Overcoming MTBE Myths
Methyl Tertiary Butyl Ether (MTBE), comprised of carbon, hydrogen and oxygen (C5H12O), has been added to gasoline since the late 1970s, originally replacing lead as an octane enhancer. In 1992, it was introduced in the federal wintertime oxyfuels program to combat carbon monoxide pollution. More recently, MTBE has been added to gasoline to make it burn cleaner in vehicle engines, reducing air pollution. The U.S. Environmental Protection Agency (EPA) has required the addition of two percent oxygen by weight in gasoline in areas of the United States, mainly on the east and west coasts, that are out of compliance with air quality standards. Refiners have made MTBE the oxygenate of choice. Reformulated gasoline's benefits to air quality have exceeded expectations.
On a parallel track, 1988 EPA requirements for improved containment of gasoline (and other products) in underground storage tanks (USTs) are being implemented. EPA reports that as of September 30, 1999, 85 percent of federally regulated USTs were in compliance. As with any petroleum product stored in USTs, accidental releases to soil and groundwater occur, often requiring remediation.
The impression among the public is that remediating MTBE requires specialized technologies and cleanup strategies that go beyond, and cost more than those required for other gasoline constituents. This is generally contrary to experience. MTBE and other gasoline constituents are routinely remediated using long-proven conventional technologies.
Remediation technologies such as groundwater extraction, soil vapor extraction (SVE), and thermal desorption work exceptionally well with MTBE.
Remediating MTBE and Other Gasoline Constituents
MTBE and other gasoline constituents are relatively mobile in the environment. Therefore, prompt release detection and source control are essential. MTBE is soluble in water, but has a low Henry's Law constant, allowing MTBE to dissolve in water and then remain in the aqueous phase. It also has a high vapor pressure, volatilizing readily from gasoline to air. It has a low tendency to adsorb to soil particles, therefore its movement in the subsurface is less retarded than many other gasoline constituents. Like other gasoline constituents, MTBE has a specific gravity less than one and a vapor density greater than one; therefore, in the gasoline phase it floats on water, and in the vapor phase it tends to sink, accumulating in basements, utility trenches, etc.
The same physical properties that make MTBE mobile in the subsurface also make it more easily recovered from the subsurface compared to other gasoline constituents. Remediation technologies, such as groundwater extraction, soil vapor extraction (SVE) and thermal desorption work exceptionally well with MTBE.
Site remediation of MTBE is generally carried out in phases. Minimizing total project cost entails optimizing the level of effort of each phase. Receptor protection and source control are the first tasks, followed by remediation of residual product and dissolved contamination. Monitored natural attenuation may be used to complete the process.
Treating MTBE, once it is recovered from the subsurface, can be accomplished with proven technologies. MTBE's low tendency to adsorb and high Henry's Law constant make it more difficult to treat using granular activated carbon (GAC) adsorption or air stripping, although these technologies are still applicable under the right circumstances and widely used at gas station sites. Use of GAC is limited to situations with relatively low MTBE concentrations. Air stripping will require higher air to water ratios than for other gasoline constituents. Other methods, such as thermal or catalytic oxidation, are equally effective for MTBE and other gasoline constituents.
Like chlorinated solvents, MTBE was initially thought to be recalcitrant to biological treatment; however, research and field data demonstrate that MTBE is biodegradable under in situ and ex situ conditions.
Bioremediation of MTBE is widely accepted. Like chlorinated solvents, MTBE was initially thought to be recalcitrant to biological treatment; however, research and field data demonstrate that MTBE is biodegradable under in situ and ex situ conditions. Bioremediation technology is evolving rapidly, and will likely become even more commonly used to treat gasoline releases containing MTBE.
Lessons Learned from Experience
ENSR has learned many remediation lessons from its extensive project experience at over 1,000 gas station sites, ranging from preliminary site assessment through full remediation and closure. This experience includes remediation at many sites where gasoline containing MTBE had been released. The majority of these sites have received official regulatory closure; nine are anticipated to be closed in the near future after additional post-closure monitoring or other relatively minor activity takes place.
Remediation typically involves some combination of source control, such as soil excavation and separate-phase product removal (31 sites) and remediation of residual product and dissolved contamination in the subsurface by groundwater pump and treat (28 sites), SVE (24 sites) and/or air sparging (5 sites). Monitored natural attenuation was used as a final remediation step at 9 sites and was used alone to close six sites.
