Title: Introduction Text: Methyl tertiary-butyl ether (MTBE) is an oxygenate that is added to gasoline in order to increase octane levels and reduce carbon monoxide and other pollutant levels in automobile emissions. Although it provides air quality benefits, MTBE releases to soil and groundwater through leaking petroleum underground storage tanks (USTs) and/or dispensing pipeline to pumps have led to adverse environmental impacts. MTBE is highly soluble and mobile in groundwater and it is more resistant to biodegradation than many petroleum hydrocarbons. Because of its unique chemical and physical properties, petroleum releases with MTBE can be more difficult and costly to remediate than those without MTBE. This Web tool provides an overview of MTBE, its chemical and physical properties, its fate and transport in the environment, and successful MTBE remediation technologies. An MTBE Treatment Technology Decision Tool is provided at the end of the tool to help you screen potential cleanup technologies for your site.

Title: Text: Oxygenates are hydrocarbons that contain one or more oxygen atoms. The primary oxygenates are alcohols and ethers. Examples of ether oxygenates include MTBE, tertiary-amyl methyl ether (TAME), tertiary-amyl ethyl ether (TAEE), ethyl tertiary-butyl ether (ETBE), and diisopropyl ether (DIPE). Examples of alcohol oxygenates include ethanol, methanol, and tertiary-butyl alcohol (TBA).

Title: Production and Use (1 of 2) Text: MTBE is a synthetic compound produced by combining methanol and isobutylene. Methanol is typically derived from natural gas and isobutylene is a byproduct of the petroleum refining process. MTBE has been used in the United States since 1979 as an octane-enhancing replacement for lead in mid-grade and high-grade gasoline. MTBE use in the United States rapidly increased in the early 1990s as a result of the Clean Air Act amend-ments for the Oxygenated Fuel (Oxyfuel) and Reformulated Gasoline (RFG) Programs. As a result of these programs, MTBE became a common oxygenate in gasoline. MTBE was added to gasoline at 15% by volume for the Oxyfuel program and at an average of 11% by volume (range 8% to 15% by volume) for the RFG program. The petroleum industry tended to favor the use of MTBE because it is relatively inexpensive to produce and very compatible with the mixture of hydrocarbons in gasoline.

Title: Oxyfuel Program Text: The Oxygenated Fuel (Oxyfuel) Program requires 2.7% oxygen (by weight) in gasoline during winter months to reduce carbon monoxide emissions.

Title: RFG Text: As part of the Clean Air Act amended in 1990, the U.S. EPA began to issue regulations that required gasoline to be "reformulated" in order to reduce emissions of ozone-forming compounds and toxic air pollutants. The Reformulated Gasoline (RFG) program required implementation in the most severe ozone impacted areas of the country. Click here for a map showing regions of the country designated as Federal Reformulated Gasoline Areas. Other areas of the country with ozone problems have voluntarily opted into the program.

Title: Production and Use (2 of 2) Text: MTBE demand and production increased from 83,000 to 161,000 barrels per day from 1990 to 1994 as a result of the Oxyfuel program, and then jumped to 269,000 barrels per day by 1997 as a result of the RFG program. In 1998, in recognition of the unanticipated environmental impacts of MTBE, the U.S. Environmental Protection Agency (EPA) appointed a Blue Ribbon Panel to investigate air quality benefits, water quality concerns, fuel supply stability, and oxygenate costs. Based on its review of these issues, the panel recommended a reduction in the use of MTBE. Several states like California, Maine, New Hampshire, Arizona, Kansas, Missouri, New York, and South Dakota have moved to ban or phase-out the use of MTBE and its production has been steadily decreasing. From 2001 to 2002, MTBE production fell nearly 2% to 75.7 million barrels per year. However, ethanol production increased by approximately 10% in 2001 as petroleum refiners began using ethanol as an alternative oxygenate to MTBE.

Title: Other Oxygenates Text: The use of MTBE in gasoline is declining because of concerns about its impact on groundwater quality. Other types of oxygenates include ethyl tertiary-butyl ether (ETBE), tertiary-amyl methyl ether (TAME), and ethanol. Ethanol is now considered a viable alternative to MTBE. Ethanol is also known as grain alcohol and is made from corn, grains, and potatoes. Ethanol has been shown to be an effective fuel additive that provides high octane, while reducing air emissions. There are several environmental factors to consider in comparison to MTBE. Ethanol is highly biodegradable unlike MTBE, but its presence may retard the biodegradation of other compounds like benzene. It is also highly soluble and may enhance the solubility of other petroleum constituents through a cosolvency effect.

Title: Chemical Properties (1 of 2) Text: MTBE is a volatile, flammable, and colorless liquid at room temperature. Its boiling point and vapor pressure make it compatible with the mixture of hydrocarbons in gasoline. This compatibility allows it to be effectively mixed with gasoline and transported by pipeline or truck. MTBE has a relatively high water solubility (~50,000 mg/L). Also, due to its low Henry's Law constant, it tends to partition into the water phase, rather than air phase. MTBE also has a lower affinity for sorption onto soil than other gasoline constituents. These unique chemical and physical properties have major implications for fate and transport, site charac-terization techniques, and the selection of treatment technologies for MTBE impacted sites.

Title: Henry's Law Constant Text: The Henry's law constant of a chemical determines the tendency of that chemical to transfer from water to air or vice versa. It is related to both the vapor pressure and water solubility of the pure chemical compound. The larger the Henry's law constant, the more volatile the compound and the more readily it moves from the water to air phase.

Title: Chemical Properties (2 of 2) Text: The hydrophobic aliphatic (C-H) portion of the MTBE molecule makes it soluble in gasoline and suitable as a fuel additive. However, MTBE and other fuel oxygenates are far more water soluble than petroleum hydrocarbon constituents like benzene, toluene, ethylbenzene, and xylene (BTEX). For example, MTBE is 30 times more soluble in water than benzene. MTBE has a lone pair of electrons on its oxygen atom, which enables it to form a hydrogen bond with nearby water molecules. Because of this hydrogen bonding between water and ether molecules, MTBE is very soluble in water and any MTBE that is released into the environment will readily partition into groundwater. MTBE is very mobile and moves with a velocity close to that of groundwater because of its high solubility and low sorption onto soils. Consequently, MTBE plumes are often very large when compared to the BTEX portion of a plume produced by a gasoline release. MTBE plumes can range up to several miles in length.

