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An Underground Storage Tank (UST), in United States environmental law, is a tank and any underground piping connected to the tank that has at least 10 percent of its combined volume underground. USTs, used to store petroleum, are regulated in the United States to prevent release of petroleum and contamination of groundwater.

Underground storage tanks generally fall into three different type:

1. Fiberglass reinforced plastic (FRP) tanks made by Xerxes, Total Containment and others
2. Steel tanks made by manufacturers in most states and conforming to standards set by the Steel Tank Insitute. Some steel tank manufacturers include Hamilton Welding, Highland Tanks, Modern Welding, and others
3. Composite material Tanks which incorporate, usually, a steel tank covered in fiberglass or a plastic compound for corrosion protection and to form an interstitial space. Composite tanks are generally made by Steel Tank Institute members and includes such types as Permatank and ACT-100.

USTs are used throughout North America at automobile filling stations and many have leaked, allowing petroleum to contaminate the soil and groundwater.

Many USTs installed before 1980 consisted of bare steel pipes, which corrode over time, and may eventually result in leakage. Faulty installation and inadequate handling may also cause leaks.

In 1984, in the United States, the Congress developed laws to minimize and prevent environmental damage, by charging owners with the task of verifying, maintaining and if the case comes to it, cleaning up sites damaged by petroleum contamination.

Legislation requiring owners to locate, remove, upgrade, or replace underground storage tanks became effective December 24, 1989. Each State was given authority to establish a program within its own jurisdiction to set up a program to compensate owners for the cleanup of underground petroleum leaks, to set standards and licensing for installers, and to register and inspect underground tanks.

Most upgrades to USTs consisted of the installation of corrosion control (cathodic protection), overfill protection (to prevent overfills of the tank during tank filling operations), spill containment (to catch spills when filling), and leak detection for both the tank and piping.

Many USTs were removed without replacement during the 10-year program and many thousands of old underground tanks were replaced with newer tanks made of corrosion resistant materials (such as fiberglass) and constructed as double walled tanks to catch leaks from the inner tanks and to give an interstitial space to accommodate leak detection sensors. Piping was replaced during the same period with much of the new piping being double wall construction and made of fiberglass or plastic materials.

Tank monitoring systems capable of detecting leaks as small as 0.1 gallons-per-hour were installed and other methods were adopted to alert the tank operator of leaks and potential leaks.

Regulations included a requirement that UST cathodic protection systems be tested by a cathodic protection expert (minimum every 3 years) and that systems be monitored to ensure continued compliant operation.

Many owners, who previously stored fuel in underground tanks, went to above ground tanks to enable closer environmental monitoring of fuel storage and to reduce costs. Many states, however, do not permit above ground storage of motor fuel for resale to the public.

The USEPA Underground Storage Tank Program is generally considered to have been very successful - with the national inventory of underground tanks reduced by more than half and most of the rest having been replaced or upgraded to much safer standards.

• Environmental Protection Agency Office of Underground Storage Tanks
• Inter-Tribal Environmental Council
• Texas Department of Environmental Quality
• New Mexico
• Oklahoma Corporation Commission
• Drury, John; Tank Bottom Inspection techniques Magnetic Flux Leakage Technical Articles