210 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



Another type of resuspension that will be considered only briefly is transfer 

 resuspension. This involves the transfer of the contaminant from its place of deposit to 

 another place where inhalation may be more probable. Unfortunately data to describe 

 this type of resuspension are extremely limited. 



Each of these types of resuspension wiU be considered in deriving the best estimate of 

 exposure to an individual in an area contaminated by the spread of transuranium 

 elements in the soil. 



Resuspension Modeling 



The three basic techniques in use for resuspension modeling are (1) the resuspension 

 factor, (2) the resuspension rate, and (3) the mass-loading approach. Each method has its 

 strengths and its weaknesses, particularly in view of the state of the technology at this 

 time. Each of the techniques is described, and its advantages and disadvantages are 

 discussed. 



Resuspension Factor 



The resuspension factor is defined as the ratio of the concentration in the air at a 

 reference height (usually 1 m) to the quantity of the contaminant per unit area on the 

 surface of the ground. The usual units are meters"^ . Its chief advantages are its simplicity' 

 and the fact that most measurements in the past have been expressed in this form; so 

 values are available for calculation.. The chief disadvantages are that it is a completely 

 empirical formulation, and thus it is difficult to extrapolate from one terrain to another, 

 and it ignores the distribution of the contamination over the are;^ and the size of the 

 contaminated area involved. Thus the denominator contains the local quantity on the 

 ground, and the numerator is an undefined function of the air concentration resulting 

 from upwind contaminated areas and activities. 



A problem common to this model, as well as to all others, is the uncertainty resulting 

 from the depth to be used in assessing the quantity per unit area to be used in the 

 denominator. For a uniform profile in the soils, the quantity per unit area will increase in 

 direct proportion to the depth of the sample used. For nonuniform profiles the estimate 

 of the quantity per unit area will also change, depending on the depth used. For wind 

 resuspension it can be assumed that the appropriate depth is small, perhaps a miUimeter 

 or less, although it is likely that this depth may be variable, depending on wind speed and 

 the degree of saltation allowed by the size of the area and the nature of the soils. For 

 mechanical disturbance the depth will be some function of the depth to which the 

 disturbance occurs, the function depending on the relative ease with which the particles 

 can escape from the soil to the atmosphere. 



Mishima (1964) has tabulated resuspension factors measured over a variety of 

 conditions. It is frequently noted that these values range over eleven orders of magnitude. 

 However, the values quoted represent both outdoor and indoor conditions, with and 

 without mechanical disturbance, and at various times after the contaminant has been 

 deposited. In a brief review of Mishima 's table, it is noted that the values for mechanical 

 disturbance range from about 2 x 10~^ to 7 x 10~^ m"^ (with one value of 10~^ m"^ 

 based on uranium contaminant with dust stirred up and sampling at a height of 1 ft 

 ignored). For periods of no activity, with relatively freshly deposited material, the values 

 generally range from 10"^ to 2 X 10~^ m~^ , whereas for aged material they range from 

 6 X 10~'° to 10~'^ m~^ . It is difficult to generalize these numbers because the exact 

 value will depend on the degree of disturbance, the placement of the sampler, the 



