United States
Environmental Protection
Agency
Office Of Solid Waste And
Emergency Response
(5401G)
EPA 510-F-98-002
January 1998
www.epa.gov/OUST/mtbe
&EPA
Office Of Underground Storage Tanks
MTBE
Fact Sheet #2
Remediation Of MTBE
Contaminated Soil And
Groundwater
Background
Methyl tertiary-butyl ether (MTBE) is a
fuel additive made, in part, from natural
gas. Since 1979, it has been used in the
United States as an octane enhancing
replacement for lead, primarily in mid-
and high-grade gasoline at concentra-
tions as high as 8 percent (by volume).
Since the mid-1980s, it has been widely
used throughout the country for this
purpose. It is also used as a fuel oxy-
genate at higher concentrations (11 to 15
percent by volume) as part of two U.S.
EPA programs to reduce ozone and
carbon monoxide levels in the most
polluted areas of the country.
Physical And Chemical
Characteristics Of MTBE
The effectiveness of remediation
methods is directly linked to the physical
and chemical characteristics of the
constituent of interest. Because MTBE
behaves differently in soil, air, and water
than other petroleum constituents, the
choice of an effective remediation
technology may be different when
MTBE is present at a site. Benzene is
most often the contaminant of concern in
gasoline because of its relatively high
solubility and its known carcinogenicity.
As a result, comparing the
characteristics of MTBE with benzene is
helpful in showing how remediation
technologies may differ when MTBE is
added to gasoline.
• MTBE is about 30 times more
soluble than benzene in water.
Pure MTBE can reach an equi-
librium concentration in water
of approximately 5 percent (i.e.,
48,000 mg/L).
• When moving from the liquid
phase (i.e., free product) to the
vapor phase, MTBE is three
times more volatile than
benzene (i.e., the vapor pressure
of MTBE is three times the
vapor pressure for benzene).
• When moving from the
dissolved phase (in water) to the
vapor phase, MTBE is about
ten times less volatile than
benzene (i.e., its Henry's law
constant is 1/1 Oth benzene).
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1 MTBE Fact Sheet #2: Remediation
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• MTBE is much less likely than
benzene to adsorb to soil or
organic carbon.
• MTBE is more resistant to
biodegradation than benzene.
When MTBE is in the soil as the result
of a petroleum release, it may separate
from the rest of the petroleum, reaching
the groundwater first and dissolving
rapidly. Once in the groundwater,
MTBE travels at about the same rate as
the groundwater whereas benzene and
other petroleum constituents tend to
biodegrade and adsorb to soil particles.
Soil Remediation
Because it has a very high vapor pres-
sure and a low affinity for sorption to
soil, MTBE can be effectively reme-
diated by two soil treatment technol-
ogies, typically without any costs be-
yond those needed for remediating other
petroleum constituents. Soil vapor
extraction (SVE) is an in situ soil
treatment technology that removes
volatile contaminants from soil in the
unsaturated zone above groundwater by
extracting the contaminant vapors with a
vacuum that is applied to the sub -
urface. Low-temperature thermal
desorption (LTTD) is an ex situ soil
treatment technology that uses temper-
atures below ignition levels to separate
volatile contaminants from soil. Be-
cause of its high vapor pressure, both
methods are very effective in removing
MTBE from soil. However, SVE and
LTTD must be used soon after a release,
before most of the MBTE moves from
the soil into the groundwater.
Bioremedial methods for soil treatment
(e.g., land-farming, bioventing, bio-
piles) are currently not recommended for
removing MTBE because it is
considered recalcitrant to biodegrada-
tion. This recommendation may change
in the future as new research examines
the efficacy of specific strains of
bacteria and/or improved methods of
biodegrading MTBE.
