The depth distribution of fallout depends greatly upon the thermocline, which is the layer of water between the warmer, surface zone and the colder, deepewater zone in a thermally stratified body of water. . \. The termocline is consequently a loyer with a large temperaf ture acadient with depth. In the area ofEniwetok Atoll, the surface layer is less than 100 meters thick. Because the temperature is fairly uniform throughout this upper layer, mixing in this layer requires only small amounts of energy and should occur easily. In contrast, transfer of materials across the thermocline layer by turbui lent diffusion is much slower since the thermocline is a layer of high stability. BEST AVAILABLE COPY : In studies during the 1958 test series at Eniwetok Atoll, analyses trrara moda hy filtratian ta dictinenich hetween particulate matter and colloidal-soluble matter. The particulate fraction was considered in these studies to be greater than 0.45 microns and the colloid-solubie fraction was considered to be smaller. At 48 hours after the nuclear explosion, the major part of the total radioactivity was concentrated at the 100-meter depth of the upper edge of the thermocline. Logically, the particulate matter would be expected to sink much more rapidly. This was confirmed by observations up to six-weeks later, all of which showed that the colloid-soluble fraction was always on order of magnitude greater in the surface zone than the particulate fraction. At the 400-meter greatest depth observed, the particulate fraction increased in time from beingan order of magnitude less until it approached the colloid-soluble fraction in magnitude at the end of this six-weeks period. 13