Results of ground-water tracer tests using tritiated water at Oak Ridge National Laboratory, Tennessee
Ground-water tracer tests were conducted at two sites in the radioactive-waste disposal area of Oak Ridge National Laboratory from 1977 to 1982. The purpose of the tests was to determine if the regolith beds had weathered sufficiently to permit the substantial flow of water across them. Regolith of the sites has developed on the Pumpkin Valley Shale and the Nolichucky Shale of the Conasauga Group, both of Cambrian age. About 50 curies of tritium dissolved in water were used as the tracer at one site, and about 100 curies at the other. Distances from the injection wells to a semi-circle of observation wells were 12 and 30 feet, respectively. Results demonstrated that ground water is able to flow through joints in the weathered bedding and that the direction of the water-table gradient is the primary factor governing flow direction. Nevertheless, the substantial lateral spread of the plume as it developed showed that bedding-plane openings, important in controlling flow in the deeper rock, can still exert a significant secondary influence on flow direction in the weathered rock. About 3,500 water samples from the injection and observation wells were analyzed for tritium during the test period. Concentrations detected spanned 11 orders of magnitude. Measurable concentrations were still present in the two injection wells and most observation wells 5 years after the tracer was introduced. Matrix diffusion may have played a significant role in these tests. The process would account for the sustained concentrations of tritium at many of the observation wells, the long-term residual concentrations at the injection and observation wells, and the apparent slow movement of the centers of mass across the two well fields. The process also would have implications regarding aquifer remediation. Other tracer tests have been conducted in the regolith of the Conasauga Group. Results differ from the results described in this report.