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).
      January 1998
     1    MTBE Fact Sheet #2: Remediation

-------
                             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

-------
                       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
January 1998
    MTBE Fact Sheet #2: Remediation

-------
                     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

-------
                      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

-------

-------