Common Myths and Misconceptions About the Behavior and Impact of MTBE Released from Petroleum Products

Common myths and misconceptions about the behavior and impact of MTBE released from petroleum products

R. E. woodward' & R. L. sloan2 '~ievvaEnvironmental Services, Inc., Houston, TX 2 Lyondell Chemical Company, Channelview, TX

Abstract

Auto fuel regulatory-mandates, to decrease the aromatic content of fuels and to reduce exhaust emissions, have led to the expanded use of additives to oxygenate auto fuels in the European Union (EU) and United States (US). The economical ether oxygenates, methyl tertiary butyl ether (MTBE) or ethyl tertiary butyl ether (ETBE) are frequently the oxygenate of choice because they deliver oxygen without increasing the Reid Vapor Pressure (RVP) or altering the fungible characteristics of autofuel. However, transport of auto fuels by common carriers that also transport heating oil and other heavier petroleum products has lead to the discovery of trace concentrations of auto fuel in many petroleum products. Subsequent leaks and spills during storage and handling of petroleum products result in the release of Auto fuel constituents to the environment. Critical review of 12 myths and misconceptions about MTBE in auto fuels reveals the concepts were conceived to rationalize early field observations andlor incomplete data sets. Closer scrutiny, in light of recent laboratory investigations, field data, case studies and world literature, indicates the myths are unsubstantiated misconceptions and assumptions about the behavior of ether oxygenates in the environment. Commonly held myths focus on four general areas of fuel and fuel oxygenates management: storageldispensing, hydrology, remediation, and health effects. Storageldispensing misconceptions address materials stability to ethers in fuel and the environmental forensics of fuel systems failure. Groundwater and hydrology myths deal with plume dynamics and the impact of fuel on drinking water resources. Remediation myths focus on the performance of traditional hydrocarbon remediation technologies, recent developments in biodegradation and natural attenuation, drivers of remedial design and remediation costs. Health effects myths address both acute and chronic exposure risk evaluations by national and international health agencies. MTBE is manageable by the same processes and precautions used for gasoline and other fuel hydrocarbons.

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Introduction h an attempt to rationalize early field observations andlor incomplete data sets, numerous myths and misconceptions have evolve about the fuel oxygenate, MTBE.

Invariably, good science has effectively debunked them leading to balanced publication of the issues [l]. This paper explores the basis of 12 myths and provides current interpretations, explanations and supporting data.

1 MTBE degrades storage1handIing 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 worldwide 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 specific instances where MTBE in gasoline caused premature failure of systems components or resulted in material incompatibility [2]. When the specific systems components were reviewed, it was concluded that the properties of the components were consistent with handling MTBE. During the early use of RFG, the US Coast Guard conducted a study of marine fueling facilities due to concerns about the integrity of specific fuel components and concluded that MTBE did not negatively impact the components marine fueling facilities [3].

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 is other gasoline components by volume. When a leak occurs, the MTBE in the gasoline is a small percentage of the hydrocarbon released to the environment. In comparison, aromatic hydrocarbons like benzene and its derivatives comprise up to 34% v/v gasoline. 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 surface water. This is very misleading since it implies that only MTBE was released to the environment. In reality over 200 chemical components of gasoline were released to the environment [4]. 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. The physical behavior of two components of gasoline, MTBE and Benzene, is compared and contrasted graphically in Figure 1. Several processes occur when groundwater plumes migrate; the chemicals in the water are diluted and dispersed; the chemicals may adsorb onto the soil particles or desorb from soil particles; the chemicals may aerobically or anaerobically biodegrade.

Figure 1. Relative Physical Characteristics and Behavior of MTBE and Benzene.

The net result is that MTBE will tend to exist on the leading edge of a typical groundwater plume; however the other gasoline components, e.g. BTEX, will tend to exist immediately behind the leading edge of the plume [5]. 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

222 Water Pollution \//I: Modclling, Mca~uringand Prediction

the plume. Several recent studies of groundwater plumes associated with gasoline releases have confirmed that MTBE and BTEX plumes frequently coincide [6].

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 less than 1. Consequently free-phase gasoline, with MTBE or without, floats on the groundwater 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 and its 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 [7].

5 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 potential 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. California Public Water Supply Wells (PWSW) impacted by MTBE, and indeed the overall number of Leaking Underground Fuel Tanks (LUFTs), have decreased since 1995 (Table 1)[8]. PWSW were sampled at three intervals during the period from 1995 to 2001 and the number of MTBE detections noted. During that period the percentage of MTBE detections decreased Erom 1.20% to 0.60% of the sampled wells. This downward trend continues, due in part to improved monitoring, tank and distribution upgrades and rapid responses to releases of gasoline.

Table 1. MTBE detection in California PWSW 1995-2001

bime Interval 1 95-98 1 98-99 1 99-01 / PWSW Sampled during period 2988 1567 3426

Total PWS Sampled 2988 4555 798 1 Total PWSW with MTBE detects 34 41 46

% PWSW with MTBE detects 1.20% 0.90% 0.60% Source: California Department of Health Services [8].

At concentrations found in groundwater near leaking underground tanks or spills, MTBE is not toxic to human beings, but its associated taste threshold may trigger

avoidance [g]. The presence of MTBE in spilled or leaked gasoline does not increase the treat that the gasoline poses to drinking water resources [l 0,l l].

