Application of RESRAD-BASELINE for a Radiological Risk Assessment for Material Disposal Areas A and T, Los Alamos National Laboratory, New Mexico - 21089

In November 1997, Congress authorized and directed the U. S. Department of Energy (DOE) to transfer certain land tracts at Los Alamos National Laboratory (the Laboratory) for other beneficial community use, including acreage at Technical Area 21 (TA-21). Radioactive solid wastes were disposed in a central pit and in two eastern trenches at Material Disposal Area (MDA) A, an inactive 1.25-acre subsurface legacy disposal site at TA-21. MDA T is a 2.2-acre radiological waste disposal site located immediately west of MDA A. It consists of four plutonium-contaminated absorption beds used to dispose of liquid wastes from 1945-1952; and a series of disposal shafts containing primarily plutonium isotopes and americium-241. MDAs A and T are situated on DP Mesa, one of many similar finger mesas on the deeply dissected Pajarito Plateau in northern New Mexico. DP Mesa is a typical east-west trending mesa on the plateau, formed by erosion of Los Alamos Canyon to the south and DP Canyon to the north. To initiate definition of final remedies the DOE's Environmental Management Los Alamos Field Office (EM-LA) committed to provide Los Alamos County with an initial radiological risk assessment (RRA) for disposed waste at MDAs A and T by September 30, 2020. The RESRAD-OFFSITE modeling software package developed for the DOE by Argonne National Laboratory, was employed to evaluate the release and transport of subsurface radiological contamination at MDAs A and T over time, and assess radiological dose for future individuals who might be exposed to this contamination subsequent to TA-21 land transfer. The disposal units at MDAs A and T are presently capped with operational soil covers having a thickness of 1.3 m and 1.7 m, respectively. The MDAs are currently fenced, which keeps people out of the immediate vicinity of the MDAs, and the MDA covers are maintained so that deep-rooted plants and burrowing animals do not significantly affect the performance of the covers by bringing radionuclides to the ground surface. The existing soil covers are assumed to be present when the surrounding lands are transferred to Los Alamos County, and present-day institutional control, which includes the maintenance of the soil covers, is assumed for the 1,000-year modeling period evaluated in the RRA. Potential radiological doses over time were evaluated for three exposure scenarios: - a commercial/industrial scenario, applicable to the mesa-top areas outside the boundary or fence of the MDAs; - a trespasser scenario, applicable inside the boundary of the MDAs; and, - a recreational (hiking) scenario, applicable to the canyons adjacent to DP Mesa. The modeling results indicate extremely low radioactive doses under the conditions evaluated, with future radiological doses likely to be more than one million times lower than the 25-mrem annual dose limit identified by DOE for protection of human health for members of the public. The natural rate of soil erosion on the relatively flat mesa top where the MDAs are situated is modeled to be approximately 0.018 mm/year, so only 18 mm of soil is estimated to be lost within 1,000 years. Given the low erosion rate, and the assumption that no human or biotic intrusion will disturb the cover or disposed wastes, the disposed wastes are effectively isolated from the environment, and the very low doses come from radon inhalation dose related to diffusion of radon-222 gas through the cover and into ambient air, and from the very small amount of high-energy gamma radiation that makes it through the intact soil cover. The initial RRA supports the conclusion that, provided current institutional controls are maintained, negligible exposures to radionuclides from disposed wastes are expected if land surrounding MDAs A and T is transferred to Los Alamos County. (authors)