API has developed guidelines for evaluating subsurface releases of gasoline containing methyl tertiary-butyl ether (MTBE) or other oxygenated fuel additives. These guidelines are presented in API Publication 4699, "Strategies for Characterizing Subsurface Releases of MTBE" (Nichols et al., 2000).
API Publication 4699 applies the principles of risk-informed decision making to the evaluation of MTBE-affected sites; in other words, it adds exposure and risk considerations to the traditional components of the corrective action process. The risk factors at a given site are evaluated through a "Conceptual Site Model", which is an inventory of all known or potential oxygenate sources, pathways, and receptors. Based on these risk factors, three levels of assessment are defined: standard, limited, and detailed. The appropriate level of assessment is initially determined based on receptor data, which can typically be obtained from a survey of nearby wells and land uses. A subsurface investigation may then be conducted to obtain information on sources and pathways. The level of assessment can be "upgraded" or "downgraded" as warranted by the resulting source and pathway information.
API Publication 4699 includes a review of the chemical properties and subsurface behavior of MTBE and other oxygenated fuel additives. It also provides an overview of characterization and monitoring issues at oxygenate release sites, as well as a detailed review of the tools and techniques used for subsurface assessment. The expedited site assessment process and the use of modern direct-push tools are particularly emphasized, since these approaches are especially well-suited for use at MTBE-affected sites. Click below to download the bulletin and/or the report.