Tritium dynamics within secondary contaminant sources in fractured shale bedrock

The disposal of low-level radioactive waste at Oak Ridge National Laboratory (ORNL) has historically involved shallow land burial via pits and trenches. For the past several decades, contaminants have been migrating from the primary waste trenches into the underlying saturated zone, which is composed primarily of fractured shale bedrock. A portion of these contaminant plumes have been moving quickly along fast-flowing fractures, while other portions of the plumes have been slowly migrating into the surrounding bedrock matrix. Contaminant migration into the matrix is both beneficial and problematic. Matrix storage alleviates the potential of large contaminant discharges from the primary waste trenches into the groundwater, yet it creates a secondary contaminant source that is difficult to remediate. The contaminant tritium is of particular concern because of the large inventories present in existing waste sites and because of its high mobility from the primary waste source. Waste Area Grouping 5 (WAG 5) at ORNL is known to be a significant contributor of tritum contamination to the local watershed. The total yearly tritium discharge from this region is on the order of many thousands of curies. Remedial action at WAG 5 is hampered by inaccurate risk assessment that is associated with the poorly defined source terms of the contaminated bedrock. The authors objectives were to (a) quantify the rate of tritum storage and depletion within secondary sources at WAG 5 and (b) define the importance of primary and secondary sources of tritium to contamination discharged to the streams along the perimeter of WAG 5