6 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, vapor management, impermeable barriers, reactive barriers and excavation. The same in situ chemical oxidation or bioremediation processes used for other hydrocarbons can destroy MTBE [12]. Technologies demonstrated to be effective for MTBE containing gasoline are summarized in Table 2.

Table 2. Demonstrated Technologies for Remediation of MTBE by process and matrix.

The physical properties and resulting behavior of MTBE, compared to benzene (Figure l), expedites remediation by some 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 and MTBE. Cataldo and

Moyer's [l31 summary of the remediation technologies used at more the 60 gasoline station sites to recover gasoline as part of source control (recovery methods) and then to remediate dissolved and residual gasoline constituents (treatment methods) confirms, by actual case studies, that traditional hydrocarbon remediation technologies are effective for MTBE. A variety of processes including, air stripping, adsorption on activated carbon or resins, biological treatment and advance oxidation have been used to remove MTBE from groundwater brought to the surface.

7 MTBE doesn't biodegrade

The capacity to biologically degrade ethers, like MTBE, is widespread in nature [14]. More than 20 organisms, with the capability to biodegrade MTBE, ETBE or TBA along with other components of gasoline, have been isolated worldwide from surface soils, aquifers, wastewater treatment plants and biofilters. Microorganisms

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with this range of capabilities include both bacteria and fungi from Alcaligenes, Arthrobacter, Burkholderia, Gordonia, Graphium, Hydrogenophaga, Nocardia, Methylobacteriurn, Mycobacterium, Pseudornonas, Rodococcus, and Sphingornonas. Oxygenate plume stabilization may be more related to population density and site conditions than to the intrinsic biological capabilities on site. Increasing evidence is being found and reported on the biological natural attenuation of MTBE in gasoline contaminated aquifers. For example, Bradley et al. [l51 reported that microorganisms indigenous to streambed 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. [5] 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. [l61 concluded that microbial degradation of MTBE might 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 [l71. Additional research is in progress nationally to further define the conditions most favorable for anaerobic biodegradation of gasoline components and MTBE.

8 Ether-oxygenate degrading organisms are not here

A common misconception concerning bioxemediation of ethers (and other compounds for that matter) is that organisms capable of degrading compounds of concern may not be present at a given cleanup site. Given the diversity of organisms capable of degrading ethers and the huge microbial populations present in soil, microorganisms capable of degrading ethers should be ubiquitous. Indeed, the observation of numerous, phylogenetically diverse microorganisms capable of biodegrading MTBE worldwide suggests the capability is geographically widespread. Nearly 20 isolates, capable of using MTBE or other low molecular weight oxygenates as a sole carbon source, have been identified to at least the genus level. Oxygenate plume stabilization may be more related to population density and site conditions than to the innate biological capabilities on site. Since ether linkages occur naturally in organic compounds ranging from essential oils of many plants, to phenolic glycosides, to nucleic acids, the ability to metabolize them, if not universal, is certainly wide spread in nature. Appropriate biostimulation may be required to increase the microbe population density of a desired strain to achieve measurable degradation rates. Since microbial population density will increase according to Von Liebig's Law of the Minimum, failure to select the appropriate stimulant(s) or to allow sufficient time for population density to increase, will produce negative evidence leading to the conclusion that capable organisms are not present at the site. Indigenous populations have been stimulated to

degrade MTBE by the addition of growth limiting inorganic nutrients and specific organic CO-metabolites including iso-pentane, propane and butane. Microcosm studies extended beyond the traditional 90-day incubation frequently show positive results for ether degradation. Further support for widespread biodegradation

capability comes from the recent discoveryldocumentation of oxygenate biodegradation under a broad range of redox potentials ranging from aerobic to anaerobic including denitrification, iron reduction, sulfate reduction, and methanogenic conditions.

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 often 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 [l81 and by rnethanogenic conditions 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 and especially on LUFTs. 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 1191. 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 [l31 has confirmed that remediation technology selection; progress and costs are very site specific. Remediation progress and costs are primarily driven by the:

226 Water Pollution \//I: Modclling, Mca~uringand Prediction

Amount and duration of the release 0 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 0 Required cleanup objectives

The specific, individual chemicals in the gasoline do not generally drive progress and cost. However, some constituents, due to toxicity, have driven progress and costs at some sites.

12 MTBE causes cancer

Most information on the toxicology of ether oxygenates comes from laboratory studies of their effect 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 [g]. Furthermore, 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. The weight of evidence suggests that ingestion of water below or close to the taste threshold is unlikely to result in adverse health effects [10]. 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 since 1979. During the treatment a tube is inserted into the gallbladder through which the MTBE is applied. The MTBE dissolves much of the fat content of the stone causing it to disintegrate. Hellstem's [20] review of the effects of this treatment on 761 patients treated in 21 centers across Europe found no toxic effects from MTBE in any of the patients.

Conclusions

Much of the confusion surrounding fuel oxygenates and MTBE is based largely on myths and misconceptions conceived to rationalized field observations or explain incomplete data sets. More thorough investigation and evaluation has revealed that the use of MTBE need not be a trade off between clean air and clean water. Like many other chemicals in use by modem society, MTBE is manageable by the same processes and precautions used for gasoline and other fuel hydrocarbons.

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