New Technical Guidance for MTBE Site Characterization

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, 1996) were originally developed to meet regulatory requirements for NAPL and BTEX characterization. However, the chemical properties and subsurface behavior of oxygenated fuel additives are substantially different from those of BTEX or other petroleum hydrocarbons, and so a reevaluation of traditional assessment procedures was warranted. In response to this need, the American Petroleum Institute has produced a technical guidance document, MTBE Site Characterization Technical Bulletin, for the appropriate characterization of sites with subsurface releases of fuel oxygenates (API, 1999). The technical bulletin uses the principles of risk-based decision making to guide the evaluation of sites affected by MTBE or other oxygenates. The publication includes an introduction to the properties and uses of MTBE, provides guidance for conducting assessments at MTBE release sites, and reviews modern assessment tools and techniques for characterizing and monitoring MTBE in the subsurface. While the primary focus of this study is on MTBE, other fuel oxygenates are also addressed. The risk-based decision making process provides a framework for determining the objectives and scope of environmental site assessments. Sites with greater risk factors warrant the most intensive assessment of contaminant sources, pathways, and receptors. Sites with fewer risk factors warrant a more limited assessment to confirm whether sources, pathways, and receptors require further investigation or corrective action. The guidance reviews the risk factors associated with oxygenate sources, pathways, and receptors. These risk factors are used to develop an initial conceptual model and to tentatively categorize a site as high, medium, or low risk. Various assessment tasks are then conducted to verify the conceptual model, and to confirm whether further investigation or corrective action is needed. Many of these assessment tasks can take advantage of improved, cost-effective methods for rapid collection and field analysis of soil, soil-gas, and groundwater samples. API (1999) provides an overview of the expedited site assessment approach and provides a detailed guide to current direct-push assessment and monitoring tools, with emphasis on their proper use at MTBE-affected sites.