leaving contamination in the subsurface may expose the landowner, property manager, or original disposer to complications that would not exist in the absence of the contamination. PRPs may be sued for natural resource damages by the resource trustee (if the underlying groundwater is no longer potable without treatment because of remaining contamination) or for personal injury and/or property damages pursuant to common law by local residents or others (if the contamination crosses property boundaries and causes injury or property damage).

At any given site, the risks and the technical, economic, and legal complications associated with residual contamination need to be compared to the time, cost, and feasibility involved in removing contamination outright. As a practical matter, the Committee did not seek to estimate the relative scope of the nontechnical impacts of leaving contamination in place, and it is probably not feasible to do so. Whether these potential consequences are likely to occur is site specific, and some implications may not materialize at some sites.

POTENTIAL FOR FAILURE OF REMEDIES AND ENGINEERED CONTROLS

The long-term management strategies for many complex sites include leaving significant amounts of contamination in place. At such sites the achievement of risk-based goals is based on a reduction of the contaminant flux (e.g., reduction in source strength) between the zone of residual contamination and the point(s) of compliance. Such flux reduction is generally accomplished by one of four approaches, possibly coupled with partial removal of source zone contamination: (1) hydraulic containment, (2) physical containment, (3) reduction of contaminant concentrations through natural processes (monitored natural attenuation), and/or (4) reduction of contaminant concentrations through an engineered reaction zone, most commonly in the form of a downgradient permeable reactive barrier (PRB) (see Chapter 4 for descriptions of these technologies). This section summarizes key concepts and tools for assessing the likelihood and consequences of failure for these approaches.

Each of the remedial strategies listed above is well established and is unlikely to exhibit “complete” failure in any meaningful sense. Rather, some degree of contaminant flux reduction is likely to be realized, even if the overall magnitude and/or spatial extent of the reduction is less than expected from design calculations. The consequences of such “partial failure” would depend both on the measures used to monitor performance and the corrective actions that are triggered by inadequate performance. There are few reports in the peer-reviewed literature that document both the failure of a long-term remedial strategy and the resulting response (although these



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