mtbe: groundwater remediation technologies

IIIIIII MTBE: Groundwater Remediation Technologies

Jeffrey A. Hassen Cheyne l? Gross

Jeffrey A. Hassen is a project director with Environmental Strategies Corpora- tion in Pittsburgh, Pennsylvania. Cheyne P. Gross is a senior engineer with Environmental Strategies Corpora- tion in Pittsburgh, Pennsylvania.

The groundwater remediation of methyl-tertiaiy-butyl ether (MTBE) presents significant challenges due to the physicocheniical properties of MTBE. The high solubility in water, low Henry's law constant, and low affinity for organic fractions in the vadose and saturated zones increase the difficulty o f groundwater remediation. This column examines the effectiveness and cost of various conventional groundwater treatment technologies to remediate MTBE-affected groundwater and discusses new developments in MTBE groundwater remediation. Where appropriate, the properties of MTBE are coni- pared to other volatile organic coni- pounds (VOCs) t o give the reader an appreciation for why the recent use of MTBE has resulted in the rapid and widespread occurrence of MTBE in groundwater.

OVERVIEW OF MTBE MTBE is a gasoljne additive used

to enhance octane levels in gasoline and as an oxygenate to decrease vehicular emissions. MTBE has been added to gasoline by petroleum com- panies to raise the octane level and improve combustion since 1979 when lead was phased o u t of gasoline. In response to the 1990 federal Clean Air Act Amendments (CAAA), oil coni- panies began using NITBE extensively

in 1992 in oxygenated gasoline a i d reformulated gasoline (RFG). Oxy- genated gasoline and RFG are two classes of gasoline that contain different amounts of oxygen as discussed below. The CAAA requires the use of RFG in areas of the United States that exceed the national air quality stan- dards for carbon monoxide (carbon monoside nonattainment areas), VOCs, and ozone emissions. Seven- teen states and the District of Colum- bia currently use RFG, either because of a congressional mandate or on a voluntary basis. Oxygenated gasoline contzdins a minimum of 2.7 percent oxygen by weight or 14.8 percent MTBE by volume. RFG contains a minimum of 2.0 percent oxygen by weight or 11 percent aromatic hydrocarbon by volume (Delzeret al., 1996). MTBE is present in over 70 percent of the gasoline sold in the United States (Happel et al., 1998).

Because of MTBE's low cost, ease of production, and favorable blending characteristics, MTBE is the most commonly used fuel oxygenate. MTBE is used in approximately 87 percent of RFG. The second most used fuel oxygenate is ethanol. Other fuel oxygenates in use include methanol, ethyl tert-butyl ether, tert-amyl methyl ether, and diisopropyl ether (Zogorski et al., 1997). MTBE is made from methanol, a by-product of the

0 2000 John Wiley & Sons, Inc. 129

JEFFREY A. HASSEN CHEYNE P. GROSS

petroleum refining process. It blends easily with gasoline and remains in solution; thus, the MTBE gasoline blend can be transferred throughout existing pipelines without risk of degradation in gasoline quality. The U.S. production of MTBE exceeds 4.5 bil- lion gallons annually for use in gasoline. This usage represents a more than threefold increase in MTBE production since the congressional mandate in 1990.

The U.S. Environmental Protec- tion Agency (USEPA) has indicated that levels of airborne toxins have been reduced by 32 percent in the northeast United States since the federal government required the use of RFG. However, its use has also resulted in unacceptable impacts to surface water and groundwater in several areas of the United States. In addition, the USEPA has tentatively classified MTBE as a possible human carcinogen, but no federal drinking water standard has been established (USEPA, 1997). The USEPA has issued a drinking water Health Advi- sory for MTBE of 20 to 40 pg/L (micrograms per liter). The advisory provides guidance to communities exposed to drinking water contaminated with MTBE. The Health Advi- sory is based on taste and odor thresholds. The advisory concentration provides a large margin of safety for noncarcinogenic effects and is in the range of margins typically provided for potential carcinogenic effects. USEPA has also placed MTBE on the Drinking Water Contaminant Candidate List (CCL). Contaminants on this list are prioritized for further evaluation within the USEPA’s drinking water program. USEPA has ranked MTBE as a chemical that needs additional occurrence, treatment. and health data.