Groundwater Investigations
And Monitoring
Because MTBE behaves differently
from petroleum hydrocarbons when
released into the environment, a reme-
dial investigation may need to be mod-
ified to properly characterize the area of
MTBE contamination. Many regu-
lators of UST programs have observed
that MTBE's relatively high solubility
allows it to dissolve into the ground-
water in "pulses" that result in rapid
orders of magnitude changes in
groundwater concentrations. Pulses,
which may be caused by the infiltration
of rain water or rising groundwater
levels, may necessitate frequent
groundwater sampling to determine
actual MTBE concentrations and levels
of risk to down-gradient receptors. The
frequency of sampling should be
determined based on the velocity of the
groundwater and the number of
monitoring wells. Determining the
impact of the selected remediation
method may be difficult without accu-
rate historical sampling data.
Groundwater Remediation
Pump-And-Treat
hi contrast with the preferred remedia-
tion techniques for petroleum hydro-
carbons such as benzene (e.g., bio-
remediation), pumping contaminated
groundwater and treating it above
ground (i.e., pump-and-treat) may more
MTBE Fact Sheet #2: Remediation
January 1998
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often be an effective remediation
technology for MTBE because MTBE
does not adsorb significantly to soil. As
a result, fewer aquifer volumes are
required to remove all of the MTBE than
are required to remove the slowly
desorbing petroleum hydrocarbons. In
addition, because it is highly soluble,
most of the MTBE mass may quickly
dissolve into groundwater, making
pumping an efficient method for
removing large quantities of the
contaminant.
As with petroleum hydrocarbons, how-
ever, diffusion is also a factor control-
ling the remediation timeframe. If mi-
cropores exist within the aquifer that are
not readily influenced by groundwater
flow, transfer of a contaminant from the
micropores to the macropores will occur
through the slow process of diffusion.
Hence, in spite of some favorable
characteristics, pump-and-treat may not
always be an efficient remediation
method for MTBE contamination.
Aquifers with high total porosity but
with low effective porosity remain
troublesome in treating any contaminant.
The physical and chemical properties of
MTBE are also important in the
treatment of MTBE above ground.
Because it does not adsorb significantly
to carbon, MTBE is not a good can-
didate for using granular-activated
carbon (GAC) to remove it from water.
GAC is about 1/3 to 1/8 as effective in
removing MTBE as it is in removing
benzene. In addition, because MTBE
"prefers" to remain in water, air strip-
pers must use a higher volume of air
than is required for benzene. Initial field
experience indicates that two to five
times more air is needed to treat the
same volume of water if MTBE
concentrations are less than 5,000 ppb.
An additional expense associated with
MTBE remediation is that more ex-
traction wells and associated equipment
(e.g., pumps, lines) may be required than
for benzene because MTBE travels
farther and faster than the rest of the
plume, resulting in a larger plume size.
The cost of treating an MTBE ground-
water plume can be significant, how-
ever, cost effective methods do exist A
1991 American Petroleum Institute
study (API Publication No. 4497) de-
termined that air stripping alone was the
most cost effective technology for
remediating water containing 20-ppm
MTBE down to a level of 10 ppb. A 25-
gallon per minute air stripping system
could achieve this level of remediation
for $9 per 1000 gallons (in 1990
dollars). If off-gas emissions were also
a concern, they could be treated for an
incremental cost increase of $7 per 1000
gallons (i.e., $16 per 1000 gallons total
cost). As an alternative, UV-catalyzed
oxidation using hydrogen peroxide could
be used to treat water and off-gases at a
total cost of $15 per 1000 gallons.
Air Sparging
Air sparging is another groundwater
remediation technology that has shown
some promise. It accomplishes reme-
diation goals by injecting air directly
into the groundwater to volatilize the
contaminants in situ. A few case studies
have shown that reductions in MTBE
levels from above 1000 ppb to less than
10 ppb are possible in less than 2 years.
However, regardless of the contaminant,
air sparging is typically only appropriate
in homogeneous sands because
heterogeneous sediments may cause
dispersion of contaminants and
channeling of air flow. In addition, air
i sparging should be less effective for
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MTBE Fact Sheet #2: Remediation
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MTBE than for benzene because more
air is needed to volatilize the MTBE.
The addition of dissolved oxygen in the
groundwater from air sparging may not
significantly increase the biodegradation
of MTBE as it would for benzene.
