EFFECTS OF POCKET GOPHER BURROWING ON CESIUM-133 DISTRIBUTION ON ENGINEERED TEST PLOTS

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 Cs-133 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-mo 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(-1)). Vegetation-bearing plots had significantly more total subsurface Cs (mu = 1.7 g kg(-1)) than plots without vegetation (mu = 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.