Several states have begun to take steps to reduce or phase out MTBE use. For example, Governor Gray Davis of California signed an Execu- tive Order in March 1999 calling for MTBE use to be phased out in Califor- nia by 2002. In addition, the USEPA commissioned a blue ribbon panel of industry experts to review public health issues associated with MTBE contamination of water supplies. The panel issued a report in September 1999 that recommended significant reduction in the use of MTBE, called on Congress and USEPA to lift the oxygenate mandate, and suggested strengthening existing regulatory pro- grams to reduce MTBE contaniina- tion of drinking water supplies across the United States.

In a March 20,2000 press release, the USEPA and the Department of Agriculture (DOA) announced ac- tions by the Clinton administration to significantly reduce or eliminate the use of MTBE and support the use of more environmentally safe alternatives such as ethanol. The USEPA and DOA released a legislative frame- work to encourage congressional action to amend the Clean Air Act, reduce MTBE use, and support a renewable fuel standard (particularly ethanol). USEPA also announced that regulatory action to eliminate or phase out MTBE has been initiated by issu- ing an Advanced Notice of Proposed Rulemaking under Section 6 of the Toxic Substance Control Act.

FATE AND TRANSPORT OF MTBE IN THE ENVIRONMENT

MTBE contamination has been reported at varying levels in air, surface water, and groundwater throughout the United States. This contamination results from point and nonpoint sources. Examples of point sources

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130 REMEDIATION/SUMMER 2 0 0 0

MTBE: GROUNDWATER &MEDIATION TECHNOLOGIES

include gasoline spills, pipeline rup- tures, and leaking aboveground and underground storage tanks. Nonpoint sources may include urban runoff, MTBE-affected precipitation, exhaust from 2-cycle boat engines, and dif- fuse groundwater discharges to surface water.

At ambient temperatures, the solubility of pure MTBE in water is about 50,000 mg/L (approximately 5 percent by weight), which is relatively high for most petroleum contaminants. For example. benzene has a solubility of 1,780 mg/L (Mackay et al., 1773). However, the solubility of MTBE in water is reduced when it is present with other organic compounds in gasoline. MTBE in a gasoline mixture that is 10 percent by weight reduces its water solubility to 5,000 mg/L at 25 "C (Squillace et al., 1777). This reduced solubility in water is caused by MTBE partitioning between the organic mixture in gasoline and water.

MTBE tends to partition weakly to the organic fraction in soils, sedi- ments, and suspended particles pre- ferring to remain in the aqueous phase. The high solubility of MTBE in water and low affinity for organic carbon in soil has contributed to MTBE migrating from a source area to groundwater at practically the same velocity as precipitation recharges a water table aquifer. The retardation factor for MTBE is close to 1 or basically equivalent to water. Thus, MTBE migrates in an aquifer at practically the same rate as the local groundwater flow velocity. In comparison, benzene, toluene, ethylbenzene, and xylenes (BTEX) compounds have retardation factors of 1.1 to 2 (Zogorski et al., 17771, which is generally why the MTBE conipo- nent of a groundwater plume en-

compasses a much larger area than BTEX compounds.

MTBE is only moderately volatile when moving from the dissolved to the vapor phase. Henry's law constant for MTBE is 0.022 at 25 "C (Robbins et al., 1773); a compound with a value of 0.05 or larger is considered highly volatile from water. In comparison, benzene has a Heniy's law constant of 0.22 at 25 "C (Howard et al., 1770), which indicates it is ten times more volatile from the dissolved phase to the vapor phase than MTBE. This explains why MTBE is more difficult than benzene to remove from groundwater using conventional air stripping o r air sparging remediation technologies.

