Approximately 10% of the 126,000 sites in the United States that currently contain contaminated groundwater are unlikely to be completely restored for decades, a new report from the National Research Council concludes.

The Nov. 8 report, sponsored by the U.S. Dept. of the Army, adds that the estimated cost to completely clean up these "complex" sites ranges from $110 billion to $127 billion. However, the figures for both the number of sites and costs are likely underestimates, the NRC said.

About half the contaminated groundwater sites deleted from the Superfund list still have contamination that will require continued oversight and funding over extended periods in order to protect the public.

"The complete removal of contaminants from groundwater at possibly thousands of complex sites in the U.S. is unlikely, and no technology innovations appear in the near-time horizon that could overcome the challenges of restoring contaminated groundwater to drinking-water standards," says Michael Kavanaugh, chairman of the committee that wrote the report and principal with Geosyntec Consultants Inc. in Oakland, Calif.

Because contaminant levels will remain above drinking-water standards at some sites despite continued active remedial efforts, Kavanaugh adds, "we are recommending a formal evaluation be made at the appropriate time in the life cycle of a site to decide whether to transition the sites to active or passive long-term management."

The NRC committee said that if a remedy at a site reaches a point at which continuing expenditure brings little to no reduction of risk prior to attaining drinking-water standards, a re-evaluation of the future approach to cleaning up the site, called a transition assessment, should occur.