However, if contaminant migration and/or plume expansion occurs prior to the detection of failure, additional costs may be incurred. In certain circumstances, the combined cost of failed MNA and implementation of an additional remedy may exceed the cost that would have accrued had the remedy originally been put in place instead of MNA. To avoid such occurrences, the monitoring program should be directed at providing confirmation of the assumptions used to extrapolate the performance of MNA, in an adaptive management mode.

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Common to all the remedies discussed above are unplanned and catastrophic events that may lead to failure of the proposed containment/treatment techniques, potentially for long periods of time. For example, natural disasters (e.g., earthquakes, floods, or other events) could cause changes in local hydrology, damage the remediation/containment system, or cause a loss of power to an active containment process. Flooding or other events could spread contamination to new areas and/or create new exposure pathways (e.g., vapor intrusion). Because contaminant migration from source zones or the plume is often slow, none of these events is likely to lead to catastrophic failure of the remedial system, but such events could lead to contaminant releases from the target capture zone if the failure is not identified and remedied. In summary, at sites where contamination remains in place, an evaluation of potential events that could lead to a failure of the long-term management approach should be performed and contingency plans developed.

IMPLICATIONS OF THE LONG-TERM NEED FOR INSTITUTIONAL CONTROLS

At every site where contaminants will be left in place (for any significant length of time), institutional controls are necessary to prevent the exposure of local residents to chemicals in groundwater and soil. At groundwater sites, institutional controls play three roles. First, they can restrict the use of contaminated groundwater. Second, they can protect the occupants of overlying buildings (or proposed buildings) from exposure to chemicals from contaminated groundwater through vapor intrusion (e.g., by requiring systems and barriers to prevent vapor from entering buildings). And third, they can prevent activities that might compromise remedies, such as penetration of landfill caps where the landfill is a source of groundwater contamination or pumping that is likely to spread contamination. If properly implemented and enforced, institutional controls allow a groundwater remedy to be protective in cases where residual contamination



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