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Methyl tert-butyl ether (MTBE)

What is MTBE? Methyl tert-butyl ether (MTBE) is one of the primary components currently being used in gasoline to meet the California Reformulated Gasoline Standards to reduce smog-forming emissions and either seasonally or year round in specific parts of the country where concentrations of ozone in the summer or carbon monoxide in the winter exceed established air quality standards. MTBE is currently the oxygenate of choice of petroleum refineries seeking to meet the California Air Resources Board (CARB) Phase 2 Reformulated Gasoline regulations and in other parts of the country due to its low cost, ease of production and favorable transfer and blending characteristics. Petroleum refineries have been using MTBE in gasoline to meet seasonal oxygenated fuel requirements since 1991. Oxygenated gasoline can contain up to15 percent MTBE by volume. MTBE has also been recorded as used in gasoline since 1979 as an anti-knock compound.

Why is there so much concern over MTBE? MTBE has recently been found in drinking water supply wells for the City of Santa Monica, California. The City has had to shut down all their wells and is currently trucking in fresh water from Los Angeles. MTBE is also showing up in other municipal supply wells, irrigation wells and groundwater monitoring wells. A recent open file report by the U.S. Geological Survey noted detectable MTBE concentrations in 27 percent of shallow wells sampled in 8 urban areas around the country. MTBE has also been found in drinking water systems in 10 northern states where MTBE has been used as either an octane booster or a oxygenate in gasoline. As more sampling of drinking water supplies and groundwater monitoring wells is done specifically for MTBE and its use in oxygenated fuels increases, its occurrence in groundwater will be found to be more widespread than is currently recognized.

Properties of MTBE: MTBE is highly soluble in water and is very mobile in the subsurface. The high degree of solubility means that MTBE can move rapidly in the subsurface and is difficult to detect without chemical analysis. MTBE does not absorb well to carbon or soil particles and does not appear to biodegrade well, although there are some encouraging reports recently to have isolated bacteria which can degrade MTBE. Indications are that MTBE is susceptible to air sparging/soil vapor extraction and can be degraded in an oxygen-enriched environment.

Because of high mobility, MTBE may be useful as a gasoline spill plume indicator, showing the leading edge of the gasoline spill plume and acting as a warning agent for hydrocarbon contamination. Furthermore, remediation and clean up projects may benefit from MTBE in fuel as a "built in" conservative tracer which can be used to validate groundwater models of the site.

How will the presence of MTBE affect UST sites? The California Environmental Protection Agency and the California Department of Health Services will soon be proposing a primary drinking water standard for MTBE. California regulators are becoming increasingly concerned that there is no system presently in place to detect MTBE contamination of drinking water from leaking underground storage tanks and is moving to create mandatory MTBE sampling at all leaking underground storage tank (LUST) sites. The California Department of Health Services is starting to gather data from public water systems to determine how widespread the problem is in these systems. EPA is considering issuing drinking water health advisories for MTBE where concentrations reach 20 to 200 parts per billion. No conclusive evidence exists that MTBE is a human carcinogen or even severely toxic at this time, however toxicity studies are being conducted by a number of State and Federal agencies. Studies to date have focused on effects derived from inhalation, not ingestion through consumption of drinking water impacted by MTBE.

Increased information on the biodegradation capabilities and fate and transport of this compound will become more available as more data is accumulated through routine groundwater monitoring of leaking underground storage tank sites and sampling of drinking water systems. For now, the regulatory agencies are simply collecting information and are not using MTBE as a site closure criteria. This could change as more information becomes available.