Executive SummaryNatural attenuation processes are commonly used for remediation of contaminated sites. A variety of natural processes occur without human intervention at all sites at varying rates and degrees of effectiveness to attenuate (i.e., decrease) the mass, toxicity, mobility, volume, or concentrations of organic and inorganic contaminants in soil, groundwater, and surface water systems. As defined in the U.S. Environmental Protection Agency (EPA) policy for monitored natural attenuation as a remediation alternative, these in situ, natural attenuation processes include biodegradation; dispersion; dilution; sorption; volatilization; radioactive decay; and chemical or biological stabilization, transformation, or destruction of contaminants. The major driver for using this approach for site remediation is the perception that monitored natural attenuation will be simpler and less costly to implement as compared to using active remediation technologies. However, this cost argument is not valid when considering all requirements described in EPA's regulatory and technical guidance for monitored natural attenuation.The objective of this review was to identify potential technical investments to be incorporated in the Subsurface Contaminant Focus Area (SCFA) Strategic Plan for monitored natural attenuation (MNA). When implemented, the technical investments will help site managers and their supporting contractors more effectively evaluate and implement monitored natural attenuation as a remediation option at U.S. Department of Energy (DOE) sites. This review included a literature search that identified a number of important resources, including the EPA's OSWER Directive 9200.4-17, Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites; EPA's Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groundwater; DOE's published guidance documents and screening tools (e.g., MNAtoolbox) for monitored natural attenuation; recently published reviews by the National Research Council on monitored natural attenuation for groundwater remediation and on the DOE Environmental Management Science Program (EMSP); EPA's Science Advisory Board (SAB) review of EPA's Research Program for monitored natural attenuation of contaminants in groundwater, soils, and sediments; and other published and Internet sources on monitored natural attenuation. Our evaluation of a bibliography generated from a search of computerized reference databases indicates that the majority of the published research pertain to natural attenuation of organic contaminants. As noted by the National Research Council, DOE's guidance and protocol for monitored natural attenuation is unique in that it is the only technical protocol to specifically address inorganic as well as organic contaminants.The outcome of this review is a set of conclusions and general recommendations on research needs, programmatic guidance, and stakeholder issues pertaining to monitored natural attenuation for the D...
