plausible that repair, rather than replacement, could be the appropriate response action. At the time of this writing, reports where installed PRBs were repaired or replaced were not located in the literature. As with the other long-term management strategies, the operating history of PRB technology is simply too short to support a robust assessment of the potential long-term management costs. However, concerns related to back-diffusion could potentially limit the application of PRB systems to sites where substantial contamination is not initially present downgradient of the installed PRB.

Monitored Natural Attenuation

Monitored natural attenuation (MNA) is most often used in conjunction with other active or engineering remedial components and is seldom employed as a stand-alone measure (EPA, 2010a). The success of natural attenuation as a remedy depends on the site-specific ability to predict the evolution of complex biogeochemical processes over an extended period of time. Because of uncertainties in long-term predictions, natural attenuation requires confirmatory monitoring, such that MNA remedies are accompanied by a detailed program of monitoring (e.g., NRC, 2000; EPA, 2004a).

Numerous protocols exist for evaluating MNA performance including a recently proposed decision framework for evaluating MNA for inorganic or radionuclide contamination (e.g., ITRC, 2010). Although focused on inorganic contaminants, the ITRC protocol contains many elements appropriate for sites with organic contaminants. In particular, the need for a contingency plan was emphasized, which provides a cleanup approach that will be implemented if “the selected remedy fails to perform as anticipated” (EPA, 2007). For MNA remedies, a suitable contingency plan might include optimization of source or plume treatments, implementation of an enhanced attenuation (EA) technology, pursuit of a technical impracticability waiver, or the use of institutional controls.

MNA systems could fail for many reasons, including temporal changes in site-specific hydrologic or geochemical conditions, the depletion of natural sources of nutrients or electron acceptors/donors, and lower-than-anticipated transformation rates. Further, the regulators may believe that there is insufficient evidence that MNA is occurring in the intended fashion. For example, it may be difficult to verify that the presence of daughter products is due to parent compound degradation and not co-contamination.

It is difficult to generalize regarding the potential cost of MNA failure, which will depend on site-specific conditions, the nature of the contingency actions, and the degree of conservatism built into the monitoring program. A properly designed monitoring program should provide “early detection” that allows for the implementation of a contingency plan prior to the point when a migrating plume would present elevated risks to receptors.

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