212 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



Oksza-Chocimowski (1977) provided a "generalized" model for the change in 

 half-time of the resuspension factor which allows the ratio of the resuspension factor at 

 time zero to the resuspension factor at a long time [R(0)/R(o°)] to vary: 



T^(t)=Aln(l+B + CtD)+-^|^ (2) 



In — -^^ 



R(oo) 



where A = constant coefficient, days 

 B = constant 



C = constant coefficient, days" 

 D = constant exponent 

 R(0') = initial resuspension factor, m~ ' 

 R(oo) = final resuspension factor, m~^ 



t = time since contaminating event, days 



This expression was evaluated for a range of values of the constants with limiting 

 values corresponding to ratios of R(0)/R(°°) of 10 and 10''. However, data are not 

 available to make adequate choices for a given area. 



In both the Anspaugh, Shinn, and Wilson and the Oksza-Qiocimowski models, the 

 constants are evaluated on the basis of data from a hmited number of areas and 

 conditions. In Anspaugh 's model, for example, it is apparent that the final resuspension 

 factor after long aging is based on wind resuspension only in a desert area. Whether 

 disturbance in the area would cause an increase is unknown, although it appears likely to 

 do so. Sehmel and Orgill (1974) made measurements downwind from the original 

 oil-storage pad at Rocky Flats and related the concentrations found to the 2,1 power of 

 the wind speed. Subsequent measurements were higher, however, owing to the digging of 

 a ditch between the oil storage area and the sampler. This work also involved increased 

 vehicular activity. This disturbance caused about an order of magnitude increase in 

 concentrations at the samplers which persisted after the work had been completed 

 presumably because of a change in the character of the surface. The half-time for decrease 

 over the next 7 montlis appeared to be about 9 months. Thus it is possible that 

 disturbance could not only increase the concentration at the time that it occurs but also 

 could result in increased wind-driven resuspension factors for some time thereafter. 



Sehmel and Lloyd (1976a) and Sehmel (1977a) have also provided data on the 

 wind-borne resuspension of a submicrometer tracer, calcium molybdate, that was applied 

 to a test area by spraying. In this experiment, using cascade impactors arranged to operate 

 in different wind conditions, they noted that the resuspension factor at 1.8 m height 

 increased as the 6.5 power of the wind speed. However, this slope was determined by 

 drawing the line through the bottom end of uneven wind-speed ranges, a procedure that 

 could well result in an overestimate. They also noted, for this system, that there seemed 

 to be little, if any, decrease in the resuspension over a period of 3 yr. Whether this is due 

 to a "preaging" by the apphcation in water is unknown. 



In concluding the discussion of the resuspension factor, it is apparent that this 

 empirical approach does not inherently incorporate many of the variables, and present 

 estimates are relatively crude. In particular, the present estimates appear to be based on 

 short-term experiments with Uttle attempt to provide a factor applicable to long-term 

 (say, 1 yr) averages. 



