ABSTRACT
The disposal of low-level radioactive waste generated by the U.S. Department of Energy (DOE) during the Cold War era has historically involved shallow land burial in unconfined pits and trenches. The lack of physical or chemical barriers to impede waste migration has resulted in the formation of secondary contaminant sources where radionucli
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des have moved into the surrounding soil and bedrock, as well as groundwater and surface water sources. At certain DOE facilities, such as the Oak Ridge National Laboratory (ORNL) located in eastern Tennessee, the extent of the problem is massive, where thousands of underground disposal trenches have contributed to the spread of radioactive contaminants across tens of kilometers of landscape. The subsurface media at ORNL is comprised of fractured saprolite and interbedded fractured limestone and shale bedrock which are conducive to rapid preferential flow coupled with significant matrix storage (Jardine et al., 2001). Fractures are highly interconnected and surround low permeability, high porosity matrix blocks. Subsurface transport processes are driven by large annual rainfall inputs (~1400 mm/year) where as much as 50% of the infiltrating precipitation results in groundwater and surface water recharge (10 and 40%, respectively). Thus, storm-flow infiltration into the media often results in large physical and geochemical gradients among the various flow regimes, which causes nonequilibrium conditions during solute transport.
