The Underground Storage Tank (UST) Program regulates the majority of USTs in Arizona, with the exception of those on Indian Lands. Nearly all underground storage tanks at these sites contain petroleum. These sites 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).

What is a "UST"?

An underground storage (UST) tank system is a tank and any underground piping connected to the tank that has at least 10 percent of its combined volume underground. The federal UST regulations apply only to underground tanks and piping storing either petroleum or certain hazardous substances.

What is "Petroleum"?

Generally, refined petroleum is divided into three categories:

• Gasoline
• High-end liquid hydrocarbon fuels such as diesel, No. 2 fuel oil, kerosene, aviation gasoline, and jet fuels
• Hydrocarbon oils, such as lubricating oils and other fuel oils.

What is MTBE?

Methyl tertiary butyl ether (MTBE) is a synthetic, flammable liquid fuel additive made principally from natural gas and other chemical compounds. It is blended with gasoline for the purpose of increasing the octane level and reducing vehicular emissions (CO2) and ozone-forming air pollutants. MTBE is a potential human carcinogen that has been detected nationwide in groundwater and some drinking water sources as a result of leaking USTs, accidental spills and overfills at gasoline pumps.

In September 2003, the U.S. EPA announced its proposed approval of Arizona's revised air quality plan for the Phoenix metropolitan area that eliminates the need for MTBE in summertime Cleaner Burning Gasoline, which is used to reduce ozone-forming emissions in the Valley.

See Also:

UST Requirements for MTBE in Groundwater
U.S. EPA Web Site

What is a "Regulated UST?"

Generally, any UST containing petroleum or hazardous substances larger than 110 gallons and operated on or after Jan. 1, 1974, with the exception of USTs used for on-site heating such as home heating oil USTs, is a regulated UST. Over a 10-year period, between Dec. 22, 1988 and Dec. 22, 1998, all operating tanks installed before the 1988 date were required to close or upgrade. The upgrade requirements included three components to prevent a release:

• Corrosion protection (for steel USTs and piping)
• Spill prevention
• Overfill prevention

Compliance with these and other operational and administrative requirements is the responsibility of the UST Program. Additional information about operating USTs can be obtained by contacting the UST Section.

What is a "Leaking UST"?

A leaking UST (LUST) is a UST that leaked some of the petroleum or hazardous substances into the soil or groundwater. Even though most USTs have been upgraded, or meet new tank standards, some still have leaks. Since 1987, ADEQ received about 8,523 UST release reports of which 7,557 are cleaned up (as of October 2009). Most of these spills occurred prior to 1998. Further information about LUSTs can be obtained by contacting the UST Corrective Action Section.

Must all LUSTs be investigated and cleaned up?

All LUSTs require an investigation and possible cleanup. Generally, releases from regulated USTs are the regulatory responsibility of the ADEQ UST Corrective Action Section. ADEQ does not regulate releases from unregulated USTs or above ground storage tanks.

Why be concerned about USTs?

Until the mid-1980s, most USTs were made of bare steel, which is likely to corrode over time and allow UST contents to leak into the environment. Faulty installation or inadequate operating and maintenance procedures also can cause USTs to release their contents into the environment.

Probably the most significant hazard associated with a UST is the potential for the regulated substance to seep into the soil and contaminate groundwater, the source of drinking water for nearly half of all Americans. A leaking UST may present other safety and environmental risks, including the potential for fire and explosion.

Sometimes USTs begin to leak due to corrosion of the tank or piping, equipment failure, improper installation and accidental spills. The spill can contaminate the soil or groundwater and occasionally migrates to surface water, sewers, private drinking water wells, or, as a vapor, into a building, such as a house. Once a release from a UST is discovered, the owner or operator of the UST must report the release, mitigate any immediate threats, investigate the extent of contamination in the soil and groundwater and implement a plan to clean up the contamination. ADEQ actively manages LUST sites that pose the greatest threat to human health or the environment.