MTBE's vapor pressure is approximately three times higher than benzene, but MTBE's Henry's law constant is significantly lower than benzene as discussed above. Therefore, in product form, MTBE is more volatile than benzene, but when dissolved in water, MTBE is much less volatile. Due to these physical properties, MTBE is more difficult to ad- dress when dissolved in groundwater than it is when trapped in the vadose zone or capillary fringe in LNAPL form. Unfortunately, MTBE has a low affinity for adsorption, therefore it migrates through the vadose zone more rapidly than BTEX. Because of MTBE's volatile nature, soil vapor extraction is an excellent technology for addressing vadose zone contamination. However, this column fo- cuses on the remediation of groundwater plumes containing both MTBE and BTEX.

MTBE does not readily biodegrade in surface or groundwater. It is persistent in the environment because the ether bond is stable under typical environmental conditions and

&MEDIATION/SUMMER 2000 131

JEFFREY A. HASSEN CHEYNE P. GROSS ~~ _________ ______ ~ ~ ~

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132 REMEDIATION/~UMMER 2000

requires acidic conditions to break down the molecular structure. In addition, the bulky nature of the twt- butyl group does not permit easy access to the ether linkage. There- fore, indigenous microbes do not tend to rapidly biodegrade MTBE. Tertiary butyl alcohol (TBA) has been documented as a transformation product of MTBE in a number of studies (Keller et al., 1998).

GROUNDWATER COLLECTION TECHNOLOGIES

Two groundwater collection technologies are discussed in this section-pump and treat and dual vacuum extraction (DVE)-both of which require ex-situ treatment of the extracted groundwater, which is discussed later in the column. Pump and treat and DVE physically remove the contaminants from the subsurface, and therefore require subse- quent treatment.

Pump and Treat Although pump and treat is not

generally considered to be an effective groundwater remediation tech- nique, it is considered effective for MTBE remediation because of MTBE’s high solubility and low adsorption to organics in soil. Fewer aquifer volumes are needed to remove MTBE mass than are required for the removal of BTEX compounds. In addition, because of MTBE’s high solubility and affinity for water, a high percentage of MTBE mass may quickly dissolve in groundwater and be re- covered by pump and treat systems.

Groundwater pumping can be performed from recovery wells or interceptor trenches depending on the hydrogeologic characteristics of the site. If the remediation goal is to prevent MTBE migration, recovery

wells will be distributed across a site based on a calculated radius of influ- ence around each recovery well. If an interceptor trench is used, it will be designed to extend completely across the downgradient edge of the plume, perpendicular to groundwater flow. Pumping rates will be set to the mininiuni value necessary to achieve hydraulic containment.

If concentration reduction or mass removal is the remediation goal, re- coveiy wells or sumps can be located at strategic locations based on contaminant source location. Pumping rates can be set to optimize the pounds of mass removed per dollar, includ- ing both capital and operation and maintenance costs. The rate of extraction will be related to the rate of diffusion of contaminant mass from the adsorbed to dissolved state. The extraction rate will also be controlled by the hydraulic conductivity of the aquifer material. An aquifer with a high total porosity, but low effective porosity will result in an increased MTBE recovery time.

Dual Vacuum Extraction DVE, also referred to as dual

phase extraction, is a combination of groundwater pump and treat and soil vapor extraction. The process involves the extraction of air and water from a network of recovery wells. Water can be extracted with a direct vacuum lift, air-entrainment using drop tubes, or submersible pump. The air and water streams are conveyed to a treatment facility and separated. The water and air streams can be treated using the technologies discussed in the Ex-Situ Groundwater Treatment Technolo- gies section of this column.