Title: Drinking Water Regulations Text: In 1997, the U.S. EPA issued a Drinking Water Advisory that recommended concentration limits for MTBE in drinking water in the range of 20 to 40 µg/L to avert unpleasant taste and odor effects. These concentrations are about 20,000 to 100,000 times lower than the range of exposure levels in which cancer or non-cancer effects were observed in animal laboratory tests. Therefore, water source protection for taste and odor also protects consumers from potential health effects. The U.S. EPA later classified MTBE as a possible human carcinogen and placed it on a Contaminant Candidate List for further evaluation. As of 2004, a national drinking water standard for MTBE has not been issued, but several states have set their own regulatory levels for MTBE in drinking water.

Title: Toxicology Text: Non-Cancer Effects: The following acute and chronic non-cancer health effects of MTBE have been reported in animal studies at or near lethal doses: ocular and mucous membrane irritation, inability to coordinate muscle movements, labored breathing, central nervous system depression, and other effects. In humans, only irritant effects have been observed; no studies are available regarding reproductive or developmental toxicity of MTBE. Carcinogenicity: The carcinogenicity of MTBE has been observed in animals by oral and inhalation routes at or near lethal doses. However, epidemiological studies of carcinogenic effects of MTBE are not available and there is no direct evidence of carcinogenicity in humans. The available animal studies have been reviewed and MTBE carcinogenicity classifications are as follows: The National Toxicology Program Board of Scientific Counselors voted in 1998 not to include MTBE on the list of compounds known to be human carcinogens. The International Agency for Research on Cancer designated MTBE as Classification: Group 3, not classifiable due to limited or inadequate available data. The U.S. EPA designated MTBE as Classification: Group C, possible human carcinogen.

Title: U.S. EPA Classification Text: The U.S. EPA through its Integrated Risk Information System (IRIS) classifies exposures into one of five categories: Group A Human Carcinogen; Group B Probable Human Carcinogen; Group C Possible Human Carcinogen; Group D Not Classifiable as to Human Carcinogenicity; Group E Evidence of Non-Carcinogenicity for Humans. The EPA classifies MTBE into Group C, possible human carcinogen.

Title: Fate and Transport (1 of 2) Text: The fate and transport of MTBE in the environment is directly related to its unique chemical and physical properties. Below are some observations about MTBE fate and transport in the subsurface: Because of its high solubility, MTBE is more readily leached from the vadose zone to groundwater when compared to BTEX. Transport by groundwater is also not retarded significantly by sorption as MTBE sorbs only weakly to aquifer soils. There is only limited transfer of MTBE to soil vapor by groundwater volatilization because of its low Henry's constant. The natural attenuation rate of MTBE is relatively slow, so an MTBE plume may continue to migrate even after the associated BTEX plume has stabilized and started to shrink. The fate of MTBE in the groundwater appears to be controlled primarily by dispersion mechanisms. Under anaerobic groundwater conditions, methanogenic biological degradation can take place at a low rate. In certain parts of the country, indigenous aquifer microorganisms degrade MTBE aerobically.

Title: Fate and Transport (2 of 2) Text: An MTBE plume will typically extend far downgradient from the source of the gasoline spill and the associated BTEX plume. This occurs because the natural attenuation of MTBE is very low given its high solubility, limited sorption onto soils, resistance to biodegradation, and other factors. In addition, fuel hydrocarbons in groundwater often cause a reduction of dissolved oxygen levels and other electron acceptors within an aquifer. This may further contribute to the apparent recalcitrant nature of MTBE compared to other gasoline constituents. For natural attenuation evaluations of MTBE, site characterization efforts should include evaluation of nutrient availability, electron donor and acceptor availability, and the presence of metabolic byproducts. Groundwater quality parameters to be monitored include dissolved oxygen, pH, redox potential, nitrate, sulfate, iron(III), methane, and hydrogen. Natural gradient tracer tests can also be used to investigate fate and transport processes within a plume; however, these tests can be expensive and resource intensive and are not generally recommended for characterization of IR sites. Given chemical properties of the tracer and initial concentrations, these tests can be used to generate field-scale data such as the velocity of a plume, the rate of dispersion of contaminants, and the rate of biodegradation. Estimating field natural attenuation rates can be challenging and the use of natural gradient tracer tests can provide important information to enhance laboratory data and plume studies. Natural gradient tracer studies were carried out over a one year period to evaluate natural attenuation of MTBE in a groundwater plume at the Naval Base Ventura County in Port Hueneme, California. This site is one of two Department of Defense (DoD) National Environmental Technology Test Site (NETTS) locations.

Title: Natural Gradient Tracer Test at NETTS Port Hueneme Text: A natural gradient tracer test was conducted on an MTBE plume at NETTS Port Hueneme to determine the rate of natural attenuation within the aquifer. The groundwater plume contains two distinct regions with both BTEX and MTBE contamination near the source zone and only MTBE contamination extending approximately 5,000 ft downgradient. Three chemical tracers were used during the tests including 1) deuterated MTBE to track biodegradation, 2) bromide as a conservative tracer to examine plume velocity and dispersion, and 3) fluorescein dye to visually monitor the tracer injection process. After a one year evaluation period, the results of the tracer test indicated that the plume velocity was as high as 1 ft/day with other flow rates in a vertical column as low as 1/3 ft/day. However, based on a non-uniform distribution of deuterated MTBE concentrations within the aquifer, it appeared that portions of the plume may have traveled at higher velocities within preferential course grain aquifer material. The dominant natural attenuation mechanism for MTBE was determined to be dispersion at this site and biodegradation was found to be minimal due to anaerobic conditions at the site (Amerson and Johnson, 2003).

Title: NETTS Description Text: National Environmental Technology Test Sites (NETTS) are funded by the DoD Strategic Environmental Research and Development Program (SERDP). The purpose of the NETTS is to demonstrate reliable, cost-effective, and proven cleanup technologies able to meet DoD contaminant characterization and remediation goals. There are two NETTS locations: the Naval Base Ventura County, Port Hueneme, California and Dover Air Force Base. These sites provide a network of well-characterized test sites where technologies can be field-tested under known conditions against established standards. Innovative in situ and ex situ characterization and remediation technologies are being demonstrated on contaminated soil, sediments, soil vapors, and groundwater at all three sites. The NETTS Port Hueneme Site is one of the leading test and evaluation locations for fuel hydrocarbon (gasoline, diesel and/or heavy fuel oil) and MTBE site characterization and remediation.

