MTBE: Groundwater remediation technologies

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<ul><li><p>IIIIIII MTBE: Groundwater Remediation Technologies </p><p>Jeffrey A. Hassen Cheyne l? Gross </p><p>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. </p><p>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. </p><p>OVERVIEW OF MTBE MTBE is a gasoljne additive used </p><p>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 </p><p>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). </p><p>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 </p><p>0 2000 John Wiley &amp; Sons, Inc. 129 </p></li><li><p>JEFFREY A. HASSEN CHEYNE P. GROSS </p><p>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. </p><p>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. </p><p>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. </p><p>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. </p><p>FATE AND TRANSPORT OF MTBE IN THE ENVIRONMENT </p><p>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 </p><p>~ </p><p>130 REMEDIATION/SUMMER 2 0 0 0 </p></li><li><p>MTBE: GROUNDWATER &amp;MEDIATION TECHNOLOGIES </p><p>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. </p><p>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. </p><p>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- </p><p>compasses a much larger area than BTEX compounds. </p><p>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. </p><p>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. </p><p>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 </p><p>&amp;MEDIATION/SUMMER 2000 131 </p></li><li><p>JEFFREY A. HASSEN CHEYNE P. GROSS ~~ _________ ______ ~ ~ ~ </p><p>- </p><p>132 REMEDIATION/~UMMER 2000 </p><p>requires acidic conditions to break down the molecular structure. In addition, the bulky nature of the twt- butyl group does not permit easy access to the ether linkage. There- fore, indigenous microbes do not tend to rapidly biodegrade MTBE. Tertiary butyl alcohol (TBA) has been documented as a transformation prod- uct of MTBE in a number of studies (Keller et al., 1998). </p><p>GROUNDWATER COLLECTION TECHNOLOGIES </p><p>Two groundwater collection tech- nologies are discussed in this sec- tion-pump and treat and dual vacuum extraction (DVE)-both of which require ex-situ treatment of the extracted groundwater, which is discussed later in the column. Pump and treat and DVE physically remove the contaminants from the subsur- face, and therefore require subse- quent treatment. </p><p>Pump and Treat Although pump and treat is not </p><p>generally considered to be an effec- tive groundwater remediation tech- nique, it is considered effective for MTBE remediation because of MTBEs high solubility and low adsorption to organics in soil. Fewer aquifer vol- umes are needed to remove MTBE mass than are required for the re- moval of BTEX compounds. In addi- tion, because of MTBEs high solubil- ity and affinity for water, a high percentage of MTBE mass may quickly dissolve in groundwater and be re- covered by pump and treat systems. </p><p>Groundwater pumping can be performed from recovery wells or interceptor trenches depending on the hydrogeologic characteristics of the site. If the remediation goal is to prevent MTBE migration, recovery </p><p>wells will be distributed across a site based on a calculated radius of influ- ence around each recovery well. If an interceptor trench is used, it will be designed to extend completely across the downgradient edge of the plume, perpendicular to groundwater flow. Pumping rates will be set to the mininiuni value necessary to achieve hydraulic containment. </p><p>If concentration reduction or mass removal is the remediation goal, re- coveiy wells or sumps can be located at strategic locations based on con- taminant source location. Pumping rates can be set to optimize the pounds of mass removed per dollar, includ- ing both capital and operation and maintenance costs. The rate of ex- traction will be related to the rate of diffusion of contaminant mass from the adsorbed to dissolved state. The extraction rate will also be controlled by the hydraulic conductivity of the aquifer material. An aquifer with a high total porosity, but low effective porosity will result in an increased MTBE recovery time. </p><p>Dual Vacuum Extraction DVE, also referred to as dual </p><p>phase extraction, is a combination of groundwater pump and treat and soil vapor extraction. The process involves the extraction of air and water from a network of recovery wells. Water can be extracted with a direct vacuum lift, air-entrainment using drop tubes, or submersible pump. The air and water streams are conveyed to a treatment facility and separated. The water and air streams can be treated using the technologies discussed in the Ex-Situ Groundwater Treatment Technolo- gies section of this column. </p><p>During DVE, the groundwater table is lowered which exposes the capillary fringe or smear zone to </p></li><li><p>MTBE: GROUNDWATER REMEDIATION TECHNOLOGIES </p><p>remediation by soil vapor extraction. The permeability of the aquifer mate- rial and the physiochemical proper- ties of the compound of concern determine the effectiveness of DVE. MTBE in the product form will be volatilized and removed from the vadose zone with a high efficiency based on its high vapor pressure, but unfortunately the majority of con- taminants present in the vadose zone or capillary fringe will likely be BTEX compounds due to the higher sorp- tion affinity and lower solubility than MTBE. Other inherent difficulties with DVE of MTBE are similar to pump and treat in that the ex-situ water treatment is more costly than for BTEX compounds. Also, DVE in- creases the oxygen levels in the va- dose zone, which is necessary for the biodegradation of BTEX compounds. Biodegradation of MTBE, however, does not readily occur aerobically as discussed later in this column. None- theless, DVE is an appropriate remediation technology for MTBE. The cost of reinediating a MTBE and BTEX plume will he based on the cost to perform the ex-situ treatment of groundwater and off gas, if re- quired. Other costs associated with installing and operating a DVE sys- tem will be the same for non-MTBE plumes. These costs are based on the physical characteristics (i.e., perme- ability) of the formation and the size of the contaminated plume. </p><p>EX-SITU GROUNDWATER TREATMENT TECHNOLOGIES </p><p>Ex-situ treatment alternatives that are discussed in this section include air stripping, liquid-phase adsorp- tion, hollow fiber membranes, and advanced oxidative processes. Addi- tionally, air sparging, DVE, and pump and treat may require treatment of off </p><p>gas. Off gas treatment technologies include vapor-phase granular acti- vated carbon (GAC) adsorption, cata- lytic oxidation, thermal oxidation, and biofiltration. The off gas treatment technology discussed is vapor-phase carbon adsorption. </p><p>Air stripping Air stripping can be used to treat </p><p>MTBE-impacted groundwater. How- ever, high air to water ratios and increased residence time are required due to the low H...</p></li></ul>


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