Wo aud xo oe 7 a w < Oo s aa: o| w 4a| ca nw 2 © ore als «< e ow = —“olo wea =< oO AIRBORNE FALLOUT DEPOSITION w a - < > Oar;a a < ad < z= g= 'ud 2 x w - z|8 tela z — oO = — TA wd 4 - le oOo]. 4 uw ao = x ox ex oOo lw Ofz rE “1 ERE > -_ ee Hy 3 —- oi oS = = Oo “= _—_—_ peo ~ - = oOo as wu — = a oe ct = «aL FE = S&S o +» = oS — z Ww © a 2 ww Or Kw _— a nt w a& Oo a z§ uu ’ . => — = —— « 2 o ROOT - 5 UPTAKE o Simplified illustration of pathwa ys of natural cycling and concentratio n processes at work in the desert ecosystem. b uw) ° = ly 288 Fig. L. uw Earlier studies of fallout from nuclear weapon tests disclosed the biologtcal significance of superficial contamination on plant surfaces resulting from radioactive airborne particle deposition (Fowler (ed) 1965; Larson et_al. 1966; Romney gt al. 1963; Russell (ed) 1966). Radioactive materials may reach the tissue of plants in two principal ways: First, airborne materials may be deposited upon the above-ground parts of plants and either adhere to their surfaces or be absorbed. Second, materials which have entered the soll may be absorbed by roots along with the nutrients on which plants depend for growth (Russell fed) 1966). The significance of the root uptake-pathway is governed largely by the biological availability of each given radionuclide during its interactions in soil. Thus, if radioactive materials are insoluble they will most Likely contaminate plant tissues only superficlally (Romney et al. 1963; Martin 1965}. Early studies done by the UCLA groups on vegetation in fallout contaminated areas at the Nevada Test Site indicated that severa! natural processes influenced the fate and persistence of fallout debris both from nuclear and non-nuclear contamination events. The period during and shortly after particulate deposition was characterized by conditions of instability largely controlled by wind activity. Rainfall, snow, and periods of calm air movement near ground level hastened later development of a quast-stable condition wherein particulate movement occurred primarily through processes governing resuspension. Vegetation contamination could be prevented or markedly reduced by protective covering during the unstable and quasi-stable periods after fallout had been deposited (Rhoads et_al. 1971; Romney et al. 1971). Measurements of fallout particles on soil and plant material sampled within downwind fallout patterns showed that a partitioning into different sized particles normally occurred during initial fallout deposition. Thus, the mean particle size generally decreased at greater distances downwind from ground zero (Larson et al. 1966). More recent studies by the NAEG in aged fallout areas at NTS and Tonopah Test Range (TTR), where plutonium was dispersed by chemical explosives, give evidence that this partitioning and patterning has continued to be reflected in the superficial contamination of the indigenous vegetation (Romney et al. 1974, 1975, 1976a, 1976b). The vegetation-to-soil inventory ratios determined in the various activity strata within several different fallout Ba terns seem to show that a greater proportion of the deposited 239-24 Pu source material has moved onto vegetation at greater distances away from ground zero. Examples are given in Table 1. Inasmuch as the activity entrapped on plant foliage primarily represents material in the resuspendable particle size range, the amount of contamination on foliage is less in proportion to the total amount of faliout activity deposited on soil at points nearer to ground zero compared to points farther away. Autoradiographs of annual plant leaf tissues collected near ground zero at Area 13 in 1976 showed that the activity present on vegetation was still in discrete particles of suspendable size range nearly 20 years after fallout had occurred (Wallace, unpudlished data). 239