- 49 - Eniwetok with the North Equatorial Current at a rate of approximately 7 to 10 miles per day. Fallout areas in the ocean from particular detonations have not been identified as discrete areas of relatively high contamination within the ocean but rather as low levels of contamination spread over a wide area. Radioisotopes in the ionic or colloidal form, if not taken up biologically, will move vertically at a slower rate than radioisotopes incorporated into particulate matter, because of gravitational force. An estimate of the movement of fallout materials through sea water was made at Eniwetok during the 1958 test series from samples of sea water taken 6, 28 and 48 hours after detonation. At six hours, the greatest amount of fallout as measured by the radioactivity of the water was at the surface and decreased rapidly to near zero at a depth of 200 feet; was still at the surface, at 28 hours, the highest value but there was enly a moderate decrease down to 200 feet; at 48 hours, the values were low at the surface and down to 200 feet but increased rapidly from 200 feet to a maximum at 300 feet, the thermocline (Lowman 1960). which coineided with Thus, in 28 to 48 hours the largest fraction of the fallout materials had moved through the stirred or mixed water layer, the water above the thermo-