Title: Biodegradation Text: MTBE and other ethers are not considered to be readily biodegradable, but biodegradation may be an effective remedial approach under specific conditions. Over the past decade, MTBE has successfully been biodegraded in laboratory and field-scale studies by pure cultures, mixed consortia, cometabolism, or just as a sole carbon or energy source. However, the process of MTBE biodegradation is still not well understood and is relatively slow. MTBE is degraded primarily through aerobic mechanisms, but there is evidence for cometabolic and anaerobic pathways. Potential intermediate products from MTBE biodegradation include tert-butyl formate (TBF) and tert-butyl alcohol (TBA). The presence of TBA alone does not necessarily indicate biodegradation because it is also used a fuel additive and is an impurity in MTBE.

Title: Aerobic Biodegradation Text: Many laboratory and field scale studies have accomplished the aerobic biodegradation of MTBE. Researchers have identified both pure cultures and native microbial consortia capable of mineralizing MTBE under aerobic conditions. In particular, the bacterial strain PM1 has been studied in great detail to determine the aerobic degradation pathway of MTBE. PM-1 grows on MTBE as sole carbon and energy source. It is a flagellated, Gram-negative rod type bacteria. It grows relatively slowly with a doubling time of 1.5 days on MTBE. PM1 was used to inoculate a test plot at the National Environmental Technology Test Site (NETTS) Port Hueneme to investigate the feasibility of using a pure bacterial culture to degrade MTBE in groundwater. The PM1 strain was isolated by University of California at Davis (UCD) from a wastewater biofilter at the Los Angeles County Sanitation District. The study showed that PM1 could degrade MTBE faster than native microbes based on laboratory microcosms. It also demonstrated the sustained presence of PM1 within the microbial community after inoculation into the test plot (Scow et al., 2001). In general, although aerobic degradation of MTBE has been proven successful, biodegradation rates tend to be slower than those observed for aromatic hydrocarbons.

Title: Cometabolic Biodegradation Text: Cometabolic biodegradation involves a reaction in which a microbe produces an enzyme to support its growth that also happens to degrade the target contaminant. The contaminant is not used as a source of energy, so the microbes require the presence of a primary substrate to supports its growth and further enzyme production. Cometabolism can take place under aerobic or anaerobic conditions. Several researches have documented the cometabolic degradation of MTBE. A propane-oxidizing bacteria has been identified that oxidizes MTBE to tert-butyl formate (TBF) and/or tert-butyl alcohol (TBA). TBA is subsequently degraded by microbes through the intermediate 2-hydroxy isobutyric acid (HIBA). HIBA is eventually metabolized to carbon dioxide. A pentane-oxidizing strain has also been reported in the literature to cometabolize MTBE (Envirogen, 2002). More information is provided later in this tool on the use of propane-oxidizing bacteria to promote cometabolic MTBE biodegradation.

Title: Anaerobic Biodegradation Text: Some researchers have reported limited biodegradation of MTBE under anaerobic conditions. The U.S. EPA conducted a study at NETTS Port Hueneme on the role of methanogenic bacteria in the biodegradation of MTBE and monitored the accumulation of methane in groundwater monitoring wells (U.S. EPA, 2001). In general, researchers have concluded that MTBE should be considered as a compound for which anaerobic biodegradation is extremely difficult.

Title: Site Characterization Approaches (1 of 2) Text: MTBE behaves differently from petroleum hydrocarbons in the environment and is likely to require a different sampling approach to characterize a site. As discussed above, MTBE can migrate about as fast as groundwater travels at a site. There have even been some cases reported of MTBE plumes separating from plumes of other gasoline constituents. This could result in an area of relatively clean water downgradient from a BTEX plume, followed further downgradient by groundwater impacted with MTBE. While plume separation may be rare, it is common for an MTBE plume to extend substantially beyond a BTEX plume in the direction of groundwater flow.

Title: Site Characterization Approaches (2 of 2) Text: One approach to characterizing the extent of an MTBE plume is to estimate the time since release using historical records, groundwater velocity, depth to groundwater, seasonal changes in groundwater flow direction, and other available information for a site. This information can be used to estimate the distance that MTBE may have traveled since its release into the aquifer and to begin to set realistic boundaries for the site investigation efforts. Because of the extended size of MTBE plumes, quick and cost effective site characterization methods are important. Remedial project managers (RPMs) should consider the use of real-time field analyses to delineate petroleum hydrocarbon plumes such as Site Characterization and Analysis Penetrometer System (SCAPS). The use of direct-push technology for sampling can also result in significant cost avoidance without compromising groundwater sampling data quality. Once the plume boundaries have been established, permanent multi-level monitoring wells can be installed as sentinel wells and compliance points.

Title: SCAPS Text: Site Characterization and Analysis Penetrometer System (SCAPS) is a technology that was developed by the Army, Air Force, Navy, U.S. Department of Energy (DOE), and the U.S. EPA to address the problems of time and expense in site characterization. It has proven to be an effective tool for rapid site characterization and assessment. SCAPS combines traditional cone penetrometer technology and samplers to provide rapid, cost effective evaluation of soil contaminants, geophysical properties and to direct traditional soil sampling and well placement. An on-site or near by environmental laboratory gas chromatograph/mass spectrometer can provide a fast turn around on sample data. SCAPS probes have been used to characterize petroleum, volatile organics, explosives and metals contaminants in over 200 locations on Army, Navy, Air Force, DOE and EPA sites. A link to more information on SCAPS is provided in the reference section of this tool.

