Methyl Tertiary Butyl Ether
MTBE- Well/ Groundwater Contamination Issues

 


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What is MTBE?

Methyl tertiary butyl ether (MTBE) is made from methanol and a by-product of the oil refining process, it was added to gasoline in an attempt to make gas burn cleaner, but studies show it has had little effect on curbing air pollution.   An ether, in general, is a compound containing an oxygen atom bonded to two carbon atoms. In methyl tertiary-butyl ether (also known as methyl t-butyl ether or MTBE), one carbon atom is that of a methyl group, -CH3, and the other is the central atom of a tertiary butyl group, -C(CH3)3.  At room temperature, MTBE is a volatile, flammable, and colorless liquid that is highly soluble in water and MTBE is 28 times more soluble than benzene.  MTBE will diffuse 8.6 times faster than Benzene. This will allow MTBE to spread laterally faster than the other hydrocarbons, creating a wider and longer plume. MTBE is 2.6 times more likely to vaporize than Benzene. MTBE is 9.2 times more likely to stay in water or to enter water from the atmosphere than Benzene. MTBE prefers to be in water 42 times more than it does air.  The Henry's Law Constant is the ratio of the concentration in air divided by the concentration in water. MTBE's low Henry's Law Constant (0.024) indicates that it greatly prefers water to air and any vapor that contacts water will preferably move into the water.

Methyl Tertiary Butyl Ether



MTBE is manufactured by the chemical reaction of methanol, generally made from natural gas, and isobutylene. The MTBE was added  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.  In the late 1970s, gasoline companies started  added MTBE in relatively low concentrations to gasoline to replace lead and to boost octane, i.e., measure of a gasoline's ability to resist engine knock .  In about 1996, the MTBE concentration has been increase d to 10 to 12 percent to enhance combustion and reduce tailpipe emissions, especially in states like California that have widespread air quality problems.  In 1990, in response to concerns over air pollution and aggressive industry lobbying, Congress amended the Clean Air Act to require the use of fuels which add oxygen to gasoline. MTBE is the best known oxygenate, and the one embraced by most oil companies although other oxygenates are used in some areas.   Approximately 31% of all US gasoline contains MTBE and MTBE makes up about 85 % of the oxygenates used nationally, other oxygenates include: ethanol and ethanol-based ethers.

In a major setback to the oil and chemical industries, on July 27, 1999 an EPA panel recommended that MTBE be reduced in use as a gasoline additive because of the toxic hazards that MTBE poses to drinking water supplies throughout the country. MTBE is a known animal carcinogen and possible human carcinogen. The reported U.S. production of MTBE in 1992 exceeded nine billion pounds - virtually all of it is added to gasoline. Oil companies reportedly make approximately $3 billion per year from MTBE production.

MTBE Transport

MTBE is highly volatile and would typically be expected to volatilize rapidly from soil surface s or water, but MTBE is soluble in water and highly mobile in soils and sorbs only weakly to aoil and aquifer material..   The solubility of MTBE in water at 25 C is approximately 5000 mg/L of a gasoline that is 10% MTBE.   Whereas, non-oxygenated gasoline has a total hydrocarbon solubility of about 120 mg/L.   In the atmosphere, MTBE is rapidly transformed into a formaldehyde compounds.   Once in the soil, the natural or alternative remediation processes have shown a limited effectiveness for remediating this chemical through bioremediation.  If found in groundwater at concentrations greater than 30 ppb, it is likely that the source is a leaking underground tank (UST) a point source.  Lower concentrations probably represent a combination of point and non-point sources, which may include atmpospheric washout and stormwater runoff from parking lots, roads, etc.. 


MTBE , Health Effect and Water Supplies

Recent testing by the U.S. Geological Survey indicated that 27 percent of the urban wells and springs tested were positive for MTBE.  With its increased use, MTBE is now being found in shallow groundwater, at very low levels in some reservoirs and 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. MTBE, like other ethers, is hydrophilic; in other words, it has a chemical attraction to water molecules. It is more than thirty times more soluble in water than other toxic compounds of gasoline.  When MTBE is released into the environment via transport accidents, leaking tanks, or simple over-filling at gas stations, MTBE travels great distances underground to the water table. MTBE can contaminate surface waters, but underground drinking water supplies are most at risk. 

The EPA has revised  its unregulated contaminant monitoring regulations to include MTBE.   In 1997, EPA released a health advisory on MTBE and indicated that at 20 to 40 ppb or less the MTBE can impart an unpleasant taste and odor to the water.  This concentration is reportedly 20,000 to 100,000 times lower than the rage of exposure levels in which cancer or noncancer effects were observed in animal experiments.  In 1998, MTBE was added to the EPAs Contaminant Candidate List.


The U.S. Environmental Protection Agency has recently proposed a "health advisory" level for MTBE in drinking water of 70 parts per billion (ppb), but other states have lower interim standards. Currently, there is no regulatory maximum contaminant level for MTBE in drinking water there is only an advisory limit. 
The advisory recommends control levels that prevent adverse taste and odor (i.e. 20 to 40 parts per billion)..

Note:  The most likely cause of the occurrence of MTBE in deep groundwater is typically steep vertical gradients caused by extraction wells, cross contamination within the aquifer, and poorly sealed wells. 

Oxygenates Other Than MTBE
EPA Underground Storage Tank Website
Listing of Certified Laboratories By State -Contact Your Local Certification Officer
Listing of Certified Laboratories in PA
Health Effects Analysis on Methyl Tertiary-Butyl Ether (MTBE)

Water Testing Packages that Included MTBE and Other VOCs
 

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Water Research Center
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