United States
Environmental Protection
Agency
Office Of Solid Waste And
Emergency Response
(5401G)
EPA510-F-97-015
January 1998
www.epa.gov/OUST/mtbe/
xvEPA
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, pri-marily in mid-
and high-grade gasoline at
concentrations 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
oxygenate 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 consti-tuents, the
choice of an effective reme-diation
technology may be different when
MTBE is present at a site. Ben-zene is
most often the contaminant of concern in
gasoline because of its rela-tively high
solubility and its known carcinogenicity.
As a result, compar-ing 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, reach-ing
the groundwater first and dissol-ving
rapidly. Once in the ground-water,
MTBE travels at about the same rate as
the groundwater whereas ben-zene 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 va-por
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
subsurface. 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 re-moving
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 infiltra-tion
of rain water or rising ground-water
levels, may necessitate frequent
groundwater sampling to determine
actual MTBE concentrations and lev-els
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
In 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
MTBE Fact Sheet #2: Remediation
January 1998
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ground (i.e., pump-and-treat) may more
often be an effective reme-diation
technology for MTBE because MTBE
does not adsorb significantly to soil. As
a result, fewer aquifer vol-umes are
required to remove all of the MTBE than
are required to remove the slowly
desorbing petroleum hydro-carbons. 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 ground-water
flow, transfer of a contaminant from the
micropores to the macropores will occur
through the slow process of diffusion.
Hence, in spite of some fa-vorable
characteristics, pump-and-treat may not
always be an efficient remedi-ation
method for MTBE contamina-tion.
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 significant-ly
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 equip-ment
(e.g., pumps, lines) may be re-quired
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 sys-tem
could achieve this level of remedi-ation
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 hydro-gen 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 homo-geneous sands because
heterogeneous sediments may cause
dispersion of contaminants and
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3 MTBE Fact Sheet #2: Remediation
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channeling of air flow. In addition, air
sparging should be less effective for
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
signifi-cantly 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 ef-fective.
New research is also showing that in
situ biodegradation may be an effective
remediation alternative; how-ever, more
information is required to determine the
specific environmental conditions that
enable significant rates of
biodegradation 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 ef-fective in treating
low-volume potable wells (e.g., for
single-family homes) with
contamination 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
release, the concentration of MTBE) and
the goals of the cleanup. At many sites,
the initial concentra-tions may be low
enough that MTBE may not be a greater
concern than the remediation of
benzene, 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 anec-dotal
information, the U.S. EPA's Office of
Underground Storage Tanks estimates
that at approximately 75 per-cent of
MTBE-contaminated sites, the
incremental cost increase of remedi-
ation will be less than 50 percent above
the cost of remediating the same
petroleum release without MTBE. At
many of these sites, costs would actually
not increase because ben-
zene 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
MTBE Fact Sheet #2: Remediation
January 1998
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percent and 100 percent. At the
remainder (approximately 5 percent) of
the sites, the additional cost of
remediating MTBE contamination may
be an unknown quantity that is greater
than 100-percent more. This situation
results when benzene has attenuated and
poses no further risk, but significant
concentrations of MTBE continue to
migrate down-gradient and contaminate
drinking water supplies. A graph of this
distribution 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 with-
out 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 ground-
water contaminated with petroleum that
does not contain MTBE. Fortu-nately,
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 understanding of
conditions that may make it an effective
option. In addi-tion to remediation of
the source area, point-of-use treatment
appears to be a common approach to
addressing MTBE when contamination
is limited to individual homes or private
wells.
Exhibit 1.
Preliminary Estimate Of The Incremental Cost Increase
of MTBE Remediation in Groundwater At LUST Sites
Number
of
Sites
75%
of
sites
\
\
X^
GreaterV
than N.
20% of ! \
sites i
Less than 5% of sites
s^
50% 100%
Percentage of Incremental Cost Increase
January 1998
5 MTBE Fact Sheet #2: Remediation
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