Title: Direct-Push Well Installation Text: Direct-push technology is a method by which a rod (hollow or solid) is "hammered" into the ground (usually through hydraulic pressure). A comparison between groundwater parameter monitoring data from direct-push installed monitoring wells and hollow stem auger drilled monitoring wells was conducted near the leading edge of the MTBE plume located at the NETTS in Naval Base Ventura County Port Hueneme, California. The purpose of this effort was to determine whether representative inorganic and organic contaminants of concern, groundwater field parameter measurements such as pH, redox, dissolved oxygen and water table elevation data could be generated using properly designed direct-push monitoring wells. No significant performance differences were observed between the direct-push wells and hollow stem auger drilled wells. Within acceptable experimental error, the performance was comparable for the hydrogeologic setting of Port Hueneme. More specifically, the chemical variability among the different well types was less than that displayed by spatial heterogeneities associated with well screen depth differences and temporal variability. Water level values also appeared to yield comparable results for the different well designs. A link to more information on direct push well installation is provided in the reference section of this tool.

Title: Multi-Level Monitoring Wells Text: Long-screen (10-ft or greater interval) conventional monitoring wells are often ineffective for discerning the details of the vertical concentration distribution in plumes and particularly for locating the highest concentration zones because the well screens provide water samples that are a mixture of waters of different composition from various depths. A new multi-level monitoring system was developed that uses custom-extruded flexible multi-chamber polyethylene tubing to monitor as many as seven discrete zones within a single borehole. This new multi-level monitoring system was tested and verified at the Naval Base Ventura County Port Hueneme, California NETTS location. A link to more information on multi-level monitoring wells is provided in the reference section of this tool.

Title: MTBE Remediation Technologies Text: Several remediation technologies have proven successful in the field for the treatment of MTBE in soil or groundwater. Many of the same technologies used for remediating petroleum hydrocarbons also work for the remediation of MTBE. However, some of these technologies are less effective and/or more costly for MTBE remediation because of its unique properties. Site-specific conditions, projected costs, and cleanup goals are some of the major factors involved in choosing a remedial technology for MTBE contamination. Click here to see a list of technologies that have been used in the field for the remediation of MTBE. At the NETTS in Port Hueneme, NAVFAC has participated in the demonstration of over 15 innovative MTBE cleanup technologies. The Navy continues to explore innovative methods for MTBE cleanup at the NETTS and other Navy sites as discussed below.

Title: Technology List Text: Groundwater: Air Sparging Bioaugmentation Biobarriers Biostimulation Chemical Oxidation Cometabolic Air Sparging Natural Attenuation Phytoremediation Pump and Treat Soil: Bioventing Soil Vapor Extraction

Title: Air Sparging: Technology Overview Text: Air sparging uses injected air to remove volatile or biodegradable contaminants in groundwater. The pressurized air travels horizontally and vertically through the saturated zone creating an underground "stripping" effect that removes contaminants from the water to the air phase by volatilization. In addition, the injection of air results in an increased level of dissolved oxygen that enhances in situ aerobic biodegradation of the contaminants. At some sites, the volatilized contaminants are collected in the vadose zone and removed from the subsurface with a soil vapor extraction (SVE) system. Air sparging/SVE performs most efficiently at sites that have homogeneous soils with high permeability. Air sparging is effective for MTBE remediation in groundwater. However, due to MTBE's relatively low Henry's constant, air sparging may require a greater flow rate of injected air and/or extended treatment duration to achieve cleanup objectives.

Title: IAS/SVE Case Study: Introduction Text: To Navigate Through This Case Study: Click on the numbered buttons on the lower left. The Navy performed an interim remedial action to reduce gasoline concentrations in soil and groundwater at the Department of Defense Housing Facility (DoDHF) Former Underground Storage Tank Site 957/970 in Novato, California. The interim remedial action consisted of in situ air sparging with soil vapor extraction (IAS/SVE). The IAS/SVE system operated at the site from June 1998 to October 1999. The remedial action was initially targeted primarily at the removal of BTEX components; however, the concentrations of MTBE in both groundwater and the extracted vapor stream were monitored closely during system operation. As remedial activities progressed, the role of IAS/SVE in effective MTBE removal became central to meeting the remedial objectives at the site.

Title: Air Sparging Case Study Text:

Title: IAS/SVE Case Study: Site Background Text: DoDHF Novato is located on former Hamilton Air Force Base (AFB) property in Novato, California, approximately 20 miles north of San Francisco. The former UST Site 957/970, covers an area of approximately 13 acres of land. Gasoline releases from former USTs at the site had impacted soils and reached the groundwater in the shallow aquifer. The MTBE plume in the shallow aquifer is shown here. The geology at the site is heterogeneous with sands and clay encountered at varying depths across the site. A shallow, thin, sandy layer is encountered at 9 ft below ground surface (bgs) with bedrock at 15 to 20 ft bgs. The aquifer zone is located in this permeable, sandy layer, throughout which the water table elevation fluctuates. The majority of sorbed gasoline constituents were found in the smear zone formed by water table fluctuations. The relative permeability of this zone, which contained the greatest levels of hydrocarbon impact, also made it the zone that is most conducive to effective air sparging.

Title: IAS/SVE Case Study: System Design Text: The goal of the interim action with IAS/SVE was aggressive treatment of “hot spot” areas and BTEX mass removal to reduce the potential of groundwater plume migration. The IAS/SVE system initially consisted of 10 air sparging wells and 6 SVE wells installed in May 1998. Sparge wells were screened as low as possible in the saturated permeable layer to allow air to traverse the maximum possible vertical distance through the aquifer. SVE wells were screened in the vadose zone and across the water table to accommodate fluctuations in groundwater levels. Subsequent groundwater monitoring events revealed areas of elevated hydrocarbon concentrations outside of the original sparge zone. For this reason, eight additional sparge wells and seven additional SVE wells were installed in October 1998 to effect cleanup in these areas.

Title: IAS/SVE Case Study: Results Text: Significant mass removal of MTBE was achieved through operation of the IAS/SVE system. Removal likely occurred through both capture of volatilized MTBE by the SVE system and biodegradation in the subsurface. It was estimated that over 10,000 kg and 450 kg of gasoline and MTBE, respectively, were removed through the SVE system. Note that these values only indicate the removal through stripping and do not account for removal as a result of biodegradation. Groundwater monitoring results show that MTBE concentrations decreased significantly in the area of the former USTs following one year of system operation. At one well located in the former UST source area, the initial MTBE concentration was 190,000 µg/L. After one year of operation, the concentration of MTBE in the same well decreased to 2,200 µg/L. This decrease represents a reduction of approximately 99% MTBE concentrations.

