What is an Underground Storage Tank (UST)? 

According to Arizona Revised Statutes (A.R.S.), a UST is a tank or a combination of tanks and connected underground pipes and impact valves used (or once used) to contain regulated substances. These systems must have at least 10 percent of the total volume of the tank and connected pipes underground to be considered USTs. 

Federal and state UST regulations apply only to underground tanks and piping storing either petroleum or certain hazardous substances. Those with sites containing regulated USTs include marketers who sell gasoline to the public (such as service stations and convenience stores) and non-marketers who use tanks solely for their own needs (such as fleet service operators and local governments).

USTs do not include any of the following:

• A farm or residential tank with 1,100 gallons or less capacity used for storing motor fuel for noncommercial purposes
• A tank used for storing heating oil for consumptive use on the premises where stored
• A septic tank
• A pipeline facility, including gathering lines, regulated under either The Natural Gas Pipeline Safety Act of 1968 or The Hazardous Liquid Pipeline Safety Act of 1979
• An intrastate pipeline facility regulated under a state law comparable to the provisions of law referred to in Subdivision D, Item i or ii
• A surface impoundment, pit, pond or lagoon
• A stormwater or wastewater collection system
• A flow-through process tank
• A liquid trap or associated gathering lines directly related to oil or gas production and gathering operations
• A storage tank situated in an underground area, such as a basement, cellar, mine working, drift, shaft or tunnel, if the storage tank is situated on or above the surface of the floor
• Pipes connected to any of the structures described in the bullets above

Under A.R.S. § 49-1001: "Petroleum" means petroleum, including crude oil or any fraction of crude oil, which is liquid at sixty degrees Fahrenheit and 14.7 pounds per square inch absolute, and petroleum based substances comprised of a complex blend of hydrocarbons derived from crude oil through processes of separation, conversion, upgrading and finishing, such as motor fuels, residual fuel oils, lubricants, jet fuels, distillate fuel oils, petroleum solvents and used oils.

What is a leaking UST? 

A UST that has had a release of a regulated substance (typically petroleum) is called a leaking UST.  This designation does not mean that the UST system continues to leak; it means that a release occurred. 

Leaking USTs may present safety and environmental risks. Millions of gallons of petroleum products and hazardous substances are stored in USTs in the United States, and leaking USTs have been identified as a major source of soil and groundwater contamination. Leaks from USTs may create a risk to soil, groundwater and surface water (drinking water resources) and air (through vapor intrusion). Contamination from the release may require extensive and expensive cleanup efforts.

How are releases addressed?

Federal and state regulations are designed to reduce the risk of and address releases to minimize environmental harm and keep the costs of major cleanups down. When a release is discovered, the owner or operator must report the release to ADEQ, mitigate immediate threats, investigate the extent of contamination and implement a plan of corrective action to address the risks.

ADEQ's UST program collaborates with UST owners and operators, along with interested stakeholders, to prevent, identify and clean up releases into the environment. In addition to conducting periodic inspections and assisting owners and operators in complying with applicable state and federal regulations, the program provides financial assistance to eligible UST owners, operators and property owners to prevent leaks and conduct environmental cleanup when they occur | Learn More >

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