TRANSURANIC AND TRACER SIMULANT RESUSPENSION 281 



nonrespirable particles and the subsequent degradation to a respirable resuspension 

 source. 



• One possible assumption for describing resuspension concerns the prediction of 

 plutonium concentration per gram of airborne soil vs. plutonium concentration per gram 

 of surface soil. However, ranges of airborne concentration per gram vs. surface-soil 

 concentration per gram indicate a wide discrepancy between airborne vs. surface soils. 

 This wide discrepancy shows that there seems to be no justification for assuming any 

 equalities between plutonium concentrations on surface soils vs. airborne soils. 



• The rate of change of average airborne radionuclide concentrations with time has 

 been described by a weathering half-life. However, the weathering half-life is not well 

 known. Data shown for radionuclides as well as for inert tracer particles indicate that the 

 half-life is from 5 months or longer rather than the often quoted 35 to 40 days. 



• At Rocky Flats the ^^^Pu/^-'^Pu ratio can be much greater on airborne 

 nonrespirable soils than on surface soils. Thus there may be a preferential resuspensioh 

 transport of '^^Pu vs. '^^Pu. For migration witliin surface soils, there are data showing 

 preferential migration with depth as well as of ^^^Pu compared with ^^^Pu. 



Botli ^^^Pu and ^^^Pu resuspension occurred on site at Rocky Flats and the Hanford 

 area, but all airborne plutonium concentrations were significantly below maximum 

 permissible concentrations (MPC's) in air. In addition, plutonium was resuspended from 

 off site near the Hanford reservation. In ail cases plutonium was deposited on each stage 

 of particle cascade impactors, which indicates that most plutonium was resuspended 

 while attached to larger host soil particles. 



Plutonium was transported on both respirable and nonrespirable airborne-soil 

 particles. In most resuspension research reported by other researchers as well as in 

 air-monitoring activifies, airbome concentrations of particles have been measured without 

 sampling both respirable and all nonrespirable particle sizes present. Only respirable or 

 near-respirable size particles are frequently measured since the usual air-sampling 

 techniques tend to keep larger, nonrespirable particles from being collected. Conse- 

 quently total airborne plutonium concentrations could be greater than normally reported 

 using most existing s^mpUng equipment systems. Results from those systems are a 

 conservative estimate (high concentration) of airborne respirable plutonium concentra- 

 tions. Nevertheless, plutonium transport on nonrespirable particles may be a significant 

 factor in total plutonium transport. Larger than respirable particles are resuspended and 

 may not travel too far downwind before redepositing again. In contrast, respirable 

 particles remain airborne for a much longer distance. Additional research is needed to 

 clarify the relative significance of plutonium transport on respirable as compared with 

 nonrespirable particles. 



Plutonium concentrations per gram of both respirable and nonrespirable airborne soils 

 discussed in this chapter are summarized in Table 1 1. Ranges are within several orders of 

 magnitude, from 2 x 10"^ to 6 x 10"^ /jCi/g for respirable ^^^Pu and from 1 x 10' '' 

 to 3 X 10"'^ //Ci/g for nonrespirable ^^^Pu. 



In all cases plutonium concentrations per gram of soil were calculated on the basis of 

 total soil samples. In addition, there is no proven method to predict the ratios of 

 concentration per gram of airborne soil to concentration per gram of surface soil. As 

 shown in Tables 2 and 4, this ratio ranges seven orders of magnitude from 10"'* to 10^. 

 This uncertainty range is almost as large as the uncertainty range of 10~^ ^ to 600 m ^ 

 for resuspension factors (Mishima, 1964; Stewart, 1967; Sehmel and Lloyd, 1976a). 



