Remediation and treatment of MTBE

Methyl tertiary butyl ether (MTBE) has been used as a constituent in gasoline since 1979, initially to increase octane levels, but more recently to meet fuel oxygen requirements mandated by the Clean Air Act. Beginning in 1992, the U.S. Environmental Protection Agency (EPA) has required the use of oxygenated fuels in carbon monoxide (CO) and ozone non-attainment areas nationwide to reduce vehicle emissions of a variety of chemicals, notably CO and hydrocarbons.

Although the air quality benefits of reformulated gasoline (RFG) with oxygenates have been demonstrated for reducing CO, ozone forming hydrocarbons and other toxic air pollutants (Spitzer, 1997; Office of Science and Technology Policy (OSTP) (White House), 1997; Kirchstetter 1999a, b), concern has been raised regarding the impacts on drinking water sources due to releases of gasoline containing oxygenates from underground storage tanks, pipelines and other emission sources such as motorized recreational vehicles.

Releases of gasoline containing MTBE to the subsurface from petroleum facilities - refineries, terminals, pipelines or service stations - have been documented throughout the United States. Over the past 10 years, the nationwide program to upgrade underground fuel tanks has eliminated many significant sources of gasoline releases to the environment.

Proper enforcement and maintenance of the tank upgrade requirements is expected to significantly reduce the number and magnitude of future gasoline releases to the environment from underground fuel tanks. However, because the upgrade requirements are not yet fully implemented or enforced, releases of gasoline containing oxygenates, including MTBE, will continue to occur, and are a continuing source of concern to water utilities, owners of private wells and other entities responsible for water quality management.

It is important to examine the remediation and treatment of MTBE contaminated soil and groundwater in the presence of conventional gasoline, as represented by the primary hazardous chemicals benzene, toluene, ethylbenzene and xylenes (BTEX).

Fate and transport of oxygenates
The fate and transport of MTBE, other oxygenates and chemicals in conventional gasoline through the subsurface are well understood (OSTP, 1997). Following an MTBE and BTEX release from an underground fuel tank, gasoline constituents travel through the vadose zone - the unsaturated zone between the ground surface and water table - and eventually reach the top of the water table in the capillary fringe as a non-aqueous phase liquid (NAPL).

MTBE has a higher vapor pressure than BTEX in gasoline, and is, therefore, expected to evaporate from the downward migrating NAPL to form a vapor phase plume. However, the high vapor density and low Henry's Law constant for MTBE will likely cause this vapor phase plume to continue moving toward the water table. The rate of this migration depends on, among other factors, soil moisture content. Once the NAPL has reached the capillary fringe, the high effective solubility - 6,000 parts per million (ppm) - and low soil adsorption coefficient will cause MTBE to partition into the aqueous phase, where it will migrate at the same rate as groundwater.

To date, only a few studies of MTBE plumes have observed natural biodegradation. Based on the Lawrence Livermore National Laboratory report (Rice, 1995, 1995), BTEX plumes have been shown to stabilize or attenuate in most hydrogeologic settings, due to the combined action of biodegradation, dispersion, and dilution. MTBE plumes, however, are expected to attenuate much more slowly than BTEX plumes, due to the observed low rate of biodegradation. Thus, MTBE plumes are expected to become larger than BTEX plumes and, therefore, threaten to contaminate a larger volume of water compared to BTEX plumes.

Remediation strategies and plume size
Despite these expectations, however, in four MTBE plume studies completed to date, MTBE plumes were not observed to elongate indefinitely (Davidson, 1995; Buscheck et al., 1998; Happel et al., 1998; Mace and Choi, 1998). Buscheck et al. reported that in over 50 percent of the plumes studied, the MTBE plume was equivalent in size to the BTEX plumes.

Similarly, the other studies showed that the majority of MTBE plumes were not substantially longer than the BTEX plumes, typically less than twice as long as BTEX plumes. Even if the results presented in these plume studies represent transient plumes, the results are significant because they indicate that if remediation or containment is implemented quickly, the volumes of MTBE- and BTEX-contaminated soil and groundwater will be similar.