Title: IAS/SVE Case Study: Summary Text: Although the interim remedial action system was designed to remove elevated levels of BTEX compounds, results showed that the system effectively removed significant MTBE mass. The MTBE removal was confirmed by the presence of MTBE in the extracted vapor stream and by the reduction of MTBE groundwater concentrations. In addition to significant groundwater concentration reductions during system operation, a decreasing trend in MTBE concentrations was observed for two years after system operation stopped. Based on the success of this technology approach and the evidence of effective indigenous biological activity to degrade MTBE at this site, biosparging was chosen as an additional MTBE treatment step at the property boundary in 2002.

Title: Biobarrier: Technology Overview Text: A biobarrier is a biologically active flow-through zone in an aquifer that is established downgradient of a gasoline spill source zone. As contaminated groundwater passes through the biobarrier, the MTBE is converted by microorganisms into innocuous byproducts such as carbon dioxide and water. The microbial population is established by bioaugmentation and/or biostimulation. Groundwater leaving the down-gradient edge of the biobarrier treatment zone should contain MTBE at concentrations less than or equal to the treatment target levels. There are several advantages and limitations associated with the use of a biobarrier for MTBE remediation.

Title: Source Zone Definition Text: The “source zone” is delineated by the presence of separate or free phase petroleum hydrocarbons.

Title: Bioaugmentation Definition Text: The addition of microbe cultures to groundwater to enhance biodegradation.

Title: Biostimulation Definition Text: The addition of amendments to stimulate the growth of indigenous and/or introduced microbes. For aerobic organisms, this includes injecting oxygen and/or nutrients into the aquifer.

Title: Advantages of the Biobarrier Technology Text: Eliminates the need for groundwater extraction, above-ground treatment, and discharge The equipment associated with the biobarrier technology is minimal MTBE is mineralized in situ to innocuous products (CO2 and H2O)

Title: Limitations of the Biobarrier Technology Text: The main factors limiting treatment efficiency are expected to be the degree of heterogeneity of the aquifer (to include the presence of clay lenses and layers), the dissolved oxygen distribution, and the MTBE-degrader activity distribution. It is only effective in certain geologic settings (e.g., permeable soil types and relatively shallow depths to groundwater). It is only effective at sites where the treatment zone can be practicably maintained in a well-oxygenated state. It is only effective at sites where either an MTBE degrading culture can be delivered, or indigenous MTBE degraders can be stimulated to a level of sufficient activity.

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Title: Biobarrier Case Study: Introduction Text: To Navigate Through This Case Study: Click on the numbered buttons on the lower left. A passive flow-through biobarrier configuration was selected for the remediation of a large-scale MTBE plume at the Naval Base Ventura County in Port Hueneme, California. The first biobarrier was installed in 2000 to degrade MTBE and other dissolved hydrocarbons leaving the downgradient edge of a gasoline-impacted source zone. This site is somewhat unique because the dissolved MTBE plume is approximately 500 feet wide and about a mile long. Dissolved MTBE concentrations upgradient of the first biobarrier were as high as 10 mg/L in the central core of the plume. The first biobarrier was installed as part of a demonstration project sponsored by the DoD Environmental Security Technology Certification Program (ESTCP) and NETTS and was carried out by NAVFAC and Arizona State University. A pump-and-treat system was also installed in 2001 at the leading edge of the plume to mitigate off-site plume migration. After the success of the first biobarrier, two additional full-scale biobarriers were installed in 2003 further downgradient as part of the NAVFAC Installation Restoration (IR) Program. These biobarriers were placed midway and just upgradient of the pump-and-treat system (see Case Study Slide 2). The Base is currently evaluating the future shutdown of the pump-and-treat system once it is demonstrated that the full-scale biobarriers can reach remedial action objectives for MTBE removal.

Title: Biobarrier Case Study: Site Background Text: The plume is the result of a release of over 10,000 gallons of gasoline that occurred in the mid-1980s from underground piping at a base service station. After the service station leak was discovered, the pipes and tanks were replaced and cleanup of the immediate vicinity was performed. The spill to a semi-perched aquifer, located 6-10 feet below the surface and extending down to a clay layer about 20 feet below the surface, resulted in a dissolved MTBE plume about 500 feet wide and 5,000 feet long. This aquifer is an anaerobic, reduced oxygen environment and is high in dissolved solids; therefore, it is not used for drinking water production. The dissolved MTBE plume greatly outdistances the residual source zone and dissolved BTEX plume by about 4,000 feet. The MTBE plume is wholly confined to a shallow perched aquifer on the base, it is not affecting a drinking water resource, and it presents no known risk to either the environment or to the public. The subsurface plume is located in an industrial area covered primarily in hardstand, so neither residents nor employees can come into contact with it.

Title: Biobarrier Case Study: System Design Text: The first 500-ft long biobarrier was designed to identify the conditions necessary to achieve the best system performance. Different operational conditions were in effect along the length of the biobarrier to better evaluate the optimal configuration. The biobarrier consisted of two different bioaugmented plots (oxygenated and seeded with two MTBE-degrading cultures) and two different types of biostimulated plots (air injection and oxygen injection). All of these configurations were shown to achieve reductions in MTBE concentrations (and BTEX, when present) to less than 5 µg/L after the barrier. It is important to note that influent concentrations, or relative dissolved hydrocarbon loadings varied along the biobarrier as well. For reference, the maximum loading was estimated to be approximately 1 g-MTBE/d per ft length of biobarrier, and maximum MTBE concentrations were on the order of 10,000 µg/L.

Title: Biobarrier Installation Video Text: This video shows the installation of the second biobarrier during the Summer of 2003. The second barrier was installed further downgradient at the mid-point of the plume and was designed to establish a stable oxygenated zone spanning the width of the barrier. As shown, the second biobarrier was installed with the piping network below ground to minimize the surface footprint. The installation of the third biobarrier near the leading edge of the plume was conducted in the Fall of 2003. All three biobarriers are currently in operation and the objective is to prevent further migration of MTBE-impacted groundwater and to shorten the time required to treat the MTBE plume. The pump-and-treat system at the toe of the plume will continue to operate until the biobarrier at the leading edge is proven effective at meeting the maximum contaminant level (MCL), which is 0.013 mg/L for the state of California.

