By George Koerner, Ph.D., P.E., CQA
The U.S. Nuclear Regulatory Commission’s Offices of Nuclear Regulatory Research (RES) and the Federal and State Materials and Environmental Management Programs (FSME) conducted the “Workshop on Engineered Barrier Performance Related to Low-Level Radioactive Waste, Decommissioning, and Uranium Mill Tailings Facilities” Aug. 3-5, 2010, in Rockville, Md.
The workshop was coordinated with several states (Texas, South Carolina, Utah, Colorado, Washington, and New York) and federal agencies (Department of Energy–DOE, Environmental Protection Agency–EPA, United States Geological Survey–USGS, and DOE’s National Laboratories).
The workshop objectives were to facilitate communication among federal and state regulators and contractors, and selected experts, on current engineered barrier issues and technical and regulatory experiences; discuss lessons learned and new approaches for monitoring and modeling; prepare recommendations to address maintenance of engineered barrier performance over time; and to identify topics for future research and the potential need to update technical guidance.
The workshop focused on engineered landfill covers and bottom liners designed to isolate waste by impeding surface water infiltration into the waste systems and by retarding the migration of contaminants from the waste disposal site. The technical topics included engineered barrier performance, modeling, monitoring, and regulatory experiences at low-level radioactive waste, decommissioning, and uranium mill tailings sites.
Surprisingly, this three-day workshop had little exposure to laboratory and field experiences using geosynthetics. There were 32 presentations and only four of them (Phaneuf, Rowe, Bachus, and G. Koerner) related to geomembranes or other geosynthetic materials.
Essentially, all of the others focused on soil-only covers, mainly evapo-transpirative (ET) types. As seen in the workshop proceedings (available from the U.S. Nuclear Regulatory Commission), ET covers have been extensively theoretically studied, along with some laboratory evaluations and limited field testing.
We, at GSI, have concerns about the long-term performance of soil-only final covers. See, for example, Heerten, G. and Koerner, R. M. (2008), “Cover Systems for Landfills and Brownfields,” Land Contamination and Reclamation, Vol. 16, No. 4, EPP Publications Ltd., United Kingdom, pp. 343-356.
The effects of total settlement and particularly differential settlement of the underlying waste with respect to ET covers is rarely addressed. Further. many low-level radioactive sites are in arid or semi-arid locations and desiccation cracking is clearly a challenge. In more-humid locations, vegetative root growth as well as animal intrusion must be considered. Sudden periods of rainfall and infiltration from sideslope saturation is yet another issue of concern.
At GSI, we believe—and the case was made at this workshop—that if the objective is a long-term, environmentally safe, and secure final cover for any type of landfill, it must include a geomembrane generally with an underlying geosynthetic clay liner, (i.e., a composite GM/GCL barrier system is recommended).
The extensive U. S. EPA study by Bonaparte, Daniel, and Koerner (“Assessment and Recommendations for Improving the Performance of Waste Containment Systems,” EPA/600/R-02/099, December 2002, 1039 pgs.), shows that such geosynthetic-related liner systems perform superbly. Furthermore, there are many additional papers on covers (e.g., most recently at the 9th International Conference on Geosynthetics in Brazil, there are papers by Youngblood, Case, and Yako) relating to the excellent long-term performance of geosynthetic barriers in landfill covers.
As an observation, there appears to be a disconnect between U.S. EPA permitted covers for hazardous and nonhazardous waste landfills (the majority of which require geosynthetic barriers) and the U.S. NRC tendencies for low-level radioactive waste covers, particularly in arid regions going in the direction of soil-only covers. This observation was evident at this workshop.
GSI is currently conducting a survey to determine how extensive (i.e., the approximate acreage) is the potential covering of low-level radioactive waste sites as well as uranium mill tailings sites.