Transuranic element uptake and cycling in a forest over an old burial ground

Abstract The consequences of returning the Savannah River Site (SRS) burial ground area to general public access at the time of completion of the SRS mission is being investigated. This study includes evaluation of the radiological impact to inhabitants of the area under a number of scenarios that include the return of the land to farming or forestry use with or without exhumation of the buried waste. In the hot, humid climate of the SRS area, it is inevitable that the open land will return to forest vegetation if the site is completely abandoned. While this has the desirable effect of limiting erosion, it can be expected that the tree roots will penetrate some distance below ground. In the SRS Low-Level Burial Ground, this would provide a vector for movement of radionuclides from waste trenches to the surface. While there have been studies of agricultural species grown on contaminated soil, at SRS and other sites, there are few studies involving forest vegetation. This study was established with the objective of determining the uptake of buried, low-level, transuranic waste from unlined earthen trenches by forest vegetation. From SRS startup in 1953 through 1974, solid waste contaminated with α-emitting transuranic nuclides was buried, unencapsulated, in earthen trenches. Burial records show that this material includes plutonium-238 ( 238 Pu), plutonium isotopes 239 and 240 ( 239,240 Pu), americium-241 ( 241 Am), and neptunium-237 ( 237 Np). In 1979, two tree plots were established, one over a trench in the burial ground and the other in an area without trenches. The alpha waste trench was cored at this time to determine the distribution of alpha contamination under the trench plot. Three species of trees were planted over the trench; loblolly pine ( Pinus taeda L.), sweetgum ( Liquidambar styraciflua L.), and willow oak ( Quercus phellos L.). All are native species. In the 2 years following establishment of the tree plots, 1979 and 1980, whole trees of each species were collected from each plot and analyzed for 239 Pu and 238 Pu. Beginning in 1986, needle samples were collected from selected pine trees in each of the plots. Because of poor growth and survival, the hardwood trees were not sampled after 1980. The results of data analysis support the conclusions that: (1) there is more 238 Pu uptake by pine tree seedlings than the other species, (2) there is greater transuranic radionuclide uptake in grown pine trees than in seedlings, and (3) there are greater concentrations of transuranic radionuclides in the grown pine trees on the trench plots than in the pine trees on the control plot. These data indicate that tree roots will extract transuranic isotopes from buried, low level waste. The amount of radioisotopes moved from the trenches to the surface is small and the level in the trees is low enough that dose from direct exposure will be very small. However, a longer term question is the amount of contamination that will move to the surface through trees and be incorporated in the surface soil after the foliage drops to the soil surface and decays. This process could contaminate surface soil that would later be used to grow food crops. A model was developed to estimate the potential for this type of transfer from the SRS alpha trenches. The results suggest that even following 100 years of transport, the transuranic, alpha dose from consuming food crops grown in the contaminated soil will be less than from naturally occurring isotopes.