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Page 2

Interagency
Assessment of
Oxygenated Fuels

J U N E 1 9 9 7

National Science and Technology Council

Committee on Environment and Natural Resources

Page 132

Oxygenated Fuels

2-52

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Page 133

Water Quality

2-53

For a compound that has a molecular weight that is similar to the mean molecular weight
of gasoline (-100 g/mol), X is closely approximated as the fraction of the compound on

m

either a weight or volume basis. For example, for a gasoline that is 10 percent by weight
MTBE, X for MTBE will be -0.10. For the dissolution of low polarity compounds (like

m

the BTEX group) from gasoline, ( will be close to 1 and equation 1 can then be
approximated as:

(2) (low polarity compounds)

The above equation assumes that dissolution is taking place into a water phase in which
there are not large amounts of alcohols or other possible cosolvents that could increase the
value of . Since values tend to decrease as the temperature decreases, predicted
values will also decrease as the temperature decreases.

Equilibrium solubility data obtained by Barker et al. (1991) indicate that equation 2 is also
valid for the dissolution of the alkyl ether oxygenates from gasoline; for MTBE, their data
indicate that ( = 1.1. For the alcohol oxygenates like TBA and EtOH, however, ( values
will be significantly larger than one.

MTBE�s high water solubility and its high concentration in an MTBE-oxygenated
gasoline can result in high concentrations in water. For example, MTBE in a gasoline that
is 10 percent (w/w) MTBE, has a solubility in water of approximately 5,000 mg/L at room
temperature. By comparison, the total hydrocarbon water solubility for a non-oxygenated
fuel is typically about 120 mg/L (Poulsen et al., 1992).

High concentrations of MTBE in ground water near gasoline-spill sites are consistent with
experimental water-solubility data. Garrett et al. (1986) and Davidson (1995a) have
observed MTBE concentrations as high as 200 mg/L. While these values are not as high
(e.g. thousands of mg/L) as might be expected for water equilibrated with gasoline
containing several percent MTBE, it is common for relatively uncontaminated water to
dilute organic compounds to concentrations below what would be calculated with equation
2 (Feenstra et al., 1996). Also, lower levels in water can result because of depletion of
MTBE in the gasoline by the dissolution process itself.

Cosolvent Effects. The high concentrations of fuel oxygenates that can occur in ground
water at spill sites have raised the question as to whether or not they could enhance the
subsurface transport velocities of the BTEX group through a �cosolvency effect.� In fact,
research has shown that cosolvency effects typically arise only when the cosolvent is
present in water at 1 percent (10,000 mg/L) or more by volume (Pinal et al., 1990, 1991).
Such concentrations are much higher than will typically be encountered in water in
equilibrium with gasolines containing alkyl ether oxygenates. Gasoline that contains 15
percent MTBE by volume, when equilibrated with water, results in no more than 7,500
mg/L (-0.75% by volume) of MTBE in the water (Barker et al., 1991).

These conclusions regarding the potential for cosolvent effects are supported by laboratory
research that employed gasoline to water ratios of 1:10 (v/v). No cosolvency effect was

Page 263

I-3

oxygenates Compounds containing oxygen (alcohols and ethers) that are

added to fuels to increase its oxygen content. Lawful use of

these substances as components of gasoline requires that they

either be �substantially similar� under section 211(f)(1) of the

Clean Air Act or approved under a waiver granted under

section 211(f)(4) of the Clean Air Act. Methyl tertiary butyl

ether (MTBE) and ethanol are the most common oxygenates

currently used, although there are a number of other possible

oxygenates.

ppbv parts per billion by volume

ppm parts per million

ppmv parts per million by volume

psi pounds per square inch, pressure

RFG Reformulated gasoline. Specially formulated fuels developed

to minimize vehicle emissions of ozone-forming and toxic air

pollutants and improve air quality. RFG contains, on

average, a minimum of 2.0 weight percent oxygen.

rpm revolutions per minute.

RVP Reid vapor pressure, fuel vapor pressure (Psi) at 100 EF

SCE sister chromatid exchange

TA tertiary-butyl alcohol

TAEE tertiary-amyl ethyl ether, a fuel oxygenate

TAME tertiary-amyl methyl ether, a fuel oxygenate

TAP toxic air pollutants (benzene, 1,3-butadiene, formaldehyde,

acetaldehyde, and polycyclic organic matter)

TBA tertiary-butyl alcohol

TBF tertiary-butyl formate

TBI throttle body fuel injection

TRI Toxics Release Inventory

TWO three-way catalysts

USEPA United States Environmental Protection Agency

USGS United States Geological Survey

UST underground storage tank

VOC volatile organic compound

WI Wisconsin

wt weight

yr year

Page 264

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For additional information, please contact:

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Executive Office of the President
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fax: 202-456-6025

Also available on the NSTC Home Page via link from the OSTP Home Page at:
http://www.whitehouse.gov/WH/EOP/OSTP/html/OSTP_Home.html

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