Hydrology of the low-level radioactive solid waste burial site and vicinity near Barnwell, South Carolina

Abstract

Geologic and hydrologic conditions at a burial site for low-level radioactive waste were studied, and migration of leachates from the buried waste into surrounding unconsolidated sediments were evaluated. The burial site and vicinity are underlain by a sequence of unconsolidated sediments of Late Cretaceous, Tertiary, and Quaternary age. These sediments are deposited over a graben which has been filled with sedimentary rocks of Triassic age. Hydraulic properties of the sediments beneath the burial site were determined by laboratory and field tests. Laboratory hydraulic conductivity values ranged from about 10^-7 to 10^-1 feet per day for the clayey sediments to nearly 22 feet per day for aquifer sands. Field aquifer tests indicate a transmissivity of about 22,000 feet squared per day for Cretaceous sediments and about 6,000 feet squared per day for Tertiary sediments. Aquifer tests indicate heterogeneity in the upper 200 feet of the Tertiary sediments. Water samples were analyzed from 51 wells, 5 streams, a Carolina bay, and rainfall at the burial site. The total dissolved solids of the ground water ranged from about 7 to 40 milligrams per liter in the upper clayey sediments to about 150 milligrams per liter in the water in the deeper calcareous sediments. The pH of the ground water ranges from 4.8 to 6.5. This slightly acidic water is corrosive to buried metal. Tritium activity greater than background was detected in sediment cores taken from drill holes adjacent to the burial trenches. High tritium activity occurred at depths above the trench floor. This indicates upward movement of water or vapor to the land surface. Tritium and organic constituents greater than background concentrations were observed in a monitoring well about 10 feet from a trench, indicating lateral migration of radionuclides from the buried waste. Traces of cobalt-60 and tritium greater than background activity were observed in sediment cores collected 5.8 feet beneath the trench floor at one site. A hydrologic model was used to simulate ground-water flow in the study area. Based on the model results the minimum time of travel for ground water to move from the burial site to the nearest stream, Marys Branch Creek, is about 50 years. Radionuclides will move more slowly than the water, and will diminish in activity, because of dispersion and radioactive decay.