Title: Biobarrier Case Study: Results Text: As shown here, the aeration and oxygenation systems for the first biobarrier were successful at raising depressed dissolved oxygen levels within the aquifer. Prior to system start-up, site-wide dissolved oxygen concentrations were below 1 mg-oxygen/L-groundwater. Afterwards, all wells within 5 feet of the gas injection row showed groundwater oxygen levels above 4 mg-oxygen/L-groundwater. This meets the level necessary to stimulate and support aerobic degradation.

Title: Biobarrier Case Study: Summary Text: The results from the first biobarrier clearly demonstrated that MTBE-impacted groundwater can be remediated along with BTEX components by aerobic biodegradation in a mixed MTBE-BTEX dissolved plume. The biobarrier system achieved an in situ treatment efficiency of >99.9% for dissolved MTBE and BTEX. After only 7 months, groundwater samples collected from downgradient monitoring wells contained less than 5 µg/L MTBE and non-detectable levels of BTEX components. In addition, groundwater contaminant concentrations did not increase in the wells to the north and south of the barrier, indicating that the contamination was not circumventing the barrier. The successful outcome of the first biobarrier led to the installation of the second and third biobarriers in 2003.

Title: Cometabolic Biostimulation Text: Cometabolic biodegradation involves a reaction in which a microbe produces an enzyme to support its growth that also happens to degrade the target contaminant. The contaminant is not used as a source of energy, so the microbes require the presence of a primary substrate to supports its growth and further enzyme production. Cometabolism can take place under aerobic or anaerobic conditions. Successful cometabolic degradation of MTBE using methane, propane, butane, and other alkanes has been reported in both laboratory and field-scale studies. Biostimulation is the process of injecting these substrates into groundwater to promote the biodegradation of MTBE or other target contaminants.

Title: Biostimulation icon Text:

Title: Biostimulation Case Study: Introduction Text: To Navigate Through This Case Study: Click on the numbered buttons on the lower left. A cometabolic biostimulation demonstration project was carried out at the NETTS at the Naval Base Ventura County, Port Hueneme, California between June 2001 and March 2002. The following joint effort between the US EPA National Risk Management Research and the Environmental Security Technology Certification Program (ESTCP) was designed to evaluate the application of in situ oxygen and propane injection for remediating MTBE contaminated aquifers. The project compared MTBE biodegradation in a Test Plot that was amended with propane oxidizing bacteria and treated with oxygen and propane to a Control Plot that received only oxygen. The project also allowed evaluation of the cost and safety of propane biosparging for MTBE remediation at the field scale.

Title: Biostimulation Case Study: System Design Text: The demonstration system consisted of a network of oxygen and propane injection points, pressurized oxygen and propane gas delivery and control systems, and ground water and soil-gas monitoring networks. In addition, tracer injection wells, ground water monitoring points and soil-gas monitoring points were installed to facilitate performance monitoring. The Test and Control Plot configurations were designed based on the range of ground water flow velocities, MTBE concentrations, and estimated oxygen requirements arising from geochemical and biological demand. Data acquired during site characterization confirmation sampling were used to finalize the design and to refine the operating characteristics of the system prior to installation. The results of the microcosm studies were also used in the system design and indicated that injection of a bacterial seed culture was required to promote rapid degradation of MTBE.

Title: Biostimulation Case Study: Results Text: MTBE concentrations decreased in both the Test and Control Plots during the demonstration. MTBE concentrations in deep monitoring wells located directly downgradient of the propane and oxygen injection systems rapidly decreased during the first two months following bioaugmentation. The maximum MTBE concentrations in the deep Test Plot wells immediately prior to bioaugmentation was 3,400 µg/L, with most wells having a concentration above 1,300 µg/L. At the conclusion of the demonstration, the maximum MTBE concentration in the deep Test Plot wells was 440 µg/L, with most wells having a concentration below 150 µg/L. In the shallow monitoring well network, MTBE concentrations in the well upgradient of the Test Plot decreased from 1,700 µg/L to 5 µg/L by the end of the demonstration. Thus, ground water entering the shallow aquifer in the Test Plot generally contained less than 250 µg/L after July 2001. This result suggests that propane and oxygen spread upgradient into the shallow aquifer and promoted MTBE biodegradation. Some of the wells in the Control Plot also had relatively rapid decreases in MTBE concentrations after oxygen injection began. Because MTBE degradation was observed in the Control Plot from oxygen injection alone, it was difficult to verify the effectiveness of propane-stimulated microbes in MTBE removal.

Title: Chemical Oxidation (In Situ) Text: In situ chemical oxidation is the injection of a strong oxidizing agent into groundwater to destroy or degrade organic chemicals. It is best suited for application near petroleum hydrocarbon source zones and not for large, dilute groundwater plumes. The selection of an oxidant is important because not all oxidants have been shown to promote the complete mineralization of MTBE. In some cases, chemical oxidation will result in the formation of byproducts from MTBE such as TBF, TBA, and other related compounds. It is believed, however, that these byproducts may be more susceptible than MTBE to subsequent biodegradation. Oxidants that have been used in the field for MTBE treatment include Fenton’s reagent, ozone, and permanganate.

Title: Fenton's Reagent Text: Fenton's reagent involves the combination of [hydrogen peroxide (H2O2) and ferrous iron (Fe2+)] to form a hydroxyl-free radical.

Title: Mineralization Definition Text: The complete conversion of an organic compound to inorganic products (principally water and carbon dioxide).

Title: Phytoremediation: Technology Overview Text: Phytoremediation is a soil and groundwater treatment technology that uses vegetation to remove, contain, or reduce the toxicity of contaminants. It can be implemented by using existing plant-life at the site or by establishing a selected plant or community of plants. The technology exploits the natural hydraulic and metabolic processes of plants such as phytostabilization, phytodegra-dation, rhizosphere degradation, hydraulic control, and phyto-volatilization. Overall, studies indicate that plants can take up and transpire MTBE in contaminated groundwater and control the migration of MTBE plumes. However, the applicability of phytoremediation remains limited due to several practical factors.

Title: Phytostabilization Text: The use of plants to increase the sequestration of contaminants and thereby reduce their bioavailability, mobility, and toxicity. Phytostabilization occurs when contaminants bind to organic matter in plant root tissue and become immobilized. Phytostablization works well with metals, but the tendency of MTBE to bind to plant organic matter is relatively low based on its chemical properties.