Remediation strategies
The goal of a remediation strategy is to capitalize on the unique properties of the target contaminant by selecting technologies that enhance removal processes that do not readily occur under normal conditions. By understanding the chemical and physical properties of oxygenates, including MTBE, it is possible to select the best remediation strategy for any particular contamination scenario (See Table 1).

Table 1

Properties of BTEX and MTBE

Physical and chemical properties

Benzene

Toluene

Ethyl-benzene

o-Xylene

MTBE

Molecular weight g/mole

78.11

92.14

106.17

106.17

88.15

Vapor density @ 1 atm; 10oC

3.36

3.97

4.57

4.57

3.80

Specific gravity @ 25oC

0.881

0.86691

0.8671

0.88021

0.7441

Water solubility mg/L

17301

534.81

1611

1751

43,000
54,300
50,0001

Vapor pressure mm Hg @ 25oC

76, 95.191

28.41

9.531

6.61

245-2761

Henry's Law constant -

0.232

0.2722

0.3362

0.2122

0.023991
0.044961
0.057221
0.12261
0.0261
0.018 @ 20oC1

Log KOC

1.18-1.991
1.50-2.161

1.56-2.251

2.94
1.98-3.041

1.68-1.831

1.091, 1.035
1.0491

Log KOW

2.362

2.732

3.242

3.102

1.201

1 OSTP, 1997. 2 Crittenden et al, 1997.

The properties of MTBE are sufficiently unique compared to the BTEX compounds - e.g., higher vapor pressure, higher solubility, lower adsorption - to indicate that remediation of MTBE is technically feasible where such technologies exist that can capitalize on these unique properties.

Six of the most commonly used remediation strategies at underground storage tank (UST) sites have been evaluated for their effectiveness to remediate MTBE relative to BTEX (Malcolm Pirnie, 1999). As was earlier indicated, the physio-chemical properties of MTBE suggest that it is highly amenable to remedial strategies that capitalize on its high vapor pressure, high solubility and low adsorption. However, hydrogeologic conditions can severely limit the success of any remediation strategy. For example, highly stratified zones or low permeable regions will impede the removal of contaminants from the subsurface, independent of the presence of MTBE.

In general, there are two types of remediation strategies: 1) phase transfer technologies, such as soil vapor extraction (SVE), air sparging, multi-phase extraction, and air or water flushing (pump and treat); and 2) destruction and/or transformation technologies such as in situ oxidation and bioremediation. The high vapor pressure of MTBE suggests that strategies using SVE and multi-phase extraction will be successful for removing MTBE. The high solubility and low adsorption indicate that strategies employing air sparging (Johnson, 1998) and flushing technologies can also be successful for removing MTBE from the saturated zone.

For destruction technologies, MTBE is an organic compound that can be oxidized, indicating that in situ oxidation will be just as effective for MTBE relative to BTEX. The predicted success of these technologies for MTBE remediation has been confirmed in several reports. In a recent survey of 48 state leaking underground fuel tank (LUFT) programs, air sparging, pump and treat, multi-phase extraction and soil vapor extraction were all listed as successful in remediating MTBE contaminated sites (Hitzig, 1998).

Bioremediation
Despite these verified successes for remediating MTBE with conventional technologies and strategies, the presence of MTBE may add significant costs to overall remediation costs for LUFT sites due to the limitations of natural attenuation. However, a number of researchers have recently succeeded in biodegrading MTBE in the subsurface under aerobic conditions. In addition, there is also limited evidence of MTBE anaerobic biodegradation in situ. These biodegradation successes suggest that strategies to enhance the natural attenuation of MTBE can be designed to effectively manage this seemingly persistent chemical.

Under certain conditions, MTBE will biodegrade in the presence of oxygen, through increased activity of a monooxygenase enzyme. This biodegradation may be the result of co-metabolic degradation in the presence of n-alkanes or benzene (Hardison, 1997; Steffan, 1997; Hyman, 1998; Koenigsberg et al., 1999), or it may result from the use of MTBE as the sole carbon and energy source (Ewies et al., 1997; Mo et al., 1997; Park and Cowan, 1997a & 1997b; Fortin et al., 1998). Regardless of the specific aerobic mechanism, the presence of oxygen has been shown to enhance the biodegradation of MTBE (Koenigsberg, 1997; Park and Cowan, 1997b; Salanitro, 1998; Yang, 1998; Koenigsberg et al., 1999).

