Pe eeee si of time, almost all the radioactive materials appear to remain in the surface waters. If the samples are taken at the original site of contamination over an extended period, the ocean currents, eddies and vertical turbulence tend to obscure the effects of gravity. However, the obscuring effects of these factors may be reduced in areas in the lee of land masses. The effect of gravity upon radioactive materials introduced into the sea was studied during the 1958 test series at Eniwetok Atoll. Measurements were made within the contaminated area less than one hour after the detonation, and a series of three sets of water samples at depths to 300 m were taken over a period of 48 hours in the same area. The radioactive material in the water samples was divided by filtration into a particulate fraction greater than 0.45 u, and the colloidal-soluble fraction, less than 0.45 4. The distribution of total radioactivity with depth at the three times is shown in Fig. 1. During the first 6 hours the major part of the radioactivity was in the top 25 m. The radioactivity decreased with depth, so that through the upper edge of the thermocline the contamination was about one-eighth that at the surface. At 4— MIXED LAYER 80+ 5 6ok —_ yy 6 _~HoOURS ----- 28 sreeeees GQ HOURS HOURS PARTICULATE ACTIVITY ae ee, 150 200 goo 160F 140 120 6& 100 HOURS 28 Hours ' 48 HOURS 80 a 60 40 20 oO eee 1 25 cana ne i 5a 1 7§ J 100 L OEPTH 159 IN 1 200 METERS I 300 Fig. 2 Distribution of particulate activity and soluble-colloidal activity with depth at 6, 28 and 48 hours after an underwater detonation 110