Title: Phytodegradation Text: The process where plant enzymes completely mineralize or partially break down contaminant compounds. There is currently no clear evidence for the complete mineralization of MTBE by plants.

Title: Rhizosphere degradation Text: Plant roots excrete sugars, acids, and alcohols that contain organic carbon that microorganisms use as a food source. This enhances microbial activity within the root zone and also enhances the potential for biodegradation of various organic contaminants. The various chemicals excreted by plants have been shown to stimulate the biodegradation of MTBE within the root zone.

Title: Hydraulic Control Text: Trees that capture and transpire water within an aquifer can act as a hydraulic barrier to prevent plume migration. The upward draw of water into the trees reduces the flow of groundwater near the tree stand and increases the residence time of the contaminant underneath the site. This mechanism has been shown to control the migration of MTBE groundwater plumes and enhance biodegradation.

Title: Phytovolatilization Text: The use of plants to evaporate or volatilize contaminants from the leaf surface of the plant once it has traveled through the plant’s system. MTBE has been demonstrated to volatilize from plants and is subsequently broken down in the atmosphere within a matter of days.

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Title: Phytoremediation Case Study: Introduction Text: To Navigate Through This Case Study: Click on the numbered buttons on the lower left. The use of trees to treat MTBE contaminated groundwater was studied at Port Hueneme NETTS from June 1999 to August 2000. Tests were conducted using a 50-60 year old Eucalyptus tree, which was located near the source zone of the 15 year old petroleum plume contaminated with BTEX and MTBE. The Eucalyptus tree was 55 feet tall and 4 feet in diameter. Near the tree, the depth to groundwater was approximately 10 feet. The aquifer was sand/gravel with groundwater flows between 0.5 to 1 foot/day.

Title: Phyto Case Study: Study Design Text: Soil and groundwater samples were collected upgradient, downgradient, and from within the rhizosphere zone of the Eucalyptus tree. Sap meters were used to determine groundwater uptake cycles of the tree. Cores were taken from the trunk and tissue samples were collected from the leaf, stem, and root for analysis of MTBE and possible metabolites. Transpiration samples were also collected from the leaves.

Title: Phyto Case Study: Results Text: Results of this field demonstration indicated a reduction of MTBE concentrations in groundwater and soil from biodegradation within the rhizosphere zone of the tree. MTBE concentrations upgradient of the tree were 100 ppb in groundwater and 42 ppb in soil. MTBE concentrations downgradient of the tree were 53 ppb in groundwater and 5 ppb in soil.

Title: Phyto Case Study: Results Text: Sap flow data demonstrated the eucalyptus tree uptakes groundwater in two cycles per day. However, samples from the tree coring effort as well as the transpiration sample results were non-detect for MTBE. These results suggest that rhizosphere degradation and hydraulic control may be potential mechanisms for phytoremediation at the site, but that there is no measurable phytovolatilization of MTBE.

Title: Limitations of Phytoremediation Text: Employing specific plant species to target particular contaminants at a site can be difficult because of adaptability problems. Climatic or seasonal conditions may interfere with or inhibit plant growth, slow remediation efforts, or increase the length of the treatment period. Phytoremediation will likely require a large area of land surface for remediation. Generally, phytoremediation may only be employed in areas with lower levels of contamination due to plant toxicity effects. The use of phytoremediation is limited by contamination depth, although there are currently studies under way which are determining the potential for fast growing, deep-rooted trees to remediate contaminated groundwater.

Title: Pump-and-Treat (1 of 5) Text: In certain circumstances, in situ treatment may be impractical or pumping of the contaminated groundwater may be required for plume containment. In this case, there are several ex situ treatment options available for the removal of MTBE from groundwater. Two of the most common ex situ treatment technologies for organic contaminant treatment are air stripping and liquid phase granular activated carbon (GAC). These conventional technologies can be used to remove MTBE from groundwater, but MTBE's chemical and physical properties can limit the cost effectiveness of these two approaches. For this reason, ex-situ, above-ground emerging and innovative treatment technologies for MTBE including hollow fiber membrane reactors, HiPOx Advanced Oxidation Process (AOP), and high energy electron injection were evaluated. A link to the NETTS Port Hueneme Web site is provided in the reference section for more information on these ex situ treatment technology demonstration projects.

Title: Hollow Fiber Membrane Reactor Text: The Hollow Fiber Membrane (HFM) unit is a compact degasification module designed for separation of volatile gases from water using a bundle of hydrophobic hollow fibers as a contacting device between clean air and contaminated water. The contaminated water is passed through the inner part of the membrane and a counter current air flow is pulled by vacuum applied to the outer part of the membrane. A concentration gradient is established between the contaminants in the ground water and clean air, leading to transfer of the volatile constituents through the membrane to the air stream. The selective physicochemical properties of the membrane allow only the organic compounds to transfer across the membrane to the air phase. A demonstration of HFM degasification coupled with Spray Aeration Vacuum Extraction (SAVE) was conducted at the NETTS located at the Naval Construction Battalion Center at Port Hueneme, California, between June 21 and June 25, 1999. The demonstration tested the efficiency of the HFM and SAVE technologies to treat groundwater contaminated with dissolved MTBE and other volatile organic compounds (VOCs). The system configuration used included SAVE, HFM, water softeners and GAC units. This treatment train consistently achieved removal efficiencies of MTBE, benzene, and toluene greater than 99.9%. During the 5-day demonstration effort, approximately 34,200 L (9,000 gallons) of contaminated groundwater was treated while continuously meeting the project’s MTBE clean up goal of 5 µg/L.

Title: HiPOx Advanced Oxidation Process Text: The HiPOx technology is similar to other chemical oxidation technologies in that it uses ozone and hydrogen peroxide to destroy organic compounds in wastewater. However, the high-precision delivery of the oxidants and the use of multiple oxidant injection ports in the HiPOx technology enhances process efficiency. In “traditional” applications of chemical oxidation technology, 2 to 3 percent ozone by weight is injected at a single point through a diffuser and allowed to bubble up, at atmospheric pressure, through the contactor. Volatile organic compounds (VOCs) are destroyed over contact times as long as 20 minutes. The HiPOx technology enhances the mass transfer of ozone into the water by using higher ozone injection concentrations (8 to 10 percent by weight), higher operating pressure (typically 35 to 45 pounds per square inch gauge [psig]), and in-line mixers to promote efficient mixing. The technology was demonstrated in application to groundwater contaminated with MTBE at the NETTS Port Hueneme during Fall 2001. Very high removal efficiencies were achieved for MTBE (>99.9 percent) but oxidation intermediates, including TBA and acetone, were present in significant concentrations in the effluent.

