that availability for bone deposition is affected to a marked degree. The rare earth elements are not considered important to the bone deposition problem. Strontium-90 fractionation was discussed earlier. The aveilability to the biosphere of strontium-90 and other elements will be discussed in later sections on biological effects. Even though fall-out occurs on arable land and the strontium-90 thus deposited finds its way into the biosphere, several environmental factors may act to reduce the availability for plant or animal uptake. Increasing acidity of the soil strongly decreases the ability of strontium-90 to filter to ground water. The presence of increased amounts of phosphate in soil further affects in an adverse manner the availability of the strontium for easy movement through the soil by dilution with water. A very low calcium content of the soil could conceivably permit more strontium to be absorbed into the plant system and finally deposited in human bones, but this is not considered likely. Fall-out into city water reservoirs has been considered an import- ant method of widespread high level contamination due to the solubility of the particulate matter in the water. The world-wide low level of contamination renders the quantity of radioactivity received from a weter supply negligible, because of modern methods of flocculation, settling, filtration and distribution of municipal water supplies. Since rain is known to scavenge particulate atmospheric radioactivity, segments of the population using rainwater directly as their drinking water supply have an increased possibility of ingesting strontium-90, put even this amount would be insignificant except in the rainfall in the immediate vicinity following a surface burst or low yield air detonation of a nuclear weapon. The problem of radioactive particles falling into tne ocean raises the question of their availability to this portion of the bio- sphere. Plankton found normally in sea water are consimied in large quantities by fish. These plankton concentrate mineral elements from the water, and it has been found that radioactivity may be concentrated 29