Myths, misconceptions and misinformation have developed concerning the recent discovery of MTBE in ground water. This paper applies good science in evaluating 11 of those myths.

1. MTBE degrades storage/handling facilities.

It has been claimed that ethers, and specifically MTBE, can prematurely degrade gaskets, seals, hoses, fittings, and valves on gasoline storage and handling facilities. MTBE has been an important component of unleaded gasoline and subsequently reformulated gasolines (RFG) for more than 20 years. MTBE containing formulations have been successfully shipped nationwide in a variety of truck transports, pipelines and rail transfer facilities. Historically, the materials of these gasoline-handling facilities have been compatible with MTBE and have tested tight. Several detailed reviews over the last three years have not revealed any instances where MTBE in gasoline caused premature failure of systems’ components or resulted in material incompatibility. When the specific systems’ components were reviewed, it was concluded that their properties were consistent with handling MTBE. Recently, the US Coast Guard conducted a study of marine fueling facilities to test the integrity of specific fuel system components and concluded that MTBE had no negative impact.

2. MTBE alone leaks from gasoline tanks.

When an underground fuel storage tank fails, all of the chemical components of the fuel are released into the subsurface soils and likely into the underlying groundwater. Typically, gasoline may contain 6% MTBE by volume, which means that 94% of what leaks into the soil and groundwater consists of other gasoline components by volume. When a leak occurs, the MTBE in the gasoline is only a small percentage of the hydrocarbon released to the environment.

The media has frequently reported that MTBE leaked from a gasoline tank or that a railcar containing gasoline, leaked and MTBE spilled onto the ground or into the surface water. This is very misleading because it implies that only MTBE was released to the environment. Gasoline contains over 200 chemical components. Therefore, all of these components were released into the environment. Indeed, many of these chemicals pose a much greater risk to the public health and the environment than does MTBE.

3. MTBE travels far beyond BTEX plumes.

When gasoline chemical components, including MTBE, contact groundwater, these chemicals will dissolve, based on their respective solubility limits and site-specific conditions. The chemicals will then migrate with the groundwater. Dissolved chemicals cannot travel faster than the groundwater but they may travel slower if their movement is retarded by adsorption to the soil. Several processes occur when groundwater plumes migrate: the chemicals in the water are diluted and dispersed; the chemicals may absorb to, or desorb from, soil particles; the chemicals may be aerobically or anaerobically biodegraded.

The net result is that MTBE will tend to exist on the leading edge of a typical groundwater plume; however other gasoline components, e.g. BTEX, will tend to follow immediately behind the leading edge of the plume. Historically, accurate interpretation of plume position and composition has been complicated by analytical detection limits for BTEX. Frequently, laboratories report MTBE but not BTEX even though chromatographic peaks for aromatics are clear. Review of the data often reveals BTEX, at some concentration, immediately behind the leading edge of the plume. Several recent studies of groundwater plumes associated with gasoline releases have confirmed that MTBE and BTEX plumes generally coincide.

4. MTBE plumes sink (or dive).

Behavior of free phase hydrocarbons in groundwater is a function of their density. MTBE and the other components of gasoline have a specific gravity of less than 1, consequently free-phase gasoline, with or without MTBE, floats on the water table. When the components of gasoline dissolve into the groundwater, they move with it through the aquifer. The addition of new water to an aquifer is called recharge. If recharge occurs from the surface, older aquifer water containing dissolved constituents may be pushed downward in the formation. Likewise, pumping of an aquifer at depth may pull the water table and constituents dissolved in the groundwater to deeper locations in the formation. In any event, dissolved constituents follow groundwater flow. For this reason it is important to conduct complete, three-dimensional characterization of plumes prior to remedial action regardless of whether or not they contain MTBE.

5. MTBE causes cancer.

Most information on the toxicology of ether oxygenates comes from laboratory studies of their effects on animals, which are often used as predictors of potential adverse health effects in humans. Several studies have shown the formation of tumors in animals exposed to high concentrations of MTBE. However, there is some doubt about the relevance of these data to assessing the carcinogenicity of MTBE to humans and whether the doses are environmentally realistic. Existing data derived from animal studies, relating to chronic carcinogenic and non-carcinogenic toxicity, are considered ambiguous and inconclusive. More importantly, human epidemiology studies failed to support the classification of MTBE as a carcinogen. No national or international regulatory agency has classified MTBE as a human carcinogen, and the available genotoxicity data suggest that MTBE is not mutagenic. Also, the International Agency for Research on Cancer (IRAC) stated (11/00) that MTBE is not classifiable as a human carcinogen. The weight of evidence suggests that ingestion of water containing MTBE below, or close to the taste threshold, is unlikely to result in adverse health effects.

When considering toxic effects, it is useful to note that free phase MTBE has been used to treat gall stones both in the UK and the US for a number of years (Schoenfield and Marks 1993). During the treatment, a tube is inserted into the gallbladder through which the MTBE is delivered. The MTBE dissolves much of the fat content of the stone causing it to disintegrate. A review of the effects of this method of treatment on 761 patients in 21 centers across Europe found no toxic effects from MTBE in any of the patients (Hellstern et al. 1998).

