MODEL FOR ESTIMATING Pii TRANSPORT AND DOSE 471 



predicting fallout interception by plants, but they did not provide quantitative methods 

 for doing so. 



Miller and Lee (1966) carried out extensive studies of fallout interception by plants. 

 The plants were cultivated in gardens near San Jose, Costa Rica, and the fallout was 

 provided by continuing eruptions of Irazu, a nearby volcano. Miller and Lee also 

 developed a comprehensive theoretical model of fallout interception by plants. The 

 model assumes different sets of constants for different fallout particle size classes, 

 different morphological characteristics of foliage, and different meteorological condi- 

 tions. Unfortunately, their model is practically unworkable, in spite of its elegance, 

 because it requires the use of constants and other parameter values that are rarely, if ever, 

 available for predictive purposes. 



The values of the interception factor determined experimentally by Miller and Lee 

 (1966) varied only sliglitly with respect to the different species of cultivated plants they 

 studied, and the only meteorological condition consistently correlated with large 

 differences in measured values of the interception factor was relative humidity. The 

 particles intercepted by plants were essentially the same sizes as those deposited on 

 adjacent soil surfaces. In both cases the mass median diameters for the volcanic dust 

 deposited as fallout were generally between 50 and 100 jLtm. Tlte weiglited averages of 

 interception factors for all the plant types tested (mostly garden vegetables) were 

 95.7 ± 66.9 cm^/g for damp exposure conditions (relative humidity greater than 90%) 

 and 47.4 ± 29.7 cm^/g for dry exposure conditions. 



Interception factors based on nuclear testing experience are about one or two orders 

 of magnitude lower. For detonations involving tlie incorporation of large quantities of 

 soil material in the initial cloud, estimates range from 1.9 to 11 .1 and have a mean of 3.7 

 cm^/g. For detonations involving the incorporation of little or no soil material in the 

 initial cloud, the estimated values are about an order of magnitude lower. 



Miller and Lee (1966) noted that the "foliar samples obtained at the weapons test 

 experiments were apparently subjected to an unknown degree of weathering before they 

 were taken, while the primary samples [in our studies] were collected at the end of a 12- 

 to 24-hr period of exposure to more or less continuous fallout from Irazu, and the weight 

 of dust deposited on leaves was often greater than the dry weight of the leaves." Heavy 

 dust deposits such as these are easily dislodged by the sliglitest mechanical disturbance, 

 and moderate rains were observed to remove more than 90% of the material deposited. 



Martin (1965) studied the interception and retention of *^Sr and ^^M by desert 

 shrubs (primarily Atriplex confertifolia and Artemisia tridentata). His estimates of the 

 plant interception factor were based on concentrations of ^^Sr and ' '^M in plant samples 

 collected 5 days after fallout deposition and estimates of the theoretical deposition rates 

 for these two radionuclides on unobstructed soil surfaces in the same locations ranging 

 from about 10 to about 100 miles downwind from the detonation point. Estimates for 

 different study areas range from 1.49 to 11.05 cm^/g, with the higher values occurring in 

 the more distant areas. The overall mean for ^^Sr was 4.09 cm^/g, and the overall mean 

 for ^^M was 4.00 cm^/g, approximately an order of magnitude lower than Miller and 

 Lee's average value for dry deposition conditions. Although the discrepancy may appear 

 to be large, it may be due to the effects of weathering during the 5 days between fallout 

 deposition and the collection of plant samples. 



Weathering Rate. To estimate the effective rates of ^^Sr and '^M loss from 

 fallout-contaminated plants, Martin (1964) coUected additional sets of plant samples at 



