Leaking Underground Storage Tanks
Federal
UST Legislation
State Support
Cleanup Status
UST Definitions
Colorado
Oil Inspect Sec
UST Data
LUST
Anatomy
Corrosion
Leaks
Site Character
Remediation
Source
Groundwater
New Tanks
Installation
Monitoring
Overview
The boom in automobile sales following World War II was closely followed by the construction of thousands of gasoline stations across the country. At these new stations, bare steel tanks were installed underground to store gasoline. The average life expectancy of a steel tank was 30 to 50 years depending on the rate of corrosion of the steel. Since the early 1980s, corrosion of steel tanks, along with faulty installation and operation, have resulted in nearly ubiquitous groundwater contamination by gasoline. Because nearly half of all Americans depend on groundwater for their drinking water (map), leaking gasoline tanks represent a significant public health hazard. Leaking gasoline tanks can also present the risk of fire and explosion because vapors from leaking tanks can travel through sewer lines into buildings.
The majority of underground storage tanks (USTs) contain petroleum products (gasoline, diesel, heating oil, kerosene, jet fuel), but many other substances classified as hazardous by the Resource Conservation and Recovery Act and the Comprehensive Environmental Response, Compensation, and Liability Act ("Superfund") are stored in USTs. Leaking USTs are called LUSTs.
In September 1999, the U.S. Environmental Protection Agency (EPA) was monitoring about 370,000 LUST sites in the United States. About 21,000 site cleanups were planned for fiscal year 2001. Cleanups are funded by the EPA's LUST Trust Fund, which is currently funded at a level of about $70 million per year.
How much gasoline does it take to contaminate drinking water? Not much. Prior to the EPA's 1988 UST regulations and their final implementation deadline in 1998, a slow leak from a 10,000 gallon gasoline storage tank at the neighborhood service station was virtually undetectable to the station operator but still quite hazardous to nearby groundwater supplies. The hazards of gasoline are mainly attributable to the BTEX compounds -- benzene, toluene, ethylbenzene, and xylenes (see below). The benzene content of typical gasoline is 0.76% by mass (gasoline composition). A spill of 10 gallons of gasoline (only 0.1% of the 10,000 gallon tank, a quantity undetectable by manual gauging and inventory control) contains about 230 grams of benzene (using a gasoline density of 0.805 grams per milliliter). The EPA's Maximum Contaminant Level (MCL) for benzene is 5 parts per billion (ppb), or 5 micrograms per liter, in drinking water. The density of gasoline is about 0.8 grams per milliliter, so the benzene in a 10 gallon gasoline leak can contaminate about 46 million liters, or 12 million gallons, of water!
Contaminants of Concern
Because most LUST sites are contaminated by gasoline, the following constituents of gasoline are the typical contaminants of concern of LUST sites.
Benzene, toluene, ethylbenzene, and xylenes together are referred to as the BTEX compounds. They are the most common hazardous components of gasoline leaks.
- benzene is the most hazardous of these compounds -- its EPA Maximum Contaminant Level (MCL) is 5 parts per billion (ppb). Long-term exposures to benzene in drinking water at levels above the MCL increase the risk of cancer.
- toluene and ethylbenzene are not considered carcinogenic (cancer-causing). Their MCLs are 1.0 and 0.7 parts per million (ppm). Over the long term, toluene and ethylbenzene damage the liver, kidneys, and central nervous system.
- xylenes are a mixture of compounds (ortho-, meta-, and para-xylene) with two methyl (-CH3) groups attached to a benzene ring. Xylenes also affect the liver, kidneys, and nervous system, but they are not considered nearly as hazardous as the first three -- the MCL for total xylenes is 10 ppm.
Methyl tertiary butyl ether (MTBE) is an additive used to increase the oxygen content of gasoline to improve air quality. In the language of the 1990 Clean Air Act, oxygenated gasoline is referred to as "reformulated gasoline" or "oxyfuel." At concentrations as low as 20 parts per billion (ppb), MTBE makes drinking water unfit for human consumption because of taste and odor ( American Water Works Association, LUST Program). Currently, MTBE is classified as a potential human carcinogen, but as yet there is no Maximum Contaminant Level for drinking water. As many as 9,000 community water wells in 31 states may be affected by MTBE contamination. The U.S. Geological Survey reports that about 20% of groundwater in areas where reformulated gasoline is sold is contaminated by MTBE. More details about air quality, health, and water quality concerns over MTBE are provided in this report by the Congressional Research Service.
MTBE is highly soluble in groundwater -- about 43,000 ppm. The high solubility of MTBE allows it to be readily dissolved into groundwater from leaked gasoline and transported over great distances. In some cases, MTBE transport has exceeded the transport distances of BTEX compounds by 10 times. Compared to MTBE, the BTEX compounds are less soluble and more readily sorbed to aquifer sediments. A study of benzene contamination of groundwater by LUSTs in Texas reports that most benzene plumes extend less than 80 meters from their source.
A sketch depicting typical groundwater contamination by a LUST