Even though reports in the scientific literature vary to some extent, several conclusions can be made regarding the biodegradation potential of MTBE in the subsurface by indigenous microbial populations:

1. The biodegradation of MTBE will occur aerobically either with MTBE as the sole carbon and energy source or co-metabolically with certain hydrocarbons;

2. The cellular yield of microorganisms utilizing MTBE as the sole organic carbon source can be expected to be very low, either because MTBE serves as a poor carbon and energy source, or because some of its metabolic intermediates may inhibit cellular growth;

3. The presence of more easily biodegradable organics in the subsurface may inhibit MTBE biodegradation; and

4. The rate of MTBE biodegradation is positively correlated with the surrounding concentration of molecular oxygen and is, therefore, likely stimulated by the introduction of oxygen in the subsurface.

Treatment
If groundwater flushing or pumping is used as the treatment technology, aboveground treatment of the extracted groundwater will be required. Typically, this is done using air stripping, possibly combined with off-gas treatment; granular activated carbon (GAC); or advanced oxidation. Each of these treatment technologies and their respective costs has been discussed in detail in a recent report published by the MTBE Research Partnership titled, Treatment Technologies For Removal of MTBE From Drinking Water (1998).

In brief, air stripping was determined to be the most cost-effective tool for MTBE removal from drinking water; however, the potential for off-gas treatment could increase costs due to increased regulatory requirements and off-gas treatment implementation costs. Air stripping is currently being used in at least two drinking water applications in the United States: LaCrosse, Kan. and Rockaway Township, N.J. (McKinnon, 1984; MTBE Research Partnership, 1998).

GAC represents the most expensive of the three options. However, GAC is a well-understood technology that will soon receive regulatory acceptance in Santa Monica, Calif., for removing MTBE from drinking water. Finally, while advanced oxidation has been demonstrated to oxidize a wide range of organic chemicals, including MTBE, there are still many uncertainties regarding its implementation, specifically the formation and fate of oxidation byproducts, and the impact of waters containing levels of bromide above 0.1 milligrams per liter (mg/L).

Conclusion
MTBE behaves differently than conventional gasoline constituents in the subsurface, primarily due to its much higher solubility and lower adsorption tendency. In addition, MTBE has not been observed to rapidly biodegrade in the subsurface, which suggests that MTBE may often behave as a conservative tracer - a nondegrading chemical injected into groundwater used to determine groundwater flow paths - until oxygen levels exceed some threshold, apparently, above 2 mg/L. However, in four recent plume studies most MTBE plumes were not observed to elongate indefinitely. Conversely, in many of the plumes analyzed, the MTBE plume was not substantially longer than the BTEX plume.

These observations suggest that if unit remediation costs are similar, MTBE should not add significant costs to remediation, despite the cost increases which are expected for characterization and treatment - e.g., air stripping, GAC and advanced oxidation - of MTBE plumes relative to BTEX plumes. The observed success of proven remediation technologies - i.e., SVE, air sparging, multiphase extraction, pump and treat, in situ oxidation - for removing MTBE from the soil and groundwater indicate that the feasibility and costs of MTBE remediation is generally comparable to BTEX remediation.

Although life cycle costs of MTBE remediation are generally similar to costs for BTEX-only plumes where active remediation is applied, cumulative costs for remediation of all MTBE sites will be higher compared to BTEX-only sites because of restrictions on the use of natural attenuation at LUFT sites containing MTBE.

Consequently, there is a substantial amount of ongoing research in the United States to develop strategies for effectively enhancing the biodegradation of MTBE in the subsurface. Recent bench- and field-scale research programs are promising, indicating that under aerobic conditions, MTBE can likely be biodegraded in situ by the use of engineered systems.

In situ biodegradation holds the potential for reducing life cycle costs of MTBE remediation, compared to the use of physical-chemical based systems. In summary, current and new technologies are capable of remediation of LUFT sites containing MTBE. Early detection of gasoline releases and rapid implementation of the appropriate technology combinations will provide protection of water resources from unintentional releases of gasoline containing MTBE or other oxygenates.

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This article originally appeared in the 04/01/1999 issue of Environmental Protection.

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