mtbe: groundwater remediation technologies
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IIIIIII MTBE: Groundwater Remediation Technologies
Jeffrey A. Hassen Cheyne l? Gross
Jeffrey A. Hassen is a project director with Environmental Strategies Corpora- tion in Pittsburgh, Pennsylvania. Cheyne P. Gross is a senior engineer with Environmental Strategies Corpora- tion in Pittsburgh, Pennsylvania.
The groundwater remediation of methyl-tertiaiy-butyl ether (MTBE) presents significant challenges due to the physicocheniical properties of MTBE. The high solubility in water, low Henry's law constant, and low affinity for organic fractions in the vadose and saturated zones increase the difficulty o f groundwater remediation. This column examines the effectiveness and cost of various conventional groundwater treatment technologies to remediate MTBE-af- fected groundwater and discusses new developments in MTBE ground- water remediation. Where appropri- ate, the properties of MTBE are coni- pared to other volatile organic coni- pounds (VOCs) t o give the reader an appreciation for why the recent use of MTBE has resulted in the rapid and widespread occurrence of MTBE in groundwater.
OVERVIEW OF MTBE MTBE is a gasoljne additive used
to enhance octane levels in gasoline and as an oxygenate to decrease vehicular emissions. MTBE has been added to gasoline by petroleum com- panies to raise the octane level and improve combustion since 1979 when lead was phased o u t of gasoline. In response to the 1990 federal Clean Air Act Amendments (CAAA), oil coni- panies began using NITBE extensively
in 1992 in oxygenated gasoline a i d reformulated gasoline (RFG). Oxy- genated gasoline and RFG are two classes of gasoline that contain differ- ent amounts of oxygen as discussed below. The CAAA requires the use of RFG in areas of the United States that exceed the national air quality stan- dards for carbon monoxide (carbon monoside nonattainment areas), VOCs, and ozone emissions. Seven- teen states and the District of Colum- bia currently use RFG, either because of a congressional mandate or on a voluntary basis. Oxygenated gaso- line contzdins a minimum of 2.7 per- cent oxygen by weight or 14.8 per- cent MTBE by volume. RFG contains a minimum of 2.0 percent oxygen by weight or 11 percent aromatic hydro- carbon by volume (Delzeret al., 1996). MTBE is present in over 70 percent of the gasoline sold in the United States (Happel et al., 1998).
Because of MTBE's low cost, ease of production, and favorable blend- ing characteristics, MTBE is the most commonly used fuel oxygenate. MTBE is used in approximately 87 percent of RFG. The second most used fuel oxygenate is ethanol. Other fuel oxygenates in use include metha- nol, ethyl tert-butyl ether, tert-amyl methyl ether, and diisopropyl ether (Zogorski et al., 1997). MTBE is made from methanol, a by-product of the
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petroleum refining process. It blends easily with gasoline and remains in solution; thus, the MTBE gasoline blend can be transferred throughout existing pipelines without risk of deg- radation in gasoline quality. The U.S. production of MTBE exceeds 4.5 bil- lion gallons annually for use in gaso- line. This usage represents a more than threefold increase in MTBE pro- duction since the congressional man- date in 1990.
The U.S. Environmental Protec- tion Agency (USEPA) has indicated that levels of airborne toxins have been reduced by 32 percent in the northeast United States since the federal government required the use of RFG. However, its use has also resulted in unacceptable impacts to surface water and groundwater in several areas of the United States. In addition, the USEPA has tentatively classified MTBE as a possible human carcinogen, but no federal drinking water standard has been established (USEPA, 1997). The USEPA has is- sued a drinking water Health Advi- sory for MTBE of 20 to 40 pg/L (micrograms per liter). The advisory provides guidance to communities exposed to drinking water contami- nated with MTBE. The Health Advi- sory is based on taste and odor thresholds. The advisory concentra- tion provides a large margin of safety for noncarcinogenic effects and is in the range of margins typically pro- vided for potential carcinogenic ef- fects. USEPA has also placed MTBE on the Drinking Water Contaminant Candidate List (CCL). Contaminants on this list are prioritized for further evaluation within the USEPAs drink- ing water program. USEPA has ranked MTBE as a chemical that needs additional occurrence, treat- ment. and health data.