Excavated soil from 19 of 29 sites was recycled at an asphalt batching plant; the presence of clay and other fine-textured materials was often the reason soil from the other 10 sites was not recycled by asphalt batching. Recovered water and vapor was treated most often by GAC adsorption (29 sites). Air stripping was used to treat recovered water at five sites, in conjunction with GAC adsorption at three sites, and alone at two sites. Catalytic and thermal oxidation was used to treat recovered vapors at five sites.
Treatment technology selection for recovered groundwater and vapors is site-specific and based on flow rate, initial VOC concentrations, required final VOC concentrations and other factors. For recovered water containing more than approximately 10,000 micrograms per liter (ug/l) of total VOCs, air stripping followed by catalytic oxidation of the stripped vapor-phase VOCs is suitable. Elevated VOC concentrations contribute enough BTUs to preclude the need for large amounts of fuel, making this technology economical.
In the 1,000 to 10,000 ug/l range of total VOCs, air stripping followed by GAC adsorption is often the best approach. For VOC concentrations less than about 1,000 ug/l, GAC adsorption and monitored natural attenuation are appropriate. For vapor-phase treatment, catalytic oxidation is often favored for total VOC concentrations greater than 75 parts per million (ppm) and GAC adsorption is often applied for lower concentrations. In cases where MTBE is the major constituent being treated, all of the above numbers can be reduced by roughly 25 percent.
It often proves effective to change technologies for the treatment of residuals as concentrations decrease with time. It is very common to start with catalytic oxidation and switch to GAC adsorption once concentrations become low enough to make GAC economical. Recently more biologically based technologies such as in situ oxygen addition by means of solid- or liquid-phase oxygen-containing compounds (five sites) have been used in remediation. This has worked very well at some sites, but not at others. It appears that high concentrations of non-target compounds (e.g., iron or total organic carbon), or incomplete source control, or low soil permeability account for cases where oxygen amendment has not been successful.
MTBE's low tendency to adsorb and high Henry's Law constant make it more difficult to treat using granular activated carbon (GAC) adsorption or air stripping.
Remediation costs for MTBE and other constituents are highly site-specific and largely a function of the time the release has gone unremediated, the size of the release, hydrogeologic conditions, and the presence of nearby sensitive receptors and pathways to them. At some sites, MTBE drives the remediation due to high concentrations or mobility; at others, benzene drives the remediation due to benzene's higher toxicity (and thus more stringent required cleanup concentrations).
Generally, releases that have impacted soil but not groundwater cost on the order of $100,000 to remediate (on average). Releases that have impacted both soil and groundwater may cost on the order of $250,000. The most costly gas station remediation is over $4 million (incidentally, MTBE is not a primary constituent at this site). There is a wide range of variability. The impact of the presence of MTBE on remediation cost is likewise site-specific. In many cases, there is little or no impact. At sites where the MTBE plume is larger than the BTEX plume, remediation costs can be higher than they otherwise might have been had MTBE not been a factor. Such sites are often in areas of relatively shallow bedrock or where soil permeability is low and the silt content is high.
There are many technologies for remediating gasoline releases containing MTBE; however, based on the positive results with physical treatment technologies such as groundwater pump and treat, SVE and air sparging, these tried and true methods are most often used. In situ oxygen amendment and other biological methods may be more extensively used in the future. Monitored natural attenuation will likely be used more often as a final polishing step.
It often proves effective to change technologies for the treatment of residuals as concentrations decrease with time.
The most important lesson learned from gas station remediation experience is the importance of release prevention, early detection, and prompt source identification and control in minimizing total costs. Gas station owners should increasingly focus more on these activities so that less remediation effort and cost are required. Historically, releases have been detected long after their occurrence; by then, plumes have grown, often extending beyond the property boundary. In conjunction with the smaller plumes of gasoline constituents that result from early detection and prompt control, in situ bioremediation may come to be more widely used at gas station sites.
Remediation of releases of gasoline containing MTBE has shown that the same technologies that are effective for treating other gasoline constituents are effective for treating MTBE. Addressing MTBE does not need to be overly complicated. It takes an understanding of specific site conditions, appropriate selection and sequencing of remedial technologies, good engineering, adequate monitoring and flexible operation and maintenance. The real key to minimizing costs in gas station remediation projects is release prevention, early detection and effective source control.
This article originally appeared in the May 2001 issue of Environmental Protection, Vol. 12, No. 5, p. 24.
This article originally appeared in the 05/01/2001 issue of Environmental Protection.