REVIEW OF RESUSPENSION MODELS 2\ 1 



meteorological conditions at the time, and the nature of the soils. However, many of the 

 measurements were made in d6sert areas with low soil moisture where resuspension would 

 be expected to be highest. 



An apparent reduction in the resuspension factor with time was indicated by Wilson, 

 Thomas, and Stannard (1960) from measurements at the Nevada Test Site (NTS). Here 

 samples were taken at three different distances from the center of a safety shot starting 

 about 1 month after the contamination occurred. The investigators noted that the data 

 were too erratic to establish half-times for the decay of the air concentration beyond a 

 very crude estimate. This estimate was made by plotting the medians of the data at each 

 sampling distance, and it provided a value of 5 weeks for the concentration half-time. 

 This value was used by Langham (1969) in assessing future hazards from plutonium 

 contamination. A somewhat larger half-life of 45 days was used by Kathren (1968) in a 

 study of acceptable contamination levels for plutonium in soils. Anspaugh et al. (1973) 

 measured the decrease in air concentration with time immediately following a cratering 

 event in Nevada and following the venting of an underground shot. For the cratering 

 event sampling was carried out for 6 weeks following the event with a measured half-time 

 of 38 days. Tlie venting experiment started 3 months after the event and continued for 9 

 to 10 months, with the most predominant radionucUdes being the isotopes of ruthenium. 

 Here a half-life of 66 days was found. 



However, the consequences of continuing such half-times over a long period were 

 pointed out by Healy (1974) and Anspaugh (1974). Healy noted that samples taken at 

 the same location as those taken by Wilson, Thomas, and Stannard (1960) but about 1 yr 

 after the conclusion of the Wilson, Thomas, and Stannard study (Olafson and Larson, 

 1961) gave values up to several orders of magnitude greater than would be predicted by 

 the 35-day half-Ufe. Anspaugh (1974) indicated that the functional nature of the decrease 

 in resuspension rate with time cannot be confidently extrapolated and that previously 

 pubHshed models should not be applied to calculations many years after the 

 contamination event. He also cited two sets of measurements at NTS where the area had 

 been contaminated with plutonium by high-explosive detonations some 20 yr earlier. 

 These studies gave values for the resuspension factor of 3 x 10"^^ m~^ and 2 x 10~^ 

 m~' . These data indicate unequivocally that resuspension does occur after this period of 

 time, although predictions using the short half-life following deposition would result in 

 unmeasurable values of air concentration. 



Anspaugh, Shinn, and Wilson (1974) used the available information to derive an 

 empirical expression for the resuspension factor which allows the resuspension factor to 

 decrease with time. In their derivation they used four constraints: (1) the apparent 

 half-time of decrease during the first 10 weeks should approximate a value of 5 weeks, (2) 

 this half-Ufe should about double over the next 30 weeks, (3) the initial resuspension 

 factor should be 10""* m~\ and (4) the resuspension factor 17yr after the 

 contaminating event should approximate 10~^ m~^. The value for the resuspension 

 factor in the aged source resulted from 23 individual air concentration measurements at a 

 location contaminated with plutonium 17 yr earlier where the average resuspension factor 

 was found to be 10~^ m"^ . An expression that approximates these constraints is given as 



R(t) = 10-^ exp [-0.15 (t)'^] + 10-^ (1) 



where R(t) is the resuspension factor (meters"^) and t is the time since the 

 contaminating event (days). 



