Hydrology of the Melton Valley radioactive-waste burial grounds at Oak Ridge National Laboratory, Tennessee

Abstract

Burial grounds 4, 5, and 6 were used sequentially from 195 1 to the present for the disposal of solid, low-level radioactive waste by burial in shallow trenches and auger holes. Abundant rainfall, a generally thin unsaturated zone, geologic media of inherently low permeability, and the operational practices employed have contributed to partial saturation of the buried waste, leaching of radionuclides, and transport of dissolved matter from the burial areas. Two primary methods of transport from these sites are by dissolution in circulating ground water, and the overflow of fluids in trenches and subsequent flow across land surface. The waste-disposal areas are underlain by the Conasauga Group (Cambrian age), a complex sequence of mudstone, siltstone, and limestone interbeds grading from one lithotype to the other, both laterally and vertically. Compressional forces that caused regional thrust faulting also caused much internal deformation of the beds. Folds, bedding-plane faults, and joints are widespread. Small solution openings have developed in some areas where the structurally-related openings have provided ingress to ground water. Both the regolith and bedrock sections of the aquifer are anisotropic, but anisotropy is more pronounced in the bedrock. The lower practical limit of ground-water circulation probably occurs at about 200 to 250 feet depth. A conceptual model of flow is described wherein the primary control on direction of ground-water flow changes from the water-table gradient in the regolith to the hydraulic head distribution within the secondary openings of the bedrock and the three-dimensional geometry of those openings. Locally, the direction of flow in the regolith also is controlled by gradients within openings in certain relatively resistant beds and structurally-related features. Hydraulic conductivities as measured by slug tests of wells range from 2.9 x 10m3 to 6.7 feet per day in the regolith, and 1.5 x 10s4 to 1.9x 10-l feet per day in bedrock at depths of 100 to 200 feet. Whiteoak Creek and its tributaries receive all overland flow from trench spillage, surface runoff from each site, and discharge of ground water from the regolith of each site. Potentiometric data indicate that this drainage system also receives ground water discharged from the bedrock of burial ground 5. By projection of the bedrock flow patterns characteristic of this site to other areas of Melton Valley, it is inferred that discharge from the bedrock underlying burial grounds 4 and 6 also is to the same drainage system. The differences in potentiometric heads and a comparatively thin saturated zone in bedrock do not favor the development of deep flow through bedrock from one river system to another. Hydraulic potentials, hydraulic conductivities, and hydrochemical data indicate that most ground water in the aquifer flows through the regolith. A smaller amount of water flows through the bedrock, where the flow paths are longer in distance and the time of travel is considerably greater. In burial ground 5 the regolith section of the aquifer has widespread contamination, principally by iron, tritium, and strontium-90. Contamination decreases substantially with increasing depth in bedrock, particularly below 100 feet. The sites have received little monitoring because they have not been considered a hazard. As improvements in plant facilities have reduced the discharge of radionuclides to the Whiteoak Creek system, the discharge of contaminants from the burial grounds has become relatively more significant. The burial grounds are now a principal source of 3H and a major contributor of 90 Sr in the discharge of Whiteoak Creek to the Clinch River.