242 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Fallout levels of ^^^Pu entering the area were estimated from the cascade impactor 

 operated in the mountains. There was no detectable ^■'^Pu activity on the 7-, 33-. and 

 1.1 -)nm impactor stages, and there was no radiochemical result for tlie l-jim stage. The 

 only detectable background plutonium activity was on the backup filter that nominally 

 collects submicrometer particles. Airborne ^^^Pu concentration at the background 

 station was 4 ± 3.5 X 10" ' ^ AtCi/cm^ , which corresponds to 0.7 ± 0.62 X 10~^ /jCi/g of 

 airborne soil on the backup filter. Error limits are the 2a radiochemical counting limits. 



In on-site research, airborne particles were separated in the sampling process into two 

 main fractions. One sample contained particles collected by gravity settling in the inlet 

 cowl section of the sampler as shown in Fig. 3. The second fraction contained those 

 particles passing through the inlet section and collected within the high-volume cascade 

 impactor. The smallest particles collected in the inlet cowl section were about lO/imin 

 diameter. This fraction was assayed for ^^^Pu and ^^^Pu. In some cases nonrespirable 

 particles were sieved into smaller size fractions, and these fractions were also assayed for 

 ^^^Pu and ^'^^Pu. Data for respirable and nonrespirable particles are discussed separately. 



Respirable Plutonium Concentrations at Rocky Flats. Airborne •^ ^ ^ Pu concentrations at 

 the three Rocky Flats sampling stations were reported (Sehmel and Lloyd, 1976b) in 

 microcuries per cubic centimeter of air and microcuries per gram of airborne soil. The 

 maximum airborne ^^^Pu concentration was 3.7 X 10^' ^ jnCi/cm^ . The maxima. u 

 ^^^Pu concentration on the airborne soil was 5 x 10~^ AiCi/g total airborne soil and 

 7 X 10~^ AiCi/g for the respirable fraction of airborne soil collected on the 2-/nm-particle 

 impactor stage. All airborne ^^^Pu concentrations were significantly less than MPC's of 

 soluble ^^^Pu in air for occupational exposure in a 40-hr work week (2 x 10~^^ 

 AtCi/cm^) or nonoccupational exposure in a 168-hr week period (6 X 10~^^ juCi/cm^) 

 (International Commission on Radiological Protection, 1959). 



Airborne plutonium concentrations were a function of both sampling height and 

 particle diameter. Airborne concentrations are shown for site AB in Fig. 4 for each 

 particle cascade impactor stage. In contrast to simple modehng concepts, airborne 

 concentrations did not always decrease with an increase in height. There were 

 unexpectedly high ^^^Pu concentrations at this site for several particle diameters and 

 heights. 



Plutonium was associated with particles collected on each particle cascade impactor 

 stage. Since there was no plutonium in the upper stages of the impactor at the 

 background mountain site, the ^^^Pu found in upper stages of impactors at Rocky Flats 

 sampling sites indicates that some plutonium was resuspended while attached to larger 

 particles. Resuspension of submicrometer particles also occurred at Rocky Flats. 



The general trend of the complete airborne ^^^Pu concentration data is a decrease in 

 concentration with increasing distance eastward from site A (Sehmel and Lloyd, 1976b). 

 As might be expected, this decrease in concentration corresponded to increasing distance 

 from the original oil storage area, which was the principal source of ground 

 contamination. However, significant deviations did occur in concentration profiles of 

 airborne '^^^Pu with both distance and height. These deviations might be attributed to 

 sampling some more-active-than-normal particles or clusters of particles. These increases 

 in average airborne ^^^Pu concentrations were present at both sites AB and B. 



As indicated in Fig. 5 for site AB, which was over 392 m from the oil storage area and 

 which was on the flat terrain, some more-active-than-normal particles or clusters of 

 particles (hot) may have been present in the 2.0-/im size range. In tliis case the 



