Effects of Pocket Gopher Burrowing on Cesium‐133 Distribution on Engineered Test Plots

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.

The Uptake and Distribution of Buried Radionuclides by Pocket Gophers (Thomomys bottae)

“…The mounds sampled in our study were created within a 48 h period before sampling so that dispersal of radionuclides from the mound through erosion could be assumed not to have occurred. [17] Intrusion into the waste cell likely did not occur, however, because the carcass did not possess higher radionuclide concentrations than off-mound soils at any other site in this sample area. A higher radionuclide concentration in gopher carcasses compared to offmound soils could also indicate intrusion into contaminated soil or a waste cell.…”

Ecohydrology Monitoring and Excavation of Semiarid Landfill Covers a Decade after Installation

“…Other work at this site has shown that pocket gophers can increase rates of infiltration and water penetration by 200 to 300% (Hakonson, 1998). Pocket gophers and other small mammals can displace large amounts of soil (Cox, 1990; Hakonson et al, 1982; Mielke, 1977; Spencer et al, 1985);, translocating it to the soil surface (Arthur et al, 1987; Gonzales et al, 1995; O'Farrell and Gilbert, 1975; Schuman and Whicker, 1986; Winsor and Whicker, 1980). Therefore, biointrusion barriers such as our gravel and cobble layers or gravel incorporated into the topsoil may be essential to even the short‐term success of any cover design.…”

“…Climate changes may affect a site's water balance directly through increased or decreased precipitation and indirectly through influences on pedogenic and ecological factors. Numerous reports have pointed out the potential for environmental processes to modify landfill covers and liners (Gonzales et al, 1995; Johnson and Urie, 1985; Link et al, 1995; Nicholson and Safaya, 1993; Waugh and Smith, 1996). Several mechanisms whereby landfill barriers are likely to fail in >100 yr are discussed by Suter et al (1993), who recommend that either perpetual care be required or that barriers be designed for long‐term integrity.…”

Source Document: The Relationship Between Pocket Gophers (Thomomys bottae) and the Distribution of Buried Radioactive Waste at the Los Alamos National Laboratory

“…However, Gonzales et al (1995) point out that the latter potentially occurs at the expense of increased channeling of water into the waste cell zone through gopher burrows. Because radionuclides have a capacity to bind to the silt-clay fraction, erosion of these particles has been inferred as the primary transport mechanism for surface-deposited radionuclides that are susceptible to erosion forces (Gonzales et al, 1995).…”

“…Mound building has been cited for potentially increasing the movement of nutrients (Litaor et al, 1996) and sediments (Black and Montgomery, 1991) offsite through overland flow, which would also affect the movement of any associated contaminants. Hakonson (1999) studied the effects of gopher burrowing activities on surface water runoff and erosion under the same experimental design described in Section 2.5 for a study by Gonzales et al (1995). Erosion from vegetated plots and plots covered with gopher mounds was less than the erosion from bare plots.…”