During DVE, the groundwater table is lowered which exposes the capillary fringe or smear zone to

MTBE: GROUNDWATER REMEDIATION TECHNOLOGIES

remediation by soil vapor extraction. The permeability of the aquifer material and the physiochemical properties of the compound of concern determine the effectiveness of DVE. MTBE in the product form will be volatilized and removed from the vadose zone with a high efficiency based on its high vapor pressure, but unfortunately the majority of contaminants present in the vadose zone or capillary fringe will likely be BTEX compounds due to the higher sorption affinity and lower solubility than MTBE. Other inherent difficulties with DVE of MTBE are similar to pump and treat in that the ex-situ water treatment is more costly than for BTEX compounds. Also, DVE increases the oxygen levels in the vadose zone, which is necessary for the biodegradation of BTEX compounds. Biodegradation of MTBE, however, does not readily occur aerobically as discussed later in this column. None- theless, DVE is an appropriate remediation technology for MTBE. The cost of reinediating a MTBE and BTEX plume will he based on the cost to perform the ex-situ treatment of groundwater and off gas, if required. Other costs associated with installing and operating a DVE system will be the same for non-MTBE plumes. These costs are based on the physical characteristics (i.e., permeability) of the formation and the size of the contaminated plume.

EX-SITU GROUNDWATER TREATMENT TECHNOLOGIES

Ex-situ treatment alternatives that are discussed in this section include air stripping, liquid-phase adsorption, hollow fiber membranes, and advanced oxidative processes. Addi- tionally, air sparging, DVE, and pump and treat may require treatment of off

gas. Off gas treatment technologies include vapor-phase granular activated carbon (GAC) adsorption, catalytic oxidation, thermal oxidation, and biofiltration. The off gas treatment technology discussed is vapor-phase carbon adsorption.

Air stripping Air stripping can be used to treat

MTBE-impacted groundwater. How- ever, high air to water ratios and increased residence time are required due to the low Henry’s law constant. Air strippers designed to treat MTBE require an airflow to waterflow ratio two to five times greater than similar BTEX treatment systems (USEPA, 1998). Therefore, air stripping units for MTBE treatment are more expen- sive to purchase and have higher operating costs than systems designed for the remediation of BTEX compounds. Increasing the temperature will increase the air stripping efficiency for MTBE due to its relatively low boiling point.

Cost comparisons performed by Keller et al. (1998) show that treating 100 gallons per minute of MTBE- contaminated water with an influent concentration of 100 pgA to an effluent concentration of 35 pgA is coin- parable to treating benzene-contaminated water with an influent concentration of 100 pg/l to an effluent concentration of 1 pg/l, when vapor treatment is not required.

Vapor-Phase Adsorption Depending on the mass of MTBE

discharged to the atmosphere and local air quality regulations, off gas treatment may be required. There is little published data for the adsorption efficiency of MTBE onto GAC in the vapor phase, but due to its affinity for water, MTBE may breakthrough

REMEDIATION/~UMMER 2000 133


134 &MEDIATION/!~UMMER 2000

GAC beds quickly in the presence of BTEX compounds (Keller et al., 1998). The treatment of off gas containing MTBE-laden air from an air stripper operating at 25 gallons per minute and reducing groundwater concentrations from 20 mg/L to 10 pg/l would cost approximately $7.00 per 1,000 gallons (USEPA, 1998).

Liquid-Phase Adsorption Liquid-phase adsorption is a treat-

ment technology that uses a media to adsorb dissolved VOCs from water. Adsorption is effective in removing organic contaminants with a high liquid to solid partitioning constant. Adsorption media is typically GAC. GAC can be produced from coconut shells, coals, or peat. Because the solubility of MTBE is high, the adsorption efficiency of MTBE for granular activated carbon is low. Further- more the presence of other VOCs and natural organic matter competes for adsorption sites and can displace MTBE. GAC is approximately one third to one eighth as effective in removing MTBE as it is for benzene. Therefore costs associated with GAC adsorption of MTBE-contaminated water will be three to eight times higher than benzene. Other adsorption media have shown a higher efficiency for the adsorption of MTBE. High-silica zeolites have shown approximately 96 percent efficiency for liquid-phase adsorption of MTBE. However, the documenta- tion of field-scale applications is limited for MTBE remediation.

