Tuesday, November 6, 2007 : 10:10 a.m.

Enhanced MTBE Degradation and TBA Production Resulting from Ethanol Releases at an Operating Retail Service Station

Barbara Mickelson, Acton-Mickelson Environmental, Inc, Jeffrey A. Johnson, Ph.D., Acton-Mickelson Environmental Inc and Timothy M. Strawn, ExxonMobil

Although the use of MTBE as an additive in gasoline has recently been reduced, it remains a widespread contaminant in the subsurface. Under anaerobic conditions, MTBE may partially degrade to tert-butyl alcohol (TBA), a compound that is more soluble and more difficult to remove from water than MTBE. Recent studies indicate that releases of ethanol  into the subsurface produce strongly anaerobic conditions. Concerns exist that recent releases of ethanol-enriched gasoline may act to promote the transformation of remnant MTBE to TBA. This paper documents data from a retail site, which include stratigraphic information, vapor and ground water sample results, and isotopic analyses, that indicate releases of ethanol have enhanced the degradation of MTBE and production of TBA. The site is an active retail facility characterized by a perched ground water unit approximately 45 feet bgs. Low levels of MTBE were reported in perched zone samples from November 1997 to February 1999, prior to the introduction of ethanol. TBA was not reported in ground water samples collected during this period. Elevated TBA to MTBE concentrations were initially detected in site wells in April 2005 with concentrations of MTBE  up to 17,000 ug/L and TBA up to 160,000 ug/L.  Elevated concentrations of TBA relative to MTBE are observed in samples from monitoring wells nearest the USTs, the likely source of the ethanol.  Similar trends are not observed in samples from monitoring wells distant from the USTs. In 2006 ethanol was detected in soil vapor and perched ground water. Vapor phase ethanol appears to have migrated within the vadose zone and dissolved into perched ground water enhancing the degradation of MTBE to TBA.  Isotopic analyses of samples from monitoring wells nearest the USTs indicate that MTBE degradation is occurring while samples from the distant monitoring wells indicate MTBE degradation has not occurred.

Barbara Mickelson, Acton-Mickelson Environmental, Inc Barbara Mickelson, President of Acton • Mickelson • Environmental, Inc., is a Professional Engineer in California, Texas, and Wyoming with 30 years experience in the assessment of contaminant impacts to soil and ground water at refineries, chemical plants, and ordnance manufacturing facilities. Her experience includes analysis of environmental risks associated with organic and inorganic substances and their remediation and mitigation. She has overseen the design and construction of water and wastewater treatment systems for municipal and industrial facilities. She has expertise in mining project permitting including surface and ground water quality issues, geomorphology, dam and embankment design and engineering.

Jeffrey A. Johnson, Ph.D., Acton-Mickelson Environmental Inc Dr. Jeffrey Johnson has over 25 years of professional experience in natural resource assessment, site characterization, remedial operations, and technical analyses. He has worked on various projects throughout North America for major petroleum corporations, legal firms, manufacturing companies, and the U.S. Environmental Protection Agency. Dr. Johnson has extensive technical skills in contaminant transport and fate of hydrocarbons, coal tar, and chlorinated solvents. For the past 10 years, he has worked extensively in the characterization and remediation of LNAPL and DNAPL sites. He holds a Ph.D. in Geology from the University of California, Berkeley, is a Certified and Registered Professional Geologist.


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Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Remediation® Conference