TRANSURANIC AND TRACER SIMULANT RESUSPENSION 283 



Airborne concentrations of ^^^Pu, ^"^^Am, ^^^Pu, ^-^"^Cs, and ^°Sr increased 

 (Sehmel, 1977c) as a function of wind speed to the one to sixth power for on-site 

 resuspension study sites. Airborne plutonium concentrations for off-site resuspension 

 increased as a power function of wind speed. Above a wind speed of about 5 m/sec, 

 airborne plutonium concentrations increased with wind speed to the third to fifth power. 

 Explanations for differences in exponents are needed. Differences might be attributed to 

 source characteristics and extent. 



In contrast, for controlled source experiments, tracer-particle resuspension rates 

 increase with about the fifth power of wind speed. This fifth power is similar to off-site 

 plutonium resuspension. 



Plutonium concentrafions on collected airborne soil at Rocky Flats ranged from a 

 maximum of twice the concentration on ground-surface soil to as low as 10""* of the 

 concentration on surface soils. The maximum reported (Krey et al., 1976b) surface-soil 

 concentrafion was 3 x 10"^ /iCi/g. Even at tliis relatively high plutonium surface-soil 

 concentrafion, airborne respirable plutonium concentrations were significantly below 

 airborne MPC's on a yearly basis. Similarly, at the Hanford resuspension study sites 

 (Sehmel, 1977c), maxim.um concentrations per gram of airborne solid for transuranics 

 were: for^^^Pu, 6 X IQ-"^ iuCi/g; for "^Pu, 1 x 10"^ /uCi/g; and for ^^ ^ Am, 7 x lO^^ 

 AtCi/g. 



There is much uncertainty in the relafionships between radionuclide concentrations 

 per gram of airborne solid and per gram of surface-soil sofids. For tlie transuranic-element 

 data reported at Rooky Flats, the ratio ranges from 10~^ to 2, and at Hanford tlie ratio 

 ranges from 0.5 to 1.5 X 10^. Uncertainties are probably complicated by spatial 

 distributions of surface contamination. 



Radionuclide particles can be resuspended either as individual particles or. more 

 probably, attached to host soil or solid particles. An average-activity, or normal-activity, 

 radionuclide particle distribution is usually collected on sampling filters. However, at 

 both Rocky Flats and Hanford, one filter sample collected in each case showed 

 significantly greater plutonium concentration than tlie maximum for all other samples 

 collected during the same time period. These anomalous higher concentrafions are 

 attributed to one or more plutonium particles of unusually higher activity than those 

 normally, or most frequently, resuspended. 



There is increasing, but still conflicting, data that ^^^Pu might be more mobile tlian 

 ^^^Pu. The isotopic-ratio data reported for airborne plutonium transport on nonrespira- 

 ble particles at Rocky Flats support the greater mobility concept. Tliis conclusion was 

 obtained by comparing airborne ^^^Pu/^^^Pu rafios on nonrespirable soil near the 

 eastem security fence and the eastern cattle fence. At the eastern security fence, airborne 



Pu/ Pu rafios were similar to ground surface ratios. However, at the eastern catfie 

 fence, the ^^^Pu/^^^Pu rafios were significantly greater than those measured on local 

 surface soils (see Fig. 11). Consequenfiy an explanafion is needed for the increased 

 ^^*Pu/^^^Pu rafio at distances from the original oil storage area. Possible explanafions 

 include preferential biodegradation of ^^^Pu compared with ^^^Pu and preferential 

 ejecfion of ^^^Pu at the eastern security fence and at the eastern catfie fence. These 

 possibilifies exist but are not definifive. Further research is needed to explain the higher 

 relative airborne ^^^Pu/^^^Pu concentrafions near the eastern catfie fence. 



As used in this chapter, the weathering half-life is the time required for airborne 

 concentrafions at a resuspension site to decrease by one-half. Weathering is probably a 



