Appendix C

Resuspension

One widely used method to determine the environmental transport of small aerosolized particles due to resuspension is utilized by the U.S. Department of Energy (DOE) computer code RESRAD. This code addresses the calculation of potential exposure due to material deposited on the ground at DOE sites formerly used in the production of nuclear weapons and fuel. The equation for the environmental transport factor (ETF) is:

ETF (g/yr) = ASR x FA x FCD x FO x FI

where

ETF = environmental transport factor for dust inhalation (g/yr)

ASR = air/soil concentration ratio = average mass loading of airborne contaminated soil particles (2 x 10-4 g/m3).

FA = area factor (dimensionless)

FCD = cover and depth factor (dimensionless)

FO = occupancy factor (dimensionless)

FI = annual breathing rate (8,400 m3/yr)

The area factor, FA, is the fraction of airborne dust contaminated. It is calculated using a mixing model for estimating the dilution of contaminated dust that is resuspended on-site by uncontaminated dust blown in from off-site. In the particular case of DU, it would be the fraction of surface soil contaminated with DU. Isotopic analysis is necessary to separate DU from natural uranium. The factor, FA, is estimated in RESRAD from:

where

A = area of the contaminated zone (m2)

DL = dilution length (3 m)

The dilution length, DL, depends on the wind speed, mixing height, resuspension rate, and thickness of the resuspendable dust layer (Gilbert et al., 1983). Estimates of the lower and upper bounds of DL are 0.03 and 250 m, respectively.

The cover and depth factor, FCD, is the fraction of resuspendable soil particles at the ground surface that are contaminated:

where

T(t) = thickness of contaminated zone at time t

dm = depth of soil mixing layer (0.15 m)

The occupancy factor, FO, depends on the time spent in the contaminated area. An upper limit is assumed to be a few percent, in zones where a DU puff occurred.

The total annual inhalation exposure (in grams of DU inhaled per year) is then estimated from the equation assuming average values and a credible range of values.

A complicating factor in the estimation of the environmental transport factor, ETF, is that natural uranium is present in all soil and resuspended airborne soil. The natural concentration averages 37 mBq/g. The EPA has measured uranium in air for many years. This includes measurements in 25 cities in the United States in their Environmental Radiation Ambient Monitoring Program (ERAMS). DU present must be evaluated in the presence of natural uranium, and isotopic values are requisite to provide sufficient information.

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