Ecological Risk Assessment of Radiological Exposure to Depleted Uranium in Soils at a Weapons Testing Facility

Abstract

The potential for unacceptable risks to biota from radiological exposure to depleted uranium (DU) in soils was evaluated at two sites where DU weapons testing had been conducted in the past. A screening risk assessment was conducted to determine if measured concentrations of DU-associated radionuclides in site soils exceed radionuclide levels considered protective of biota. While concentrations of individual radionuclides did not exceed acceptable levels, total radionuclide concentrations could result in potentially unacceptable doses to exposed biota. Thus, a receptor-specific assessment was conducted to estimate external and internal radiological doses to vegetation and wildlife known or expected to occur at the sites. Wildlife evaluated included herbivores, omnivores, and top-level predators. Internal dose estimates to wildlife considered exposure via fugitive dust inhalation and soil and food ingestion; root uptake was the primary exposure route evaluated for vegetation. Total doses were compared with acceptable dose levels of 1.0 and 0.1 rad/day for vegetation and wildlife, respectively, with potentially unacceptable risks indicated for doses exceeding these levels. All estimated doses were below or approximated acceptable levels, typically by an order of magnitude or more. These results indicate that current levels of DU in soils do not pose unacceptable radiological risks to biota at the sites evaluated.

Keywords:

• Depleted uranium
• ecological risk assessment
• ecological dose modeling
• radiation exposure
• biota dose limits.

The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S. Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Work supported under a military interdepartmental purchase request from the U.S. Department of Defense through U.S. Department of Energy Contract W-31-109-Eng-38.

Notes

^a Activity concentrations (Bq/g) of uranium isotopes, U-238, U-235, and U-234 were calculated on the basis of the total uranium concentration and the average activity ratios among the isotopes. Activity ratios for each sample were first calculated, then the ratios were summed across all the samples to obtain the average values. The average activity ratios for the KS Impact Area were U-238:U-235:U-234 = 1:0.016:1.15. The average activity ratios for the G-6 Test Area were U-238:U-235:U-234 = 1:0.0357:1.09. Therefore, the activity concentration of U-238 (Bq/g) can be obtained by dividing the total uranium concentration (μ g/g) with 81.37 for the KS impact area, and with 81.61 for the G-6 test area.

^b Maximum of the sampling data used to derive the percentile values.

^c Minimum of the sampling data used to derive the percentile values.

^a CEPA, 2002;

^b estimated using allometric equations from USEPA, 1993; food ingestion rate based on dry weight;

^c Beyer et al., 1994;

^d estimated using allometric equation in Harestad and Bunnel, 1979;

^e assumed;

^f based on Ord's kangaroo rat (Garrison and Best, 1990);

^g assumed negligible on basis of receptor foraging mode;

^h Drewek, 1980;

ⁱ Schoener, 1968;

^j Mautz and Nagy, 1987;

^k Turner et al., 1969;

^m based on tiger rattlesnake (Crotalus tigris);

ⁿ Rodriguez-Robles and Greene, 1999.

^a Source: USDOE, 2002.

^b Root uptake transfer factor was calculated with the correlation with Kd:ln (Kd) = 2.1 + 0.56 × ln(B_iv), where B_iv is the root uptake transfer factor, and 2.1 and 0.56 are coefficient values for sandy soil (Baes et al., 1994).

^a The dry-weight insect tissue concentrations were assumed to be the same as the soil concentrations. The water content in tissues was assumed to be 0.65. The tissue concentrations for the wildlife receptors were calculated assuming that all the food ingested and air inhaled were contaminated. Exposure parameters are listed in Table 2.