an alternative project at the Savannah River Site designed to develop cost effective and environmentally protective technical solutions for the challenge of large and complex plumes of chlorinated volatile organic compounds (cVOCs) at DOE sites. The project builds upon the current U.S. Environmental Protection Agency (EPA) protocol and directive (EPA, 1998(EPA, , 1999 for monitored natural attenuation (MNA). The DOE effort focuses on three major technical thrust areas: mass balance, enhanced attenuation (EA), and innovations in characterization/monitoring. Each of these three topics is supported by a broad-based group of recognized technical experts and each group currently is developing an interim report related to the topic area. By introducing the concepts of mass balance and EA, along with the technical basis and documentation for these concepts, the project is working toward a goal of providing new and powerful tools for transitioning from active remediation to a protective long term monitoring state. This project is a departure from classical MNA in that its central theme is to take an active part, as needed, to achieve a favorable balance between the release of contaminants from sources (source loading) and processes that destroy or retard migration of contaminants in resultant plumes (attenuation capacity of the system). OBJECTIVESThe objective of this document is to explore the realm of enhancements to natural attenuation processes for cVOCs and review examples that have been proposed, modeled, and implemented. We will identify lessons learned from these case studies to confirm that enhancements are technically feasible and have the potential to achieve a favorable, costeffective contaminant mass balance. Furthermore, we hope to determine if opportunities for further improvement of the enhancements exist and suggest areas where new and innovative types of enhancements might be possible. TARGET AUDIENCEThe intended audience for this reference guide includes state and federal regulators, stakeholders, and environmental managers, as well as environmental technology professionals. Although this diverse group is technically astute, not all users of the document will be subject-area experts for the various technologies discussed. This fact poses a significant challenge to achieve an appropriate level of technical detail. To the extent possible, enhancement technologies are covered succinctly in the main body of the document. However, supplemental information on many enhancements has been included in Appendix A so that readers seeking additional information and references can find them easily. WSRC-STI-2006-00083, Rev ORGANIZATION OF THE REPORTAn overview of the purpose of the document is presented in Section 1. The conceptual framework and organizing principles of enhancements to natural attenuation of cVOCs in the subsurface are presented in Section 2. This section contains a summary discussion of different classes of enhancements and the concept of enhancement zones is introduced. The objective of Sectio...