Hollow Fiber Membranes Hollow fiber membranes (HFM)

can be used to strip MTBE from groundwater. However, few field- scale tests have been performed using the HFM technology. The HFM

process involves passing contaminated water through a microporous fiber while applying a vacuum coun- tercurrently. As water is pumped through hollow fibers, MTBE vola- tilizes and diffuses within gas-filled pores due to the large concentration gradient.

HFM can be performed using a sweep gas to draw MTBE from the gas-filled pores. Studies have shown that mass transfer of VOCs could be ten times higher than packed tower air stripping when using HFM with a sweep gas. Other advantages of HFM over air stripping are lower airflow requirements, which translates into smaller blowers and vapor-phase treatment units. Air and water flow rates can be independently controlled to maximize mass transfer. Also, there is very little water carryover due to the hydrophobic nature of the fibers, which makes vapor-phase treatment more efficient.

One of the drawbacks of this technology is that buildup of pre- cipitated metals within the filter pore spaces causes reduced efficiency within the treatment system. The buildup can be washed with acid to open the flow pathways. As is the case with air stripping, HFM transfers the mass from the dissolved phase to the vapor phase. Depend- ing on the local air regulations and the total emissions, off gas treatment may be required.

Advanced Oxidation Processes Advanced oxidation processes

(AOP) using ozone, hydrogen peroxide, ultraviolet (W), or combinations thereof have been successful in removing VOCs from water. The ozonation process creates a hydroxyl radical (OH) that reacts with organic compounds. The hydroxyl radical can


fully oxidize MTBE to carbon dioxide and water (Keller, 1998). This process, however, is affected by the groundwater chemistry of the influent water. Typical compounds that interfere with the oxidation process are carbonates and bicarbonates. Other organics will also reduce the efficiency of the MTBE oxidation. The generation of by-product compounds is also a concern. Studies have shown that ozonating MTBE can create tert-butyl formate (TBF) or tert-butyl acetate (TBA). Both TBF and TBA are more toxic than MTBE; therefore , thorough stoiciometric in- vestigations are required before imple- menting ozonation.

Cost Comparison of Ex-Situ Treatment Technologies

A comparative cost analysis of air stripping, GAC, AOP, and HFM is provided in Exhibit 1. This analysis indicates that HFM is the most cost- effective technology, for flow rates of 10 to 100 gpm, when no air treatment is required. However, air stripping is the most cost-effective technology for high flow rates (i.e., greater than 100 gpm) if no air treatment is required. GAC appears to be the most cost-effective at all flow rates if air treatment is required and the influent concentration of other organic compounds is low. If the influent has high concentrations of other organic compounds, and air treatment is required, air stripping is the more cost-effective for flow rates greater than 100 gpm.

IN-SITU GROUNDWATER TREAT- MENT TECHNOLOGIES

The groundwater treatment technologies discussed in this section are air sparging, chemical oxidation, natural attenuation, and enhanced bioremediation. Of the four

in-situ treatment technologies, in- situ chemical oxidation, enhanced bioremediation, and natural attenuation involve chemical breakdown of the contaminants in the subsurface, and therefore do not require secondary treatment alternatives. Air sparging; however, may require off gas collection and treatment, because air sparging transfers the contaminant mass from the dissolved phase to the vapor phase. Vapor treatment technologies may include vapor-phase GAC adsorption, thermal oxidation, catalytic oxidation, or biofiltration. Vapor-phase GAC adsorption was previously discussed in this column.

