API PUBL 4699-2000

The 1990 Federal Clean Air Act Amendments (CAAA) mandated the use of cleaner burning gasoline in metropolitan areas throughout the United States. One effect of this legislation was an increase in the use of oxygenated compounds in gasoline, particularly methyl tertiary-butyl ether (MTBE). During the 1990s, MTBE was used more commonly in gasoline, and at higher concentrations, than in previous decades.

As oxygenate usage has grown, however, so have concerns regarding the potential Impacts of these compounds on groundwater quality. Traditionally, the greatest concerns associated with gasoline releases have been non-aqueous phase liquids (NAPL) and dissolved-phase benzene, toluene, ethylbenzene, and xylenes (BTEX). The assessment techniques commonly used by industry (API, 1996a) were originally developed to meet regulatory requirements for NAPL and BTEX characterization. However, many fuel oxygenates are more mobile and persistent in groundwater than BTEX or other petroleum hydrocarbons. These differences have important implications for subsurface assessment; for example, a small surface release of conventional fuel may have no apparent impact on shallow groundwater, while an equivalent release of oxygenated fuel may result in unacceptable concentrations of dissolved-phase MTBE. Furthermore, dissolved-phase MTBE plumes may be more difficult to detect and delineate than dissolved-phase BTEX plumes. In light of these differences, a reevaluation of traditional assessment procedures is warranted.

This study uses the principles of risk-informed decision making to guide the evaluation of sites affected by MTBE or other ether oxygenates. The risk-informed decision making framework uses Information about sources, pathways, and receptors to guide site management decisions to reduce or eliminate risks from exposures to chemicals released In the environment.

Section 2 of this publication provides a brief review of MTBE and other fuel oxygenates, including summaries of current environmental, analytical, and regulatory issues. General background Information about the chemical structure and properties of fuel oxygenates is included in Appendix A. Section 3 provides a risk-based guidance for conducting assessments at MTBE release sites, while Section 4 reviews modern assessment tools and techniques for characterizing and monitoring MTBE in the subsurface. The primary focus of this study is on MTBE, but the guidance outlined in Sections 3 and 4 should be broadly applicable to other ether oxygenates, which share generally similar properties. This guidance is not necessarily appropriate for ethanol or other alcohol oxygenates, since alcohols are generally more biodegradable than ethers.

The guidance provided in this document is not intended to supercede any local, state or federal policies regarding the assessment of oxygenate releases. Rather, it provides a framework for making assessment decisions to supplement existing policies and procedures, and to provide a framework where agency-specific guidance does not exist.

It should be noted that any and all comments regarding technologies and comments herein are for Informational purposes only. Neither the contractor nor API endorses such products or technologies.