API PUBL 4779-2007

Releases of MtBE-containing gasoline have led to the detection of MtBE in soil, surface water, and groundwater. Methyl tert-butyl ether may be removed from environmental media by some of the active or passive strategies typically employed for gasoline remediation, including Monitored Natural Attenuation (MNA). However, based upon the physical and chemical properties of MtBE, the data needs and evaluation procedures for selecting and implementing an MNA strategy for MtBE differ from those typically used for other gasoline constituents. This technical protocol addresses data collection, evaluation, and interpretation procedures that consider the properties of MtBE when evaluating natural attenuation.

Natural attenuation refers to all biological and abiotic processes that dilute, remove, degrade, or detoxify chemicals in the environment (USEPA, 1999 and 2001). Natural attenuation has become an accepted strategy for many classes of chemicals provided that risks can be managed effectively and the remedial timeframe is comparable to other practicable alternatives. Evaluation of the performance of natural attenuation strategies relies upon monitoring networks that can quantify changes in chemical concentration and/or mass and related geochemistry and hydrology that influence, or are products of, attenuation processes. This remedial approach is often referred to as MNA.

This protocol provides guidance to those interested in assessing MtBE natural attenuation, and those with the responsibility of reviewing such work. This manual is designed to:

• Present the basic scientific principles relevant to the evaluation of MtBE natural attenuation;
• Develop a framework for assessing the feasibility of incorporating MtBE natural attenuation into an overall site strategy;
• Identify those data that can be used to assess MtBE natural attenuation;
• Provide a concise technical reference for relevant chemical properties, analytical methods, and field sampling techniques;
• Provide protocols and guidance for data interpretation; and
• Provide guidance on the presentation of natural attenuation data/information to facilitate regulatory and other stakeholder review and acceptance of MNA remedies. This protocol is not a prescriptive manual to be followed step by step for all sites with MtBE. Rather, the protocol helps stakeholders identify and quantify attenuation mechanisms and assess whether these mechanisms provide for sufficient MtBE natural attenuation in a particular environment. This protocol may be used at any stage of a site evaluation, from site discovery to remedy re-evaluation.

The material presented in this protocol is intended to support work plan development, site investigation, data analysis, and decision making for regulators, site managers, and practitioners. The remainder of this document is organized as follows:

• The remainder of Section 1 summarizes the regulatory status of MNA for MtBE and discusses attenuation mechanisms specific to MtBE;
• Section 2 presents a tiered approach that can be used by stakeholders to interpret several lines of evidence to evaluate natural attenuation on a site-specific basis;
• Section 3 provides methods for field data collection and chemical analysis; and
• Section 4 describes data evaluation and presentation methods for the three tiers of data.