Title: High Energy Electron Injection Text: The high-energy electron injection (E-Beam) technology destroys organic contaminants in groundwater through irradiation with a beam of high-energy electrons. The injection of accelerated electrons into an aqueous solution results in the formation of three primary reactive species: aqueous electrons and hydrogen radicals (•H), which are strong reducing species; and hydroxyl radicals (•OH), which are strong oxidizing species. These reactive species can destroy most organic compounds to non-detectable concentrations. However, oxidation byproducts such as acetone, aldehydes, and glyoxals, may be formed in significant concentrations. The capabilities of the E-Beam technology for treating groundwater contaminated with MTBE and with BTEX were demonstrated in the summer and fall of 2001 at the source zone of the Naval Exchange Gasoline Station site at the NETTS Port Hueneme. Treatment goals were established for the demonstration based primarily on California MCLs for drinking water. Results of the two-week steady-state operation indicated that MTBE and BTEX concentrations in the effluent were reduced by greater than 99.9 percent from influent concentrations.

Title: Air Stripping (2 of 5) Text: Air stripping partitions volatile organic compounds from groundwater by exposing a large surface area of contaminated water to clean air flow. Various types exist including bubble diffusion strippers, low profile air strippers, spray towers, aspirated air strippers, and packed towers. Packed tower air strippers are the most commonly used. The typical packed tower air stripper includes a spray nozzle at the top of the tower to distribute contaminated water over the packing in the column, a fan to force air countercurrent to the water flow, and a sump at the bottom of the tower to collect decontaminated water. Off gas treatment is often required and can include the use of thermal oxidation or vapor-phase carbon adsorption.

Title: Air Stripping & MTBE (3 of 5) Text: Air stripping is a technology that is very effective for the removal of volatile organic compounds like BTEX, but it is less effective for the removal of MTBE. Because of its lower Henry's constant, MTBE removal requires higher air to water ratios than for BTEX removal. This results in higher capital and operation and maintenance costs for air strippers designed for adequate MTBE removal. Despite higher relative treatment costs, air stripping is a proven technology that can successfully remove MTBE from water given properly designed systems. Both packed tower strippers (above 100 gpm) and low profile air strippers (below 100 gpm) have been demonstrated to meet treatment objectives for MTBE removal from groundwater.

Title: Granular Activated Carbon (4 of 5) Text: Liquid phase GAC adsorption is another conventional ex situ treatment technology for removing organic contaminants from groundwater. It involves pumping groundwater through one or more vessels containing activated carbon. The GAC removes contaminants from the water stream by sorption until available active sites are occupied. Carbon is “activated” by being processed to create porous particles with a large internal surface area (3,200 to 27,000 square feet per gram of carbon) that attracts and adsorbs organic molecules as well as certain metal and inorganic molecules. As the available surface sites become occupied, the contaminant concentration in the GAC effluent increases. When contaminant concentrations in the effluent exceed specified action levels, the carbon can be regenerated in place, removed and regenerated at an off-site facility, or removed for disposal.

Title: Granular Activated Carbon & MTBE (5 of 5) Text: The effectiveness of MTBE removal with GAC is limited by its high solubility, which results in a tendency to remain in solution rather than adsorb onto the solid GAC surface. In addition, natural organic matter (NOM) and other organics will compete with MTBE for adsorption onto GAC. Other more strongly sorbed organics can result in the desorption or the displacement of previously sorbed MTBE. Testing is recommended to evaluate site specific conditions. GAC will be more cost-effective at sites with lower MTBE concentrations, lower NOM levels, and lower levels of other organic contaminants. GAC is not cost-effective for the treatment of high volumes or for MTBE concentrations that exceed 300 ppb. Low flow rates and low MTBE levels make the use of GAC more economical because of decreased carbon usage rates. It is recommended that GAC vessels be operated in series of two or more vessels. Also frequent sampling and testing of influent, mid-fluent, and effluent water is suggested to the monitor for MTBE breakthrough.

Title: MTBE Treatment Technology Decision Tool Text:

Title: References Text: General National Environmental Technology Test Site Port Hueneme Web Site U.S. EPA's MTBE Homepage U.S. EPA's MTBE Fact Sheet # 1 U.S. EPA's MTBE Fact Sheet # 2 U.S. EPA's MTBE Fact Sheet # 3 American Petroleum Institute's MTBE Resource Page MTBE Research at UC Davis USGS MTBE Homepage Regulations Groundwater Oxygenate Cleanup Levels for LUST Sites AEHS 2003 State Summary of Cleanup Standards State Drinking Water Regulations and Guidelines for MTBE Site Characterization Strategies for Characterizing Subsurface Releases of Gasoline Containing MTBE, API Publication 4699 SCAPS Performance Comparison Direct Push versus Drilled Wells Remediation Processes/Technologies U.S. EPA's CLU-IN; MTBE Treatment Profiles Other References Amerson, I. and R.L. Johnson. 2003. “Natural Gradient Tracer Test to Evaluate Natural Attenuation of MTBE Under Anaerobic Conditions,” Ground Water Monitoring and Remediation, Winter 2003. Envirogen, Inc. 2002. ESTCP Technology Demonstration Draft Final Report. In Situ Remediation of MTBE Contaminated Aquifers Using Propane Biostimulation. September 13, 2002. EPA. 2001. NETTS Project/Demonstration Summary. Natural Attenuation of MTBE in Groundwater Under Methanogenic Conditions. NCBC-46-00. Scow, K., MacKay, D., and P. Johnson. 2001. NETTS Project/Demonstration Summary: Direct Injection of a Bacterial Culture to Biodegrade MTBE-Contaminated Groundwater. NCBC-38-99.

Title: Contact Information Text: For more information about MTBE remediation, please contact: NFESC POC (805) 982-1656 PRTH_NFESCT2@navy.mil