Air Sparging Air sparging is the process of in-

jecting air beneath the groundwater surface to strip VOCs from groundwater or aid in aerobic biodegradation. The aquifer properties and the physiocheniical properties of the contaminant of concern determine the effectiveness of air sparging. Air sparging is only applicable to homog- enous aquifer material either natural or engineered through the construc- tion of an air sparging trench. High or low permeability lenses will inhibit the effectiveness and may result in the spreading of contaminants. Also, as aquifer permeability increases, efficiency increases. If the aquifer is amenable to air sparging, the contaminant’s Henry’s law constant will determine the efficiency of air sparging. As discussed earlier, MTBE has a relatively low Henry’s law constant. Some contaminants with low Henry’s law constants, such as ketones, are amenable to air sparging due to aerobic biodegradation. However, MTBE does not biodegrade rapidly under aerobic conditions as discussed earlier.

REMEDIATION/SUMMER 2000 135

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A few studies have shown that reductions in MTBE levels from above 1,000 ppb to less than 10 ppb are possible in less than two years. How- ever, remediation of MTBE hy air sparging will he more costly than BTEX compounds, due to the higher airflow rates that are required. Costs to install and operate an air sparging system are based on the physical characteristics (i.z., permeability) of the aquifer material and the size of the contaminant plume.

In-Situ Chemical Oxidation In-situ chemical oxidation is the

process of injecting an oxidizer into an aquifer to chemically oxidize VOCs. Oxidizers used for this process include hydrogen peroxide, ozone, and potassium permanganate. Factors that determine the efficiency of this reme- dial option include the ability to obtain adequate mixing of the oxidizer within the contaniinated groundwater plume. The potential to obtain adequate mixing is affected by the permeability of the aquifer material. As is the case with AOP, incomplete oxidation of MTBE may result in the generation of TBA o r TBF.

Natural Attenuation Studies have shown that ben-

zene, toluene, ethylbenzene, and xylenes (BTEX) groundwater plumes are stabilized and remediated as a result of the natural attenuation of these hydrocarbon compounds. As a result, natural attenuation is now recognized as an acceptable compo- nent of risk-based remedies at petroleum release sites. However, early studies on the natural attenuation of MTBE indicated that the compound was nonbiodegradable or veiy recalcitrant with respect to natural attenuation in groundwater by indig-

enous niicroorganisins. These studies have reported little or no MTRE biodegradation under a variety of aerobic and anaerobic ( denitrifying, sulfate-reducing, iron-reducing, methanogenic) conditions.

Later studies have shown that MTBE is very slowly degraded under aerobic and anaerobic conditions. Yell M Novak (1974) showed in laboratory studies that MTBE degradation was found to be limited to environ- ments with low organic content in soils with a pH of approximately 5.5. MTBE degradation did not occur in more organic rich soils likely due to the presence of other organic material for mici-obiznl activity. Yeh and Novak in the same studies concluded that the natural attenuation of MTBE occurred under methanogenic conditions. Morinile et al. (1974) also concluded in 1abOldtOiy studies that MTBE biodegradation occurred under methanogenic conditions. Both studies found no biodegrddatioll of MTBE under sulfate-reducing or n - trate-reducing conditions. The USEPA (2000) evaluated the natural attenuation of MTBE under methanogenic conditions in the laboratoiy and calculated the rate of attenuation o f MTBE under similar conditions within a known source area impacted by a gasoline spill containing BTEX and MTBE. The laboratory study concluded that MTRE concentrations were reduced from 3,110 pg/L to less than 40 pg/L after 490 days of exposure to the substrate. The biodegradation was minimal until the reinoval of BTEX was complete. The source area with an average groundwater concentration of 1,200 pg/L, MTBE source of 46 kg, and flux away from the source of 2.76 kg/year, was estimated to take approximately 60 years to biodegrade to 30 pg/L More research is needed

REMEDIATION/~UMMER 2000 137


to determine the optimum conditions for the natural attenuation of MTBE.

Enhanced Bioremediation Despite the resistance of MTBE to

indigenous microbes, studies have demonstrated that bacterial popula- tions and some pure bacterial strains, when isolated from biotreated slud- ges and other sources, have the ability to biodegrade MTBE. Recent research by the University of California, Davis, has shown promise with the use of laboratory-cultured naturally occurring microorganisms, which have biodegraded MTBE under aerobic conditions within laboratory studies. Field-scale testing of this micro- organism is planned for spring 2000.

