290 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



Several studies approached the problem of particle retention by using smiulated quartz 

 fallout material containing adsorbed fission products. Witherspoon and Taylor (1'569) 

 found that over a 33-day period 88- to 177-^(m-diameter (MMD)* particles were more 

 effectively entrained by pine foliage than by oak. Although after only 1 lir wind 

 resuspension accounted for a 90% reduction in the number of particles in oak leaves as 

 compared with a 10% reduction in pine, the first rainfall (to + 1 day) accounted for a 

 15% reduction of particle activity remaining at 1 hr. Retention half-times reported for 

 periods of to 1, 1 to 7, and 7 to 37 days were 0.12, 1.4, and 25 days for oaks, and 0.26, 

 4.5, and 21 days for pines, respectively. 



A similar study by Witherspoon and Taylor (1970) presented data for the retention of 

 44- to 88- and 88- to 175-iL(m-lVIMD particles by various agricultural plants. These studies 

 indicate that average wind speeds of 0.5 mph over the initial 12-hr period following 

 contamination are more effective in removing the smaller particles (21.1% vs. 15.8%) 

 and that average wind speeds of 1.1 mph over a 12- to 36-hr period resulted in a higher 

 loss of the larger particles (21 .6% vs. 15.4%). Subsequent to to + 6 days, varying amounts 

 of rainfall resulted in a marked reduction in retention of particles of both size ranges. The 

 resuspension behavior of these relatively large particles is in keeping with theoretical and 

 empirical measurements on the inertial forces within the boundary layer required to 

 resuspend spores from leaves (Aylor, 1975; 1976; Aylor and Parlange. 1975). 



Subsequent studies by Witherspoon and Taylor (1971) with 1- to 44-|Um-MMD 

 simulated fallout particles showed longer weathering half-lives for 1- to 44-/jm particles 

 (17.9 days) than those reported earlier for 44- to 88- (15.7 days) and 88- to MS-iim (15.1 

 days) particles. Loss rates were also less affected by time or rainfall after a particle 

 residence time of 7 days. This suggests that particle size does, in fact, play an important 

 role in the extent of toliar retention. 



Although these studies aid in our understanding o^ the interception and retention of 

 larger particles (>10jL(m) analogous to close-in fallout, questions arise as to the behavior 

 of fallout particles of submicron size. Both Iranzo (1968) and Romney et al. (1975) 

 reported that plutonium-containing material resuspended in field situations is difficult to 

 remove from contaminated foliage; as much as 50%- is tenaciously held on foliar surfaces. 

 This suggests that retention is affected by factors other than the passive association of 

 particles with relatively flat foliar surfaces where only inertial forces influence their 

 removal or resuspension. 



In studies o\~ 6.77 ± 0.02-ium-AMAD+ uranine particles, where the primary particle 

 had an MMD in the submicron range. Wedding et al. ( 1975) have shown that deposition is 

 related to the roughness o^ the leaf surface. By analogy, the leaf-roughness factors 

 affecting deposition should also atTect retention. The etTect of wind and rainfall on 

 foliady deposited PbCI; particles (1- to 3-iUm MMD) was evaluated by Carlson et al. 

 (1976). These studies showed that lead particles remained fixed to leaves under 

 controlled conditions for up to 4 weeks after fumigation; reentrainment wind speeds of 

 up to 6.7 m/sec were inefTective in removing surface deposits. Losses due to simulated 

 rainfall were proportional to the amount of rainfall; mists were more efTective than 

 droplets in removing lead deposited on leaf surfaces; only 15 and 5% of the foliar 

 deposits, respectively, were leachable. 



*Mass median diameter (MMD) assumed; particles pliysically measured. 

 tActivity median aerodynamic diameter. 



