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USA| The Uranium Mill Tailings Remedial Action (UMTRA)
Project designed and constructed disposal cells at 19 sites across the
country. In order to meet the remedial action standards, certain technical
specifications were identified leading to the conclusion that the remedial
action would result in acceptable disposal performance. Disposal performance
criterion addresses 1) stability, 2) radon control, 3) water resources
protection, and 4) site cleanup. Prior to initiating remedial action, the
Department of Energy (DOE) developed a site-specific remedial action plan
(RAP) that is divided into sections dealing with geologic stability, geotechnical
stability, surface water hydrology and erosion protection, water resources
protection, radon attenuation, and site cleanup. Specific characterization
data, laboratory test results, design calculations, technical analysis
were included as appendices.
UMTRA disposal caps/covers have evolved over time. This evolution relates to changes in U.S. Environmental Protection Agency (EPA) standards as well as to changes in politics, economics, and the state of technology over the projects 20-year life. The stability and radon control design features have not changed significantly over time. Two significant features that have evolved over time involves cover design and mill subpile contaminated soil removal considerations. Both of these remedial action elements are a function of a water resources protection strategy as related to disposal performance. In the very early days of UMTRA, it was not apparent that contaminated groundwater and water resources protection was a significant issue. As the program evolved and matured it became more evident that ground water cleanup and protection were an important element to consider when demonstrating that the performance of the disposal unit would comply with the groundwater protection standard for disposal. The performance assessment uses quantitative analysis of infiltration, leaching, and contaminant transport to demonstrate that the estimated concentration of each hazardous constituent in the uppermost aquifer at a point of compliance is less than or equal to prescriptive or risk-based standard. The associated assumptions in the demonstration are often verified after disposal by long-term monitoring. UMTRA-style caps are designed to perform under steady-state equilibrium conditions and are based on the Law of Mass Conservation (whereas: inflow equals outflow plus or minus changes in storage). These designs are intended to limit infiltration through the cover. Long-term moisture storage in the encapsulated tailings and continued seepage out the bottom of the cell is avoided. After EPA published draft ground water quality standards in 1987, the UMTRA Project refined the cover design approach and placed greater emphasis on designing low-permeability compacted soil layer (fine-grained radon barrier) with a saturated hydraulic conductivity less than 10-7 cm s-1. Beginning in the early 1980s some cover design people believed that in the absence of regular maintenance, ecological succession on engineered covers is inevitable. Thus, they advocated exploiting beneficial ecological changes and designed covers that improve rather than degrade over the long term as inevitable natural processes act on the system. The recently completed cover design at the Monticello, Utah CERCLA site is the product of this evolution of thinking. These designs have certain potential advantages over compacted clay layers, but can add considerable cost to the disposal. Only long-term monitoring of the disposal performance will determine advantages and disadvantages of design feature philosophy. Contaminated soils at the mill sites are often extensive due to wind blown and water borne forces. These contaminant scenarios generally are not the source for continued groundwater contamination. With the assumption that the tailings will be removed or encapsulated in a long-term engineered disposal cell, there often continues to be a secondary source term present to contaminate groundwater. These continued source terms are the mill site subpiles soils that underlain the tailings. The UMTRA Project, in most cases, always used radium –226 –228 only, as the constituent to measure when performing excavate control. When the activity of radium –226 –228 was below the soil cleanup standard of 15 picocuries (pCi) per gram ( 1 pCi = 0.037 becquerel), the bottom of the excavate was complete (clean). Now that 19 disposal cells have been built by the UMTRA Project, the DOE is addressing groundwater cleanup as the second phase. As additional detailed characterization is completed at these former mill sites, it is becoming apparent at some of the sites that using only radium as the constituent to measure for excavate of the subpile soils can leave behind other hazardous constituents that can be transported to groundwater and cause contamination. Many of these constituents common to uranium mill tailings do not have established soil cleanup standards. These constituents vary in their solubility characteristics. Some of the more common constituents that have leached into subpile soils include: uranium, nitrate, sulfate, molybdenum, vanadium, and arsenic. The holistic approach integrates public involvement, planning the conceptual and final disposal design, alternatives analysis, institutional controls, and long-term monitoring and surveillance into a check list outline and associated guidance such that remedial action can achieve long-term goals and objectives. |
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