Several states have begun to take steps to reduce or phase out MTBE use. For example, Governor Gray Davis of California signed an Execu- tive Order in March 1999 calling for MTBE use to be phased out in Califor- nia by 2002. In addition, the USEPA commissioned a blue ribbon panel of industry experts to review public health issues associated with MTBE contamination of water supplies. The panel issued a report in September 1999 that recommended significant reduction in the use of MTBE, called on Congress and USEPA to lift the oxygenate mandate, and suggested strengthening existing regulatory pro- grams to reduce MTBE contaniina- tion of drinking water supplies across the United States.
In a March 20,2000 press release, the USEPA and the Department of Agriculture (DOA) announced ac- tions by the Clinton administration to significantly reduce or eliminate the use of MTBE and support the use of more environmentally safe alterna- tives such as ethanol. The USEPA and DOA released a legislative frame- work to encourage congressional action to amend the Clean Air Act, reduce MTBE use, and support a renewable fuel standard (particularly ethanol). USEPA also announced that regulatory action to eliminate or phase out MTBE has been initiated by issu- ing an Advanced Notice of Proposed Rulemaking under Section 6 of the Toxic Substance Control Act.
FATE AND TRANSPORT OF MTBE IN THE ENVIRONMENT
MTBE contamination has been reported at varying levels in air, sur- face water, and groundwater through- out the United States. This contami- nation results from point and nonpoint sources. Examples of point sources
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MTBE: GROUNDWATER &MEDIATION TECHNOLOGIES
include gasoline spills, pipeline rup- tures, and leaking aboveground and underground storage tanks. Nonpoint sources may include urban runoff, MTBE-affected precipitation, exhaust from 2-cycle boat engines, and dif- fuse groundwater discharges to sur- face water.
At ambient temperatures, the solu- bility of pure MTBE in water is about 50,000 mg/L (approximately 5 per- cent by weight), which is relatively high for most petroleum contami- nants. For example. benzene has a solubility of 1,780 mg/L (Mackay et al., 1773). However, the solubility of MTBE in water is reduced when it is present with other organic com- pounds in gasoline. MTBE in a gaso- line mixture that is 10 percent by weight reduces its water solubility to 5,000 mg/L at 25 "C (Squillace et al., 1777). This reduced solubility in wa- ter is caused by MTBE partitioning between the organic mixture in gaso- line and water.
MTBE tends to partition weakly to the organic fraction in soils, sedi- ments, and suspended particles pre- ferring to remain in the aqueous phase. The high solubility of MTBE in water and low affinity for organic carbon in soil has contributed to MTBE migrating from a source area to groundwater at practically the same velocity as precipitation recharges a water table aquifer. The retardation factor for MTBE is close to 1 or basically equivalent to water. Thus, MTBE migrates in an aquifer at prac- tically the same rate as the local groundwater flow velocity. In com- parison, benzene, toluene, ethyl- benzene, and xylenes (BTEX) com- pounds have retardation factors of 1.1 to 2 (Zogorski et al., 17771, which is generally why the MTBE conipo- nent of a groundwater plume en-
compasses a much larger area than BTEX compounds.
MTBE is only moderately vola- tile when moving from the dissolved to the vapor phase. Henry's law con- stant for MTBE is 0.022 at 25 "C (Robbins et al., 1773); a compound with a value of 0.05 or larger is considered highly volatile from wa- ter. In comparison, benzene has a Heniy's law constant of 0.22 at 25 "C (Howard et al., 1770), which indi- cates it is ten times more volatile from the dissolved phase to the vapor phase than MTBE. This explains why MTBE is more difficult than benzene to remove from groundwater using conventional air stripping o r air sparging remediation technologies.
MTBE's vapor pressure is approxi- mately three times higher than ben- zene, but MTBE's Henry's law con- stant is significantly lower than ben- zene as discussed above. Therefore, in product form, MTBE is more vola- tile than benzene, but when dis- solved in water, MTBE is much less volatile. Due to these physical prop- erties, MTBE is more difficult to ad- dress when dissolved in groundwater than it is when trapped in the vadose zone or capillary fringe in LNAPL form. Unfortunately, MTBE has a low affinity for adsorption, therefore it migrates through the vadose zone more rapidly than BTEX. Because of MTBE's volatile nature, soil vapor extraction is an excellent technology for addressing vadose zone contami- nation. However, this column fo- cuses on the remediation of ground- water plumes containing both MTBE and BTEX.
MTBE does not readily biode- grade in surface or groundwater. It is persistent in the environment be- cause the ether bond is stable under typical environmental conditions and
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requires acidic conditions to break down the molecular structure. In add