factors could have the effect of making biological transport of plutonium to man progressively more important relative to physical transport and inhalation. These considerations are interesting because most assessments of the potential hazards of environmental plutonium attributed little importance to biclogical transport and ingestion compared to physical transport and inhalation. The values of the interception factor determined experimentally by Miller and Lee varied only slightly 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 voicanic dust deposited as fallout were generally between 50 and 100 um. The weighted 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 percent) and 47.4 + 29.7 cm*/g for dry exposure conditions. Foliar Deposition Estimation of depvsition velocity, V,, 1t.e., the ratio of surface deposition surfaces. rate to air concentration, was discussed earlier as applied to soil for different Deposition velocities have also been determined experimentally to various kinds of plants, and several kinds of vegetation, with respect more conveniwas it In the present study, however, aerosols and particulates. the product of air ent to base estimates of air to plant deposition rates on vegetation) concentration, soil/air deposition velocity, and a plant (or The plant interception factor is defined as the amount interception factor, divided by the initially deposited per gram (dry weight) of plant material amount initially deposited per unit area of soil surface. on of airborne Much of the available information concerning the intercepti fields radionuclides by plants has come from studies made in the fallout Romney et al. (1963) summarized produced by nuclear test detonations at NTS. found that levels early studies in the vicinity of the Nevada Test Site. They (a) downwind distance of fallout deposition on plants varied with respect to: midline of the from the detonation point, (b) lateral distance away from the with different fallout field, (c) a variety of morphological features associated surfaces near plant species, and (d} the level of fallout deposition on soil ip between the contaminated plants. White there was no constant relationsh there was a fallout concentrations on plants (dpm/g) and on soils (dpm/m*), and in the fraction of goo¢ correlation between radioactivity in plant samples Apparently, the larger fallout samples with particle sizes less than 44 um. and were not ag readily particles were deposited close to the detonation point were deposited intercepted by plants as were the smaller particles which were apparently more farther downwind: or, if they were intercepted, they In the laboratory, Romney et al. easily removed by weathering processes. ity on fallout(1963) found that 50 to 90 percent of the gross radioactiv water or a wetting agent contaminated plants could be removed by washing with ed the possibility of such as versene. These and similar studies demonstrat they did not provide quantitative predicting fallout interception by plants, but methods for doing so. studies of fallout interception by Miller and Lee (1966) carried out extensive near San Jose, Costa Rica, and plants. The plants were cultivated in gardens s of IrazG, a nearby volcano. the fallout was provided by continuing eruption theoretical node! of a Miller and Lee also developed a comprehensive s lants. The model assumes different sets of constant nb t morphological characteristics iitferent fallout particle size classes, differen Unfortunately, their ns. of foliage, and different meteorological conditio elegance, because it requires model is practically unworkable, in spite of its are rarely if ever the use of constants and other parameter values which available for predictive purposes. Interception factors based on nuclear testing experience are about one or two orders of magnitude lower. For detonations involving the 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 ne soil material in the initial cloud, the estimated values are about an order of magnitude lower. | | Miller and Lee 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-hour period of exposure to more or less continuous fallout from IrazG, 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 slightest mechanical disturbance, and moderate rains were observed to remove more than 90 percent of the material deposited. ; l Martin (1965) studied the interception and retention of 89¢r and !31yz by desert shrubs (primarily Atriplex confertifolta and Artemisia tridentata). His estimates of the plant interception factor were based on concentrations of 89sr and '3lr 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 em?/g, with the higher values occurring in the more distant areas. The overall mean for 99Sr was 4.09 cm?/g, and the overall mean for !3!1 was 4.00 em? /g: approximately an order of magnitude lower than Miller and Lee's average value for dry deposition conditions. Aithough the discrepancy may appear to be a large one, it is easily accounted for by the effects of weathering during the 5 days that elapsed between the time of initial fallout deposition and Martin's first collection of plant samples. Weathering Rate . To estimate the effective rates of ®9sr and #311 loss from fallout-contaminated plants, Martin (1964) collected additional sets of plant samples at intervals of 10, 15, 30, and 60 days after fallout. Factoring out the less of !3!] due to vaporization and the losses of both radionuclides due to radioactive decay, the data indicated that the weathering (environmental) half-time for these two radionuclides increased with respect to time after fallout. During the interval from 5 to 15 days after fallout, the average weathering half-time was about 20 days. The value obtained for the interval from 15 to 30 days after fallout 634 635