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Drinking Water

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Concerns about MTBE

With these air quality benefits, why is there concern with the use of MTBE?

A growing number of studies have detected MTBE in ground water throughout the country; in some instances these contaminated waters are sources of drinking water. Low levels of MTBE can make drinking water supplies undrinkable due to its offensive taste and odor.

Is MTBE harmful to humans?

The majority of the human health-related research conducted to date on MTBE has focused on effects associated with the inhalation of the chemical. When research animals inhaled high concentrations of MTBE, some developed cancers or experienced other non-cancerous health effects To date, independent expert review groups who have assessed MTBE inhalation health risks e.g., Interagency Assessment of Oxygenated Fuels) have not concluded that the use of MTBE-oxygenated gasoline poses an imminent threat to public health. However, researchers have limited data about what the health effects may be if a person swallows (ingests) MTBE. EPA's Office of Water has concluded that available data are not adequate to estimate potential health risks of MTBE at low exposure levels in drinking water but that the data support the conclusion that MTBE is a potential human carcinogen at high doses. Recent work by EPA and other researchers is expected to help determine more precisely the potential for health effects from MTBE in drinking water.

EPA reviewed available health effects information on MTBE in its 1997 Drinking Water Advisory guidance and decided that there was insufficient information available to allow EPA to establish quantitative estimates for health risks and as such would not set health advisory limits. The drinking water advisory document indicates that there is little likelihood that MTBE in drinking water will cause adverse health effects at concentrations between 20 and 40 ppb or below.

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Drinking Water Quality

Has EPA set a drinking water health standard for MTBE?

EPA has not set a national standard for MTBE, although some states have set their own limits. EPA will issue a secondary drinking water standard, based on taste and odor, by late Fall 2000. This taste and odor standard will serve as a guideline that states may adopt. In December 1997, EPA issued a Drinking Water Advisory that states concentrations of MTBE in the range of 20 to 40 ppb of water or below will probably not cause unpleasant taste and odor for most people, recognizing that human sensitivity to taste and odor varies widely. The advisory is a guidance document that recommends keeping concentrations below that range. EPA also reviewed the available information on health effects in the 1997 advisory and stated that there is little likelihood that MTBE concentrations between 20 and 40 ppb in drinking water would cause negative health effects.

EPA is continuing to study both the potential health effects and the occurrence of MTBE, and it is on a list of contaminants (Contaminant Candidate List) for which EPA is considering setting health standards. As a means of gathering occurrence information, beginning in 2001, EPA will require all large drinking water systems and a representative sample of small systems to monitor and report the presence of MTBE (Unregulated Contaminant Monitoring Regulation).

How do I know if I have MTBE in my water?

It is possible your water would taste and/or smell like turpentine if MTBE is present at levels around or above 20-40 ppb (some people may detect it at even lower levels). Though you cannot currently purchase a home testing kit, you can determine if your water contains MTBE the following ways. If your drinking water is supplied by a public water system, you can contact the system directly and ask whether they monitor for MTBE and what levels, if any, have been detected. In 2001, most public water systems will be required to monitor for MTBE. If you have a private well, you may want to have your well water tested. Your local health department may be able to tell you if MTBE has been found in water in your area. If you want to get your water tested, call the Safe Drinking Water Hotline (800-426-4791) or go to http://www.epa.gov/safewater/faq/sco.html to get the phone number for the office in your state that certifies drinking water laboratories.

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Occurrence of MTBE in Water

How does MTBE get in drinking water sources?

There are opportunities for MTBE to leak into the environment (and potentially get in drinking water sources) wherever gasoline is stored, and there are opportunities for it to be spilled whenever fuel is transported or transferred. While federal and state programs minimize the potential for leaks and spills, no system is foolproof.

Contamination of drinking water sources can occur from leaking underground and above ground fuel storage tanks, pipelinees, refueling spills, automobile accidents damaging the fuel tank, consumer disposal of "old" gasoline", emissions from older marine engines, and to a lesser degree, storm water runoff, and precipitation mixed with MTBE in the air (EPA's Office of Ground Water and Drinking Water) or (USGS report).

How widespread and at what levels is MTBE contamination in water supplies?

Although there are no nation-wide data sets from which to fully characterize MTBE contamination of water, a growing number of studies to-date have detected MTBE in drinking water supplies throughout the country. Current data on MTBE levels in ground and surface waters indicate widespread and numerous detections at low levels of MTBE, with a more limited number of detections at higher levels (only about 1 percent of concentrations are more than 20 parts per billion (ppb) as discussed in the 1999 Blue Ribbon Panel Report on Oxygenates in Gasoline). Studies have shown that MTBE is detected in water roughly five times more often and at higher concentrations in areas of the country where federal RFG is sold (i.e., where there is an oxygenate mandate).

When MTBE is detected, the levels are typically below 20 ppb which is lower than EPA’s Drinking Water Advisory. However, releases from petroleum storage tanks, and pipeline breaks or other point sources can cause high concentrations of MTBE in water. When such releases occur, the resulting localized concentration can be much higher than the EPA's advised taste and odor acceptable range (EPA's Office of Ground Water and Drinking Water).

What is the status of the drinking water contamination in Santa Monica, CA, the city with the first significant incidence of MTBE contamination?

In 1996, the city of Santa Monica learned that two of its drinking water wellfields, Charnock and Arcadia, were contaminated with MTBE at levels as high as 610 ppb and 86 ppb respectively. In response, the two wellfields, representing 50 percent of the city's drinking water supply were shut down and the city began purchasing replacement water. This incident was the first major water contamination which brought public attention to MTBE.

EPA's Region 9 and the Los Angeles Regional Water Quality Control Board (RWQCB) are pursuing a joint enforcement action at the Charnock wellfield in Santa Monica. Site-specific clean-up is underway. At the smaller Arcadia wellfield, the RWQCB has the lead while EPA provides technical support and field oversight of the clean-up.

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Movement and Disposition of MTBE in the Environment

What happens when MTBE gets into the environment?

Because MTBE dissolves easily in water and does not "cling" to soil very well, it migrates faster and farther in the ground than other gasoline components, thus making it more likely to contaminate public water systems and private drinking water wells. MTBE does not degrade (breakdown) easily and is difficult and costly to remove from ground water.

How long will MTBE remain in water?

MTBE is generally more resistant to natural biodegradation than other gasoline components. Some monitoring wells have shown little overall reduction in MTBE concentration over several years which suggests that MTBE is relatively persistent in ground water. In contrast, studies of surface water (lakes and reservoirs have shown that MTBE volatilizes (evaporates) relatively quickly.

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Additional Information

You can access additional documents on how MTBE affects drinking water from the Office of Ground Water and Drinking Water.

You can also call the Safe Drinking Water Hotline at 800-426-4791 for information and assistance about EPA's drinking water regulations, the wellhead protection program, source water protection and related guidance, and public education materials.

Local Information will tell you whom to contact in your area for more information on MTBE in drinking water.

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