Research 

RESEARCH

Should MTBE be Banned or Limited to 3% by Volume in Gasoline?

Matthew C. Small, Hydrogeologist,

Mineral Engineering, U.C. Berkeley, U.S. Environmental Protection Agency

U.S. Environmental Protection Agency (EPA) statistics indicate that over 1 million petroleum underground storage tanks (USTs) have been closed nationwide in the last 10 years, leaving about 920,000 regulated USTs currently in service. Unfortunately, many of these tanks are known to have leaked, contaminating soil and ground water. EPA estimates there are currently over 350,000 confirmed releases or leaking underground storage tanks (LUSTs) in the U.S. 

Until recently the chemical of greatest concern at these LUST sites was benzene,  a naturally occurring hydrocarbon refined from crude oil, found at all gasoline release sites.  Benzene content in gasoline typically ranges between 1% and 4% by volume. Benzene is classified as a known human carcinogen, and even low level exposures are considered unacceptable (5 ppb federal MCL and 1 ppb California MCL in drinking water).  When benzene is released to the environment, it tends to dissolve slowly and move relatively slowly.  Once released, benzene also breaks down (degrades) fairly quickly, minimizing the potential for human exposure.

With the advent of the 1990 Clean Air Act requirements for increased oxygen content in reformulated gasoline to reduce automobile tailpipe emissions, methyl tertiary butyl ether (MTBE) has been added to gasoline at levels up to 14% by volume (11% in California).  Because MTBE has only recently been added to gasoline in high volumes, it is not found at all gasoline release sites.  However, when released, MTBE tends to dissolve quickly and migrate with ground water without breaking down.  These properties along with the relatively high concentration in the gasoline source maximize the potential for human exposure.  Though the toxicity of this compound is currently being debated (U.S. EPA has set a consumer acceptability advisory level of 20-40 ppb), it is clear that there is a great potential for human exposure to this chemical in ground water.

This presentation will discuss recent work undertaken in cooperation with EPA scientists to evaluate the potential environmental impacts of banning MTBE versus limiting MTBE content in gasoline to 3% by volume.  The evalutaion begins with an analysis of the potential for ongoing undetected leaks from underground storage tanks.  Even USTs that are in full compliance with EPA regulations can release hundreds to thousands of gallons of gasoline per year.  Based on the potential for ongoing releases of MTBE, the evaluation continues with a series of simulations and calculations of potential plume lengths under conditions of natural ground water flow and transport.  Plume lengths were simulated over a range of hydraulic conductivities to simulate a range of geologic settings.  Predicted plume lengths vary by 10% between the 3% and 11% MTBE simulations.  The final portion of the analysis develops a new approach for using capture zone modeling to predict impacts to pumping wells.  This approach uses latin hypercube analysis to simulate a range of geologic settings, source sizes, and source locations to estimate impacts to shallow drinking water wells from MTBE.

PROJECTS

Preliminary Risk Evaluation, Tuba City, Arizona

Co-wrote Preliminary Risk Evaluation to support issuance of a unilateral administrative order (UAO) for site assessment and remediation.  Reviewed existing documents, interviewed local experts, performed source-pathway-receptor analysis, and performed risk calculations.  Coordinated technical input and review from other sections as needed.  Provided draft language used by office of regional counsel to create UAO.

Risk-Based Corrective Action (RBCA), U.S. EPA Region 9.

Participated in creation of ASTM standard for "Risk-Based Corrective Action (RBCA) at Petroleum Release Sites".  Publicized RBCA standard by writing articles for national publications and presenting at conferences throughout the U.S.  Worked with EPA Region 9 state underground storage tank (UST) programs (Arizona, California, Hawaii, and Nevada) to implement RBCA for setting cleanup standards at leaking UST sites.  Provided training, program review, and technical oversight.  Currently participating as member of technical oversight committee for demonstration of RBCA at Military bases in California.  Currently working to implement RBCA locally in City of Oakland in cooperation with Oakland Redevelopment and Mayor's Office.

U.S. EPA Region 9, Indian Lands Underground Storage Tank Program, Arizona, California, and Nevada 

Worked to design and implement underground storage tank (UST) program on Indian Lands in U.S. EPA Region 9 (Arizona, California, and Nevada).  Routinely coordinated with Tribal personnel, EPA headquarters, EPA regional staff, State UST program staff, and the regulated community  to solve problems, facilitate communication, provide funding, and solve problems.  Participated in design and operation of database for tracking leaking USTs.  Reviewed consultant reports for site assessments, corrective action, and site closure.  Participated in EPA National and Regional workgroups for a variety of issues related to USTs on Indian Lands.  Provided training to Tribes and regulated community on UST installation, leak detection, site assessment, risk-based corrective action, and remediation.  Managed contracts to provide additional training, create a consultant's manual, and provide data validation.  Managed 8001 demonstration grant to Navajo Nation.

Hydrogeologic Investigation and Contamination Remediation, Livermore, California

Managed day to day operations.  Wrote project proposal and budget.  Maintained contact with clients and regulators.  Supervised Junior level staff.  Constructed and interpreted geologic sections to determine aquifer configuration.  Collected and interpreted pump test data to determine aquifer characteristics.  Performed analytical modeling of ground water flow for capture zone estimation.  In charge of potentiometric surface mapping, ground water quality sampling, and data trend interpretation.  Designed, installed, and developed monitoring and extraction wells.  Logged subsurface materials and collected soil samples for chemical analysis.  Assisted with installation and operation of ground water extraction and carbon filtration system.  Produced regulatory compliance reports submitted to the California Regional Water Quality Control Board.

Hydraulic Testing and Modeling, Superfund Site (CERCLA), Livermore, California

Performed and interpreted aquifer tests, to determine aquifer parameters.  Developed a method for applying Rose diagrams to determine preferred directions of hydraulic communication.  Developed computer code for plotting Rose diagram data.  Determined parameters for input to analytical and numerical ground water flow and contaminant transport computer models.  Conducted sensitivity analysis for modeling parameters.  Performed computer modeling using CAPTURE, PLUME, and CFEST codes.  Produced reports detailing potential for contaminant migration based on modeling results, submitted to the U.S. Environmental Protection Agency and the California Regional Water Quality Control Board.

Hydrogeologic Investigation, Superfund Site (CERCLA), Mountain View, California

In charge of collection, compilation, and interpretation of water chemistry data, and production of isoconcentration contour maps.  Created and maintained computer database for water chemistry data.  Determined aquifer characteristics from potentiometric surface and pump test data.  Developed computer programs used to produce contour maps and three dimensional mesh surface plots of chemical concentration and water elevation data.  Produced sections of reports submitted to the U.S. Environmental Protection Agency and the California Regional Water Quality Control Board.

Hydrogeologic Investigation and Well Installation, Sunnyvale, California

In charge of potentiometric surface data interpretation, and collection of water samples for chemical analysis.  Logged subsurface materials, collected soil samples for chemical analysis.  Designed, installed, and developed monitoring and extraction wells.  Interpreted geologic sections from well logs to help determine aquifer configuration.  Produced sections of reports submitted to the U.S. Environmental Protection Agency and the California Regional Water Quality Control Board.