Storage Tank (UST) Section

UST Spill & Overfill Prevention

Revised On: Dec. 13, 2023 - 4:12 p.m.

All UST systems that are filled with more than 25 gallons of a regulated substance at any time must have spill and overfill prevention equipment systems.

What is spill prevention?

Spill prevention prevents a release of a regulated substance to the environment and involves capturing the spill of a regulated substance during a transfer from a truck to a tank by a bucket that fits around the fill pipe. Spill prevention devices are called Spill Buckets or Catchment Basins and should be sized so that they can hold more than the amount of product contained in the delivery hose.


Device should be kept free of product, water and/or debris and should show no signs of damage.

Required Documentation

Must show the spill bucket was installed by an ADEQ-certified service provider for the installation and retrofit of a UST.

Talk to your ADEQ-certified service provider about spill prevention equipment and maintenance options that suit your UST system | Learn More >

What is overfill prevention?

Overfill prevention involves a device, set at the appropriate activation level, that prevents a UST system from overfilling. An overfill prevention device is required on any UST system that is filled with more than 25 gallons of product at any one time.

Overfill Devices
  • Automatic Shutoff Device/Flapper Valves — Automatically shuts off flow of fuel at 95 percent, or less, of tank capacity. 
  • Overfill Alarm — Alarm activates when the fuel reaches 90 percent, or less, of tank capacity, or is within one minute of being overfilled.
  • Ball Float Valve — Restricts vapor flow out of the vent line at 90 percent, or less, of tank capacity, or within 30 minutes before overfilling. As of Oct. 13, 2015, these are not allowed on new tank installs, or when replacement is needed on existing tanks. Owners and operators must use one of the other overfill prevention methods listed in 40 CFR Part 280, such as Flapper Valves and Overfill Alarms | View 40 C.F.R. Part 280 >
    If replacing a Ball Float Valve with a Flapper Valve then the Ball Float Assemblies must be removed unless the Flapper Valve is set below the activation level of the Ball Float. For equipment installed after Oct. 13, 2018, all overfill prevention equipment must be tested every three years.

Required Documentation
  • For overfill equipment installed before Dec. 31, 1996, ADEQ requires the installer's signature, company name, and overfill device activation level.
  • For overfill devices installed after Dec. 31, 1996, provide documentation that the overfill device installation and activation level were completed by an ADEQ-certified installation and retrofit service provider.

It is important to provide certified documentation stating the appropriate activation level. If the activation level was not previously certified by the installer, then an ADEQ-certified individual performing installation and retrofit or tank tightness testing can verify and certify the activation level.

Maintenance Checklist

Spill Prevention Device
  • Keep documentation of certified installation
  • Note the size (gallons)
  • Keep device free of product, water and debris
  • Check for and address any visible damage

Overfill Prevention Device
  • Keep documentation of certified installation and activation level
  • Note type of overfill device
  • Verify the appropriate activation level