REMEDIATION Winter 2007 Monitored Natural Attenuation Forum: MNA of Metals and Radionuclides Bruce E. Rittmann Robert G. Ford Richard T. Wilkin Jess W. Everett Lonnie Kennedy While the natural attenuation of many organic compounds is established and accepted by the regulated and regulatory communities, there is some debate whether monitored natural attenuation (MNA) of metals and radionuclides is a reasonable remedial alternative to consider. Do you consider MNA to be a reasonable remedy to consider for metals and radionuclides and, if so, what are the important fate, transport, and monitoring considerations? PANEL MEMBER: BRUCE E. RITTMANN, DIRECTOR OF THE CENTER FOR ENVIRONMENTAL BIOTECHNOLOGY, ARIZONA STATE UNIVERSITY, TEMPE, ARIZONA In most cases, natural attenuation of metals and radionuclides involves their immobilization as a mineral precipitate or surface complex. If the precipitate or complex is stable over the long term, immobilization can prevent migration to sensitive receptors and be a natural attenuation remedy. Very insoluble precipitates offer the highest potential for having long-lasting stability. Examples include Cr(OH) 3 , UO 2 , PuO 2 , and NpO 2 . The low-solubility precipitates involve reduced forms of the metal or radionuclide (i.e., Cr(III), U(IV), Pu(IV), and Np(IV)). Normally, contaminated water or soil contains these elements in oxidized forms that do not form insoluble precipitates. Thus, successful immobilization requires reduction. Bacterially driven reduction of the metals are possible directly through enzymatic catalysis or indirectly, usually through bacterial reduction of Fe 3+ to Fe 2+ , which then reduces the metal or radionuclide. This means that natural attenuation of metals and radionuclides usually requires that bacteria and a bioavailable electron donor be present. This situation led the National Research Council (NRC; 2000) to classify the likelihood of successful of natural attenuation as “low” or “moderate” for metals and radionuclides. Likelihood of success relates the probability that a given site will have the conditions needed to drive the detoxification mechanism (NRC, 2000). For comparison, benzene, toluene, ethylbenzene, and xylene (BTEX) was placed in the “high” likelihood category. In the case of most metals and radionuclides, the factor keeping the likelihood at the low level is the lack of a naturally occurring electron donor. This means that the potential for natural attenuation of most metals and radionuclides should be assessed through the presence of bioavailable electron donor and capable metal-reducing bacteria. c  2007 Wiley Periodicals, Inc. Published online in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/rem.20156 121