Fact Sheet: MTBE


Methyl tertiary butyl ether (MTBE) is a chemical added to gasoline to promote more complete combustion and reduce emissions of carbon monoxide (CO) and organic compounds. The chemical adds oxygen to the gasoline, which increases the temperature at which it burns in the engines and reduces the amount of harmful byproducts in the vehicle’s exhaust.

Because it mixes readily with gasoline, is easily transported, has a low production cost and high octane rating, MTBE has become the oxygenate of choice for most gasoline producers who face state and federal mandates to produce less-polluting gasoline.

MTBE and Water Supplies
With its increased use, MTBE is now being found in shallow groundwater, at very low levels in some reservoirs and, to a much lesser extent, sources of drinking water. The most likely sources of the groundwater contamination are leaking underground storage tanks and pipelines. Although MTBE is readily mixed with gasoline, it does not appear to be easily absorbed by soil. As a result, the MTBE moves from the leaking gasoline source into the water where it is dissolved. California currently has a massive effort underway to replace by December, 1998 existing underground storage tanks with ones that are much less likely to leak. The Lawrence Livermore National Laboratory is currently studying how MTBE moves through the soil to groundwater.

The U.S. Environmental Protection Agency has recently proposed a “health advisory” level for MTBE in drinking water of 70 parts per billion (ppb) and the California Department of Health Services (DHS) has established an interim action level of 35 ppb. The interim action level is an advisory indicator level which, if exceeded triggers the water supplier to notify customers of the presence of MTBE in the drinking water. Currently, there is no regulatory maximum contaminant level for MTBE in drinking water.

MTBE Testing Requirements and Results
In 1996 the DHS Division of Drinking Water and Environmental Management, the regulator of drinking water quality in California, asked the systems it regulates to begin testing their water supplies for MTBE and made the testing mandatory in February of 1997.

To date, more than 1,800 drinking water sources have been tested for MTBE contamination and only 21 of those sources reported detectable concentrations of MTBE. DHS has a detection level for reporting of 5 ppb. Eleven of these sources are reservoirs that allow motorboat activity. This means that unburned MTBE in engine exhaust and gasoline spills from boats and marinas are the likely sources of MTBE in reservoirs.

Only two municipal drinking water sources, the city of Santa Monica and California Water Service of Marysville, have detected MTBE above the DHS interim action level of 35 ppb. In Santa Monica, two drinking water well fields near leaking underground storage tanks have been shut down and the city is buying replacement water from another utility. MTBE levels in the Marysville well dropped to low concentrations shortly after the initial high finding.

The Department of Health Services is working with other state and federal government agencies to monitor MTBE levels in drinking water and evaluate the best way to prevent any possible health consequences for the water consumer.
— From the California Department of Health Services