Chemical, geological, and hydrological factors governing radionuclide migration from a formerly used seepage trench: a field study. Environmental Sciences Division publication No. 2202

Abstract

A total of 3.2 x 10/sup 7/ L of intermediate-level liquid wastes (ILW) generated from routine Oak Ridge National Laboratory operations, were disposed in Trench 7 between 1962 and 1966. The disposed ILW contained about 10/sup 7/ GBq of fission nuclides (primarily /sup 137/Cs and /sup 90/Sr), activation products (primarily /sup 60/Co), actinides (primarily /sup 232/Th and /sup 238/U decay series nuclides), and transuranics (primarily /sup 241/Pu-/sup 241/Am). Gamma-log profiles of the wells near ILW Trench 7 indicate that the waste liquids seeped along discrete layers parallel to bedding and along the strikes of faults and folds. Although most of the radioactivity has been retained by sorption reactions with the trench fill, soils, and weathered bedrock, groundwater characteristics in the vicinity of ILW Trench 7 are still greatly influenced by the constituents of the waste liquids disposed two decades ago. Radionuclide concentrations in the groundwaters near the north end of ILW Trench 7 undergo seasonal variations, with the lowest activities occurring in fall and winter and the highest activities occurring in the spring and after prolonged rainstorm events. The rise in radioactivity in these wells correlates with a rise in the groundwater level and a concurrent increase in groundwater pH. Although ILW Trench 7 has worked effectively to retain most of the disposed radioactivity, two suspected transport pathways from the trench to a nearby seep area have been identified and both appear to be associated with fault zones. Only the pathway near the north end of the trench appears to be leaching alkalinity and thus affecting the retention capacity for /sup 90/Sr. A groundwater interceptor at the north end of ILW Trench 7 is one possible remedial action to help maintain a dry alkaline environment and ensure the integrity of the trench for nuclide retention.