No abstract

Very low levels of radionuclides exist on soil surfaces from atmospheric fallout following weapons testing or from stack discharges, and from exposure of some of the older waste storage and disposal sites worldwide. Biological factors including vegetation and animal burrowing can influence the fate of these surface contaminants. Animal burrowing introduces variability in radionuclide migration that confounds estimation of nuclide migration pathways, risk assessment, and assessment of waste burial performance. A field study on the surface and subsurface erosional transport of surface-applied t33Cs as affected by pocket gopher (Thomomys bottae) burrowing was conducted on simulated waste landfill caps at the Los Alamos National Laboratory in north central New Mexico. Surface loss of Cs, adhered to five soil particle size ranges, was measured several times over an 18-too period while simulated rainfalls were in progress. Gophers reduced Cs surface loss by significant amounts, 43%. Cesium surface loss on plots with only gophers was 0.8 kg totalled for the study period. This compared with 1.4 kg for control plots, 0.5 kg for vegetated plots, and 0.2 kg for plots with both gophers and vegetation. The change in Cs surface loss over time was significant (P < 0.01). Relatively little subsurface Cs was measured in plots containing only gophers (0.7 g kg-t). Vegetation-bearing plots had significantly more total subsurface Cs (p 1.7 g kg -l) than plots without vegetation (l~ = 0.8 g kg -1). An average of 97% of the subsurface Cs in plots with vegetation was located in the upper 15 cm of soil (SDR1 + SDR2) compared with 67% for plots without vegetation. Vegetation moderated the influence of gopher activity on the transport of Cs to soil subsurfaces, and stabilized subsurface Cs by concentrating it in the rhizosphere. Gopher activity may have caused Cs transport to depths below that sampled, 30 cm. The results provide distribution coefficients for models of contaminant migration where animal burrowing occurs.

Material Disposal Area G at the Los Alamos National Laboratory is a lowlevel radioactive waste storage facility. The noticeable presence of pocket gopher mounds and cast soil on closed waste burial sites of various types resulted in the need to understand possible interactions between gophers and radioactive waste at Area G. In our study, pocket gophers, mound soil, off-mound surface soil, and vegetation were collected at Area G and at off-site background locations. The samples were analyzed for 241 Am, 238 Pu, 239 Pu, 3 H, and total U. A comparison of radionuclide concentrations in mound soil to surface soil and in gophers to soil and vegetation implied that gopher activity is generally not resulting in the upward transport of radionuclides. Concentrations of 241 Am, 238 Pu, 239 Pu, and 3 H in some of the gopher, soil, and vegetation samples were higher than background at some of the sites, however, gophers at only one site within Area G had 3 H concentrations that resulted in an estimated dose to gophers that could impact their health. Relationships in radionuclide concentrations between the four media (pocket gophers, mound soil, off-mound surface soil, and vegetation) were examined by conducting correlation tests. Correlations were highest for Am 241 and 238 Pu, however, only the 238 Pu relationship may be accurate enough to be used in predicting concentrations. The relationship in radionuclide concentration between pelts and carcasses was highly variable-carcasses, including the gastrointestinal tract, contained between 51% and 575% of the radionuclide concentration on pelts. Data generated by this study are valuable for ecological risk assessments. Further investigation through modeling and monitoring may be necessary to determine if the 3 H shafts are a source of environmental 3 H levels that are of ecological concern. Future research should include modeling the transport of radionuclides through ecological receptors within and around Area G. This should include investigations of transfer to high-level carnivores, especially raptors. w3z-tGp-0~6 w-i fWw I!os Wd t 12 B85 t 'L 1 J~4uJOOOa 'hpsmy~. This report has been reproduced directly from the best available copy. It is available electronically on the Web (httn:l/www.doe.govlbridge).