236 Health Physics Table 2. Total gammaactivity leached by water from local fallout particles as a function of test conditions and leaching time (Crocker et al. 1965). Test condition Leaching time in days Fraction dissolved Coral-surface tests Coral-surface tests Surface-water (barge) 4-8 20 2-3 0.08 0.23 0.60-0.70 tests August 2010, Volume 99, Number 2 Table 3. Solubility of various radionuclides from simulated fallout particles (from LeRoy et al. 1966). Soluble fraction leached out in 0.1 N HCl Particle characteristics’ used in the study Local fallout simulated particles: Microspheres of fabricated glass particles containing (or 0.1 N NaOH) 856r B4ag 33Ra 0.06 (0.08) 0.01 (0.05) 0.05 (0.07) carrier-free tracers, where balloon tests (0.65—0.85), where the fireball did not interact with the ground surface. Particle solubility in neutral, alkaline and acid solutions from continental detonations ranged from less than 0.01 to 0.5 of total radioactivity depending onparticle size and location with respect to the detonation site (LeRoy et al. 1966). Russian and U.S. studies, following ground-surfacetests, determined that the soluble fraction of fallout debris in the close-in zones within 25 km from point of detonation generally ranged from 0.005—0.10 of total radioactivity (Cohn et al. 1954; Gordeev 1999). Surface orientation of individual nuclides in fallout particles plays a dominant role in the degree of leaching (Norman and Winchell 1970). Therefore, volatile nuclides absorbed preferen- tially on particle surfaces are expected to be more soluble than refractory nuclides. In summary, as the distance from ground zero increases, the degree of fractionation (R/V) decreases, the particle size of deposited fallout decreases, and the solubility increases. One of the few radionuclide-specific solubility studies was conducted using simulated fallout particles spiked with *Sr, '"Cs, and '’Ba tracers (LeRoy et al. 1966). For simulated local fallout (close-in) particles, the fraction leached by 0.1 N HCI or 0.1 N NaOH was very small, ranging from 0.01 to 0.08 for those tracers. For simulated particles of distant fallout, 0.38, 0.72, and 0.91 of the total activity was leachedby acidic solution for Cs, Sr, and Ba tracers, respectively. Muchless activity was leached by alkaline solution, however (Table 3). In the environment, **?**°Pu was measured in seawater and lagoon sediment from the Bikini and Enewetak Atolls where the nuclear tests in the Marshall Islands were conducted (Noshkin et al. 1987). The fraction of plutonium mobilized by seawater at any given time was estimated at about 0.001 of the total sediment inventory. Following contact of fallout debris with the surroundings, additional nuclide separation can occur. For example, soluble radionuclides may be preferentially leached out by sea water in deep and surface water detonations. It was found that the adsorption of *’Sr ions onto soil decreases with increasing soil acidity (Maxwell et al. 1955; Adams et al. 1958). The presence of phosphate ions in the soil resulted in markedly increased 10% of activity could be leached out in dilute HCl Distant location fallout simulated particles: Fabricated particles containing 0.72 (0.15) 0.38 (0.12) 0.91 (0.23) carrier tracers, Sr and Ba in oxide form and Cs as silicate. The mix was calcined at 1,000°C, 50-100% of activity could be leached out in dilute HCl “Particle size ranged from 30—40 ym and particle specific activity was about 1.85 MBq per gram. adsorption of strontium onto soil particles. Mild corrosion of the metallic surfaces of fallout particles could, over time, increase surface area, creating more favorable conditions for radionuclide leaching. In general terms, the interaction of radionuclides with various environmental conditions can either decrease or increase their solubility/mobility and biological availability over time. The ratio of biologically available activity to total radioactivity in local fallout is reported to be generally small. LeRoy et al. (1966) fed 10 healthy human volun- teers 1-wk-old particulate fallout collected about 1.5 km following the land-surface detonation of the 1962 Small Boy at the NTS. An averagefraction of only 0.032 of the total gammaradioactivity was absorbed by the alimentary tract; the range was 0.003 to 0.088. Some of the observed variability in alimentary tract absorption in that study was due to several factors including large variation in fallout particle sizes and radionuclide content, uncertain whole-body counting geometry, as well as variation among individuals in terms of age, sex, diet, and lifestyle. Absorption of radionuclides by humans, either through ingestion or inhalation, is likely controlled by dissolution of the fallout particle matrix. Therefore, radionuclide absorption in humans from fallout particles may be different from that generally associated with simple compounds of a given element as often used in most animal uptake studies (CRP 1996). It should be recognized, however, that the biological availability of fallout debris as expressed by varioustransfer parameters (e.g., f, values) are element-specific and cannot be simply predicted on the basis of solubility tests alone