Bioremediation
Although MTBE is generally believed to
be resistant to biodegradation, pre-
liminary research has shown that bio-
degradation may be an effective reme-
diation option under specific condi-
tions. Bioreactors, an ex situ form of
bioremediation, have shown some initial
promise. Additional research and
development are continuing to make
them more reliable and cost effective.
New research is also showing that in
situ biodegradation may be an effective
remediation alternative; however, more
information is required to determine the
specific environmental conditions that
enable significant rates of biodegrada-
ion to occur.
Point-Of-Use Treatment
Because MTBE groundwater plumes
commonly travel farther than benzene
plumes, MTBE may be more likely than
the remainder of the petroleum release to
impact drinking water wells. As a
result, many states have been treating
contaminated groundwater at the point
of exposure and at the source area of the
plume. In New Jersey, regulators have
found that GAC is effective in treating
low-volume potable wells (e.g., for
single-family homes) with contam-
ination levels below 300 ppb. If high-
volume potable wells are involved (e.g.,
for restaurants, industrial sites) or if
concentrations exceed 300 ppb,
miniature air strippers may be a more
cost-effective option. Manufacturer
specifications should be consulted for
any treatment unit and followed up with
adequate levels of influent and effluent
monitoring.
Incremental Cost Increase Of
MTBE Groundwater
Remediation
The incremental cost increases for UST
corrective action activities that involve
MTBE versus ones that do not contain
MTBE vary widely depending on the
history of the release (e.g., how long the
release has been occurring, whether
MTBE was contained in the initial re-
lease, the concentration of MTBE) and
the goals of the cleanup. At many sites,
the initial concentrations may be low
enough that MTBE may not be a greater
concern than the remediation of ben-
zene, resulting in no cost increase. But,
when an MTBE plume is much larger
than the benzene plume and impacts
drinking water wells ahead of it, MTBE
will be the driving force in remediation
efforts, potentially resulting in a very
high incremental cost increase.
Based on limited research and anecdotal
information, the U.S. EPA's Office of
Underground Storage Tanks estimates
that at approximately 75 percent of
MTBE-contaminated sites, the
incremental cost increase of remedi-
ation will be less than 50 percent above
the cost of remediating the same petro-
leum release without MTBE. At many
of these sites, costs would actually not
increase because benzene might still
pose the greatest risk, thus driving the
remediation effort. At 20 percent of the
sites, the incremental cost increase
would be between 50 percent and 100
percent. At the remainder (approx-
imately 5 percent) of the sites, the
additional cost of remediating MTBE
MTBE Fact Sheet #2: Remediation
January 1998
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contamination may be an unknown
quantity that is greater than lO&percent
more. This situation results when
benzene has attenuated and poses no
further risk, but significant concentra-
tions of MTBE continue to migrate
down-gradient and contaminate drinking
water supplies. A graph of this distri-
bution is presented in Exhibit 1.
Conclusion
Remediation of MTBE-contaminated
soil generally does not pose an addi-
tional concern when a petroleum release
has occurred because MTBE can often
be removed from soil without additional
time or expense. But remediating
MTBE-contaminated groundwater can
be problematic. MTBE's high solubility
in water, low rate of adsorption to soil,
and low rate of biodegradation can make
treating groundwater contaminated with
MTBE more expensive than treating
groundwater contaminated with
petroleum that does not contain MTBE.
i Fortunately, there are proven treatment
technologies available. Pump-and-treat
is usually the most cost effective
.' method, but in some cases air sparging
may be appropriate. Other existing
technologies may also prove effective as
more case studies are reported. The
' potential for in situ biodegradation of
MTBE is widely believed to be low, but
new research may clarify our under-
standing of conditions that may make it
an effective option. In addition to reme-
diation of the source area, point-of-use
treatment appears to be a common
i approach to addressing MTBE when
contamination is limited to individual
homes or private wells.
Exhixt 1. Preliminary Estimate Of The Incremental Cost Increase
Of MTBE Remediation in Groundwater At LUST Sites
Number
of
Sites
75%
of
sites
than ,
20% of '
sites
Less than 5% of sites
50% 100%
Percentage of Incremental Cost Increase
January 1998
MTBE Fact Sheet #2: Remediation
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