Research funded by the Na- tional Science Foundation is under way to measure the kinetics of MTBE degradation by a recently isolated MTBE degrading bacterium (ENV 735) under different micro- biological and geochemical conditions in aquifer microcosms. The data from the study will be used to evaluate the feasibility of using ENV 735 and other similar microorganisms for in-situ bioremediation in contaminated aquifers.

Bioreactors have also shown some encouraging results. Research and development is being conducted on biofilm reactors to evaluate reliable and cost-effective designs (Converse & Schroeder, 1998).

CONCLUSIONS MTBE has proven to be an effec-

tive gasoline additive to aid in the reduction of vehicular carbon monoxide and VOC emissions. Due to the low cost of production and blending characteristics, MTBE has quickly risen to be the most popular fuel oxygenate used in gasoline. However, once

MTBE is released into the environment, its high solubility in water, low rate of adsorption to organic material in soil, and recalcitrant characteristics, make treating groundwater impacted by MTBE more problematic than the remediation of other petroleum hydrocarbon compounds.

Pump and treat is one the more effective technologies to control the continued migration of MTBE. A limited number of aquifer volumes is needed to remove MTBE because of its affinity for groundwater. Once extracted from the Subsurface, several technologies can be used to remove MTBE from groundwater. The most cost-effective technologies include air stripping and GAC. HFM is another technology that has proven effective in laboratory tests, but has seen limited field-scale application.

The potential for in-situ biodegradation of MTBE by natural attenuation was originally believed to be ineffec- tive; however, recent research has shown limited degradation under aerobic and anaerobic conditions. More research is needed to determine the specific environmental conditions necessary to promote natural attenuation and to predict the rate of degradation. Recent laboratory studies have reported success using laboratory-cultured naturally occurring microorganisms. Field-scale studies of cultured microorganisms are currently under way. Lastly, the successful application of ex-situ bioremediation technologies such as biofilters and bioreactors have been reported during pilot- and full-scale limited application.

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138 REMEDIATION/SUMMER 2000


California, Davis, Department of Civil and Environmental Engineering.

Delzer, G.C., Zogorski, J.S., Lopes, T.J., & Bosshart, R.L. (1996). Occurrence of the gasoline oxygenate MTBE and BTEX compounds in urban stormwater in the United States, 1991-1995 (Water Resources Investigation Report 96-4145, 8 p.). United States Geologi- cal Survey.

Happel, A.M., Beckenbach, E.H., 81 Halden, R.U. (1998). An evaluation of MTRE impacts to California groundwater resources (UCRL- AR-130897, 68 p.). Lawrence Livermore Na- tional Laboratory, University of California.

Howard, P.H., Sage, G.W.. Jarvis, W.F., & Gray, D.A. 1990. Handbook of environmental fate and exposure data for organic chemicals (Vol. IV). Chelsea, MI: Lewis Publishers, Inc., 578 p.

Keller, A,, et al. (1998). Health 81 environmental assessment of MTBE (Vols. I-V) (A Report to the Governor and Legislature of the State of California). University of California, Davis.

Mackay, D., Shiu, G. W.Y., 81 Ma, K.C. (1993). Illustrated handbook of physical-chemical properties and environmental fate for organic chemicals-volatile organic chemicals (Vol. 111). Chelsea, MI: Lewis Publishers Inc., 916 p.

McCarthy, J.E., & Tieinann, M. (1998). MTBE in gasoline: Clean air and drinking water issues. Congressional Research Service, Envi- ronment and Natural Resources P o k y Divi- sion, 9 p.

Mormile, M.R., Liu, S., & Suflita, J.M. (1994). Anaerobic biodegradation of gasoline oxygenates: Extrapolation of information to mul- tiple sites and redox conditions. Environmen-

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