Many sites with chlorinated organic contamination in groundwater throughout the nation have gone through extensive remedial evaluations and actions. The remedial alternatives for many of these sites include high-energy treatments such as pump-and-treat systems. After years of operation, the effectiveness of these high-energy processes has begun to diminish without remedial objectives being met. Other more effective remedial alternatives need to be implemented; however, there is a lack of guidance available to regulators and the environmental community regarding how and when to transition these sites to lower-energy remedial alternatives and eventually to monitored natural attenuation (MNA). To answer this need, the ITRC Enhanced Attenuation: Chlorinated Organics (EACO) Team developed this guidance, which includes a protocol to assist in a smooth transition (or a bridge) between aggressive remedial actions and MNA, and thus the concept of enhanced attenuation (EA) was born. EA is that "bridge," incorporating three important features: the evaluation of mass balance, defined as the relationship between mass loading and attenuation capacity of an aquifer; a decision framework that provides guidance for site decisions, and a toolbox of potential EA technologies (known as "enhancements") that optimize aquifer conditions to provide a sustainable treatment or, at least, minimize the energy needed to reduce contaminant loading and/or increase the attenuation capacity of an aquifer. The decision framework provides direction to regulators and practitioners on how to integrate EA into the remedial decision process. The EA approach is consistent with the current regulatory environment and can be accommodated within a broad range of regulatory programs such as those that follow the Comprehensive Environmental Resource Conservation Liability Act or state dry cleaner regulations. This new remedial framework and decision process will accelerate the environmental cleanup progress on a national scale and may reduce overall costs, while still providing protection to human health and the environment. Briefly, the EA decision framework achieves the following: * facilitates transition of contaminated sites through the remediation process * complements MNA and expands remediation opportunities * encourages energy efficiency and develops the best solutions for the environment EA provides an organized, scientific, and structured yet broadly usable approach to implement specific treatment technologies ("enhancements") at appropriate sites and at appropriate times. this guidance discusses contaminant mass loading, aquifer attenuation capacity, and remediation treatment sustainability. These concepts and working methodology support all EA processes. While the underlying EA principles are not new, the EA concept was developed to address situations where natural attenuation processes, rates, or capacity are not sufficient to meet remedial goals. Specific elements considered in the EA decision framework include risk, remediation time frame, and cost criteria. Transitioning between source-zone treatment and MNA and/or between MNA and slightly more aggressive methods can be sequenced spatially as well as temporally. The EA decision framework also allows for situations where a site currently undergoing MNA may require enhancements due to changes in acceptable remediation time frames, cost, risk, or other conditions at the site.