bsaPTER \7 so that it disintegrates a ship, some of the activity will become associated with heavier particles (of the disintegrated ship) than are formed for a true free-water burst, and more rapid mixing of the radioactive particles with the water and their penetration to greater water depths will probably occur. Exact rates and depths of fallout penetration are difficult to predict, but estimates can be made by comparing results at tests in the Pacific, where the differences in rates and depths of fallout penetration resulting from barge and island shots were probably primarily due to differences in particles sizes. All the waterborne radioactive particles resulting from a surface or shallow subsurface burst will be distributed initially in the upper water layer, often referred to as the “mixed leyer," that may be from less than 30 meters to more than 150 meters thick, depending on the geographic location. The temperature of this layer is quite uniform from the sea surface to the bottom of the layer, or to the thermocline, below which the temperature decreases rapidly with depth. When a substance of soluble or colloidal nature, or one having about the same density ase water, falls on the ocean surface, it becomes distributed into the mixed layer fairly rapidly, often within a few hours. However, because of the sharp increase mo in density below the mixed layer, little further downward penetration of particles of this type occurs. For an underwater burst so deep that the bubble undergoes one or more pulsations before reaching the surface, some activity probably will be distributed along the path of bubble migration, particularly at bubble minima, some activity will be thrown into the air and mixed with the plumes and base surge, and some will remain in the water at the surface where the bubble breaks through, resulting in a region of contaminated water about surface zero. The distribution of the radioactive particles at later times, for such a shot, will be dependent on the burst depth, the water depth, the thickness of the mixed layer, and the prevailing winds and water currents. 17.7.3 Water-Contamination Data Some references* give water-contamination data obtained following (1) land and water-surface shots at Operations Castle and Redwing; (2) the shallow underwater shot, Bikini Baker at Operation Crossroads; (3) the shallow bottom shot, Umbrella at Operation Hard- tack; (4) the moderately deep shot Wahoo at Operation Hardtack; and (5) Wigwam, the deep underwater shot. Deta indicate that both the nature and distribution in water of the radioactive particles resulting from bursts over land surfaces are different from those of particles resulting from bursts over water surfaces, and that these characteristics are affected by the kind and mass of material engulfed by the fireball. *References 31, 32, 33, 40, 42, 64, 65, and 76 through 84. 17-91 ee ee re ee erre re ee