by swimming upward. Because of the effect of gravity on particles, the soluble and insoluble fractions would probably act as two independent systems as far as concentration or dispersal processes are concerned. In the waters of the open sea, the naturally-occurring trace elements zine, cobalt, ruthenium, manganese, caesium, strontium, iodine, and possibly zirconjum and cerium are present for the most part in solution. The small amount OF UPPER : OF OCLINE COLLECTED 23 DAYS AFTER SURFACE SHOT — “ Y, 5 COLLECTED 4 BAYS AFTER OVER -WATER jl-- SHOT 100 METERS in solution plus colloid fraction from and one from an over-water detonation e over-water detonation, the total he surface, decreased gradually at face activity, and rose to a value the upper edge of the thermocline. t one-tenth of that at the surface. rface was associated with particles, f the activity was in the particulate vity was associated with particles. e matter at the surface is probably he area contaminated with fallout particulate activity at and in the ue, to a large degree, to very small hrough the thermocline because of vould be present in fallout from the > activity at the thermocline would ynly approximately one-fourth that illout area from the surface detonayn; thus the larger particles present lepths below those sampled by the ulate form are introduced into seasher because of their high solubility sure as they sink through the water xey may be precipitated as particles in the sea or with accompanying - inorganic particles. The particles, i to be removed from the mixed organisms tend to offset this effect of natural iron occursin the colloidal and particulate form and may have associated with it a limited amount of manganese and zirconium. All the abovenamed elements except caesium, strontium and iodine, however, when introduced to the sea in fallout, would most probably be present in the particulate form (36). The fallout elements strontium, caesium and iodine, which occur mostly in the soluble form, were found in the least amount in plankton. In contrast, the radioactive fission products with the least solubility, Zr® and (‘el44, were present in plankton in the greatest amount (36, 7). LOWMAN (36) reported that the non-fission-product radioelements zinc, cobalt, and iron were present in plankton from one survey in approximately equal amounts of about: 24°% each and manganese at a level of less than one per cent. All of these elements introduced as fallout would exist initially in the insoluble form. The physical state of any given element in sea-water will depend upon whether or not the solubility product of its least soluble compound has been exceeded (2, 10), and precipitation will occur when it is exceeded. In order to predict the form that a given radicelement will assume in sea-water, the ionic activity of the compoundslikely to be present must be known. Very few data are available concerning the activities of the ionic forms of the fission-product elements in the sea. Limited information is available, however, for the neutron- induced radicelements found in fallout from thermonuclear devices. Precipitation of a radioelement in the sea will occur only when precipitation of its stable element occurs normally or is induced by accompanying non-radioactive débris swept up into the fireball of the weapon at the time of detonation. This débris at the Entwetok Test Site may be calcium compounds from the islands and reefs, materials from nearby structures associated with the nuclear devices or from the nuclear devices themselves, or salts from evaporated sea water. Several of the abundant fission products belong to the rare-earth series, and most of these elements are very insoluble in sea-water. In addition, seawater is probably naturally saturated with rare-earth elements, so that any added material will be present in the particulate form (10). Radioactive contamination may also occur in the sea in particulate form as a result of another process which is neither precipitation nor coprecipitation, but rather involves the adsorption of radioactive ions to organic detritus in the sea water. This process may be distinguished from the adsorption of radioactive ions or particles on to microorganisms. Both organic detritus and microorganisms have exposed polar groups with which transition elements such as manganese, iron, cobalt, nickel, copper and zinc, and anions such as zirconium, ruthenium, tungsten, neptunium, uranium, tellurium and molybdenum, may become complexed. A difference between organic detritus and microorganismsis an increased tendency of the former to sink to the depth of the sea, thus removing the radiocelements from the biosphere. However, for the most part organic detritus probably sinks at a slower rate than particles of calcium compounds, and would thus tend to be removed from the surface waters at a slower rate than radioelements coprecipitated with calcium oxide or scavenged by calcite. 117 La ws _ te, Peet aA gag bbe e™