METHYL tertiary-butyl ether, more commonly known as MTBE, is a chemical Janus. It benefits air quality by making gasoline burn cleaner, thus reducing automobile emissions. But it can also find its way into groundwater supplies and give drinking water an unpleasant taste and odor. At present, more than 20 public drinking water wells in California have ceased water production for this reason. Worse yet, the health effects of MTBE are uncertain-the U.S. Environmental Protection Agency currently classifies MTBE as a possible human carcinogen. Since 1992, MTBE has been the compound of choice for U.S. oil refineries required by the federal Clean Air Act to add an oxygenate to gasoline to help reduce air pollution. However, some MTBE has appeared in drinking water wells throughout the U.S. This discovery has sparked a national controversy between the need to reduce air pollution (especially in heavily populated areas) and the necessity to safeguard precious water resources from contamination. In an effort to resolve this controversy, the U.S. Environmental Protection Agency (EPA) formed a 14-member panel of MTBE experts from government, the oil industry, academia, regulatory agencies, and environmental groups to explore the environmental and public health effects of MTBE and make policy recommendations by July 1999. Anne Happel, an environmental scientist at Lawrence Livermore, is a member of this EPA blue-ribbon panel. She leads a multidisciplinary team in the Environmental Restoration Division studying MTBE contamination of groundwater from leaking underground fuel tanks (LUFTs) throughout California. The team's goal is to help water quality regulators, public health specialists, and MTBE users understand more about how MTBE enters and behaves in groundwater so they can better manage its use, prevent harm to humans, and protect limited groundwater resources. The team has estimated how often MTBE escapes into groundwater through gasoline release and traced the behavior of MTBE in groundwater. The team is currently designing a data management system to target LUFTs most in need of remediation because of the risk they present to drinking water sources. The database will allow those responsible for water quality to better manage the cleanup of leaking tank sites and strategically protect drinking water from MTBE. The study results to date have provided the project sponsors-the California State Water Resources Control Board, the U.S. Department of Energy, and the Western States Petroleum Association-with fundamental information for effective management of California's groundwater resources. They will also be used to help make legislative decisions and set policy regarding MTBE's use as a gasoline additive in California and nationwide. Analyzing Field Data Scientists know that MTBE behaves differently in groundwater from other petroleum products such as benzene. Unlike petroleum hydrocarbons, it is highly water soluble, not easily adsorbed to soil, and resists biodegradation. Thus, with widespread use, MTBE has the potential to occur in high concentrations in groundwater, travel far from leak sources, and accumulate to become a hazard on a regional scale. To investigate these potentialities, the Livermore project team designed a study of MTBE subsurface plumes based on statistical analysis of historical data from California LUFT sites. Researchers investigated data collected at leaking tank sites throughout California to gain insight into MTBE movement from actual gasoline releases. They examined the frequency of MTBE contamination of groundwater at LUFT sites and public water wells throughout California and analyzed the behavior (mobility and attenuation) of MTBE plumes as compared to benzene plumes at LUFT sites.

The historical data used present some inherent limitations. Happel says that data from actual leaking tank sites are filled with real-world complexity, uncertainty, and variability. For example, a leaking tank site may have had multiple past releases, each formulated with different quantities of MTBE; the ages of the releases are also unknown; and estimates of their volume are uncertain. When natural variability is added into the analyses-for example, MTBE transport can vary in different geologies, or it can fluctuate because of the elevation and gradient of the groundwater surface-it is easy to see that data from these sparsely monitored individual sites are less than ideal for precise, quantitative contaminant transport research, which relies on data from large, heavily monitored sites. The project team overcame some of these limitations by treating data from a large number of sites as a statistical population. Similar to an epidemiological survey, this approach allowed them to deduce general trends in the behavior of MTBE and other petroleum hydrocarbons. The first data analyzed were from 236 LUFT sites located in 24 counties where groundwater had been monitored for MTBE prior to the beginning of 1996, earlier than legally required. The Livermore team began by assessing how well standard Environmental Protection Agency analytical methods (EPA 8020 and EPA 8260) performed for detection and quantification of MTBE in groundwater samples in the presence of dissolved gasoline. This evaluation enabled the team to quantify the margin of error in the historical data collected using the EPA methods so that the data could be interpreted, presented, and used with appropriate caveats and qualification. The project team found that the groundwater of 78 percent of these 236 sites contained detectable levels of MTBE. Given that at least 13,278 of the 32,409 regulated LUFT sites are known to have contaminated groundwater, the project team inferred that more than 10,000 LUFTs may have released MTBE into groundwater. These conclusions are consistent with recent work in which data were collected from over 4,000 sites throughout California. The Conclusions They Reached While the inferred 10,000 sources of MTBE contamination were the focus of journalistic reporting on MTBE problems, that number was an estimate of the extent of contamination and only one of the findings from the overall investigation. The project team also measured MTBE plume lengths and compared them with the lengths of benzene plumes-benzene is currently the petroleum compound of greatest regulatory concern-to determine the overall plume migration of the two compounds. Finally, team members analyzed the behavior of MTBE groundwater plumes over time. They were fortunate to obtain MTBE data for 29 sites in San Diego County collected since the beginning of 1992 by an oil company that had analyzed for MTBE while sampling for other hydrocarbons. The team's work confirmed and quantified what other informal, piecemeal studies had hypothesized, namely, that MTBE is a frequent and widespread contaminant in shallow groundwater throughout California, that MTBE plumes are more mobile than hydrocarbon plumes, and that MTBE may attenuate primarily through dispersion because it resists biodegradation. Put together, these conclusions point to a compound that may progressively accumulate until it contaminates groundwater resources on a regional scale. The team's findings substantiate the need for MTBE regulation and help provide the initial regulatory boundaries.

More Insights to Come Given the widespread distribution of MTBE in groundwater at leaking tank sites throughout California, the State Water Board is asking Lawrence Livermore to develop a statewide geographical information system to manage the threat of MTBE contamination to public water supplies. This system will allow regulators for the first time to "triage" sites by targeting manpower and resources for analysis, characterization, and remediation of leaking tank sites closest to drinking water supplies. The Livermore team has designed a system that will provide detailed information on leaking tank sites and public water supplies to multiple regulatory agencies. Furthermore, access over the Internet will overcome current limitations for obtaining and sharing data among multiple regulatory agencies, industry, and other stakeholders. Happel explains that the goal is to give all interested parties oversight management of leaking tank sites by providing them with access to LUFT data and on-line tools to analyze the data. "We believe that this system has the potential to dramatically transform the way regulators and industry make cleanup decisions and establish priorities for managing cleanup." The team also will be performing more studies of MTBE biodegradation. All the while, it will be leveraging information and technologies from other projects in Livermore's Environmental Restoration Division to further its MTBE work. The team's insights will be valuable contributions to revising MTBE regulations. -Gloria Wilt

Key Words: gasoline releases, geographical information system, groundwater, leaking underground fuel tanks (LUFTs), methyl tertiary-butyl ether (MTBE), statistical analysis, water quality.

For further information contact Anne Happel (925) 422-1425 (happel1@llnl.gov).