6. MTBE is a threat to drinking water resources.

Any chemicals, metals or other toxic substances are a potential threat to drinking water supplies if they are released in a drinking water recharge area. The actual threat is based on the properties of the specific chemicals, metals, etc. and on the concentration of those constituents.

MTBE is not toxic to human beings. The presence of MTBE in spilled or leaked gasoline does not increase the threat that the gasoline poses to drinking water resources.

7. MTBE can’t be remediated.

MTBE responds to the same types of physical, chemical and biological treatment processes effective with other hydrocarbon contamination. Gasoline plumes containing MTBE can be managed by traditional approaches of hydraulic control, impermeable barriers, reactive barriers and excavation. The same in situ chemical oxidation or bioremediation processes used for other hydrocarbons also destroys MTBE.

Indeed, the physical properties and resulting behavior of MTBE expedites remediation by conventional, physical processes. Classic treatment technology like pump and treat is particularly effective at removing MTBE from the saturated zone due to the high solubility, low Henry’s constant and low adsorption coefficient of MTBE in ground water. In the unsaturated zone, the low vapor pressure of MTBE makes soil vapor extraction (SVE) a particularly effective approach to removing the components of gasoline as well as MTBE.

A variety of processes including air stripping, adsorption on activated carbon or resins, biological treatment and advance oxidation has been successfully used to remove MTBE from groundwater brought to the surface.

8. MTBE doesn’t biodegrade.

The capacity to biologically degrade ethers, like MTBE, is widespread in nature. More than 20 organisms, with the capability to biodegrade MTBE along with other components of gasoline, have been isolated worldwide from surface soils, aquifers, wastewater treatment plants and biofilters.

Increasing evidence is being found and reported on the biological natural attenuation of MTBE in gasoline contaminated aquifers. For example, Bradley et al. (ES&T, 1999, 33(11): 1877-1879) reported that microorganisms indigenous to stream-bed sediments at two gasoline-contaminated groundwater sites have been shown under laboratory conditions to be capable of significant mineralization of MTBE (73%). Reisinger et al, (Soil, Sediment and Groundwater, MTBE Special Issue, March 2000, 43 – 46) concluded that the attenuation of MTBE in contaminated groundwater plumes by biological attenuation shows rate constants and half-lives that are nearly identical to those of benzene. USGS researchers, Bradley et al.( ES&T, 2001, 35(4):658–662), concluded that microbial degradation of MTBE may be higher than previously thought. They reported that the potential for MTBE biodegradation in surface water systems is high, even at sites with no history of MTBE exposure.

While defined biodegradation pathways are predominantly aerobic, recent evidence indicates that some organisms indigenous to the subsurface can utilize MTBE as a carbon and energy source by reducing iron in the presence of humates or under methanogenic conditions. Additional research is in progress nationally to further define the conditions most favorable for anaerobic biodegradation of gasoline components and MTBE.

9. MTBE won’t naturally attenuate.

By the US EPA’s definition, the process of natural attenuation includes both destructive (mass reduction) and non-destructive processes. Destructive processes include biological degradation and abiotic chemical degradation. Non-destruction processes include dilution, adsorption, dispersion and volatilization.

Aerobic biodegradation of MTBE occurs when the concentration of other degradable substrates becomes limited and sufficient dissolved oxygen is present. Consequently, biologically-based natural attenuation at the leading edge has been used to explain many mature, static plumes. Recent investigations into biological degradation of MTBE under anaerobic conditions have verified biodegradation by ferric iron reduction in the laboratory and by methanogenic conditions (http://www.epa.gov, search MTBE, methanogenesis) in the field.

10. MTBE remediation costs significantly more than BTEX remediation.

MTBE has received considerable public scrutiny over the last several years. This has resulted in increased focus on gasoline spills and leaks; especially on leaking underground storage tanks (LUSTs). There is increased emphasis on assessment and remediation of gasoline spills and leaks. It is true that some gasoline spills and leaks were ignored in the past, but today all leaks and spills must be assessed and remediated. There are those who “blame” this increased emphasis on remediating gasoline spills and leaks on MTBE. This is invalid because gasoline does not belong in groundwater. Numerous case studies over the last few years have confirmed that the presence of MTBE in gasoline does not significantly impact the cost for assessment and remediation. The site assessment, design and remediation – are generally independent of the gasoline components.

11. MTBE always drives remediation design, progress and cost.

A review of over 60 gas station sites with MTBE in soils and groundwater has confirmed that remediation technology selection; remediation progress and remediation costs are very site specific. Progress and costs are primarily driven by:

• Amount and duration of the release
    
• Physical nature of the subsurface
    
• Concentrations of the gasoline components in the soils and groundwater
    
• Rate and direction of chemical migration
    
• Nearest receptors and exposure pathways
   
• Required cleanup objectives

Specific, individual chemicals in the gasoline do not generally drive progress and cost. On the other hand, benzene, due to its toxicity, has driven progress and costs at some sites.

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