DNA 1240H-2 total amount of radioactive material produced by the explosion remained in the water. Fallout from the mushroom cloud caused @ radioactive rain to fall in an area within the lagoon, and it was estimated that the largest part of the radioactive material was deposited on the surface of the weter by that rain. In general, vertical diffusion of radioactive waterial in the lagoon wes very slow. :Gamma dose rates above the surface ‘of the lagoon near surface zero vent fram about 400 r/2h br (~ 17 r/hr) at H+ l hr, to about 65 r/2k hr (2.7 r/hr) at H+ & hr, and to less than 0.1 r/2h hr (0.008 r/hr) at 5 days after shot. However, at that ‘time, the water was still sufficiently radioactive to seriously con- taminate the evaporators and hulls of nontarget ships within the lagoon .©5 ration Hardtack: Some water-contamination records ere available from the underwater shots of Operation Hardtack, in May and June 1958. Both, underwater and surface GITR dose rate data are available, as well «8 some water sample data. At Shot Umbrella, a relatively shallow burst on the bottom 150 ft), ship records are availablefrom only one operating underwater GITR (gamma-intensity-time recorder). The GITR, suspended from a boom extending over the fantail of the DD 593, was located at about 12 ft underwater and 7900 ft from surface zero. Tabulated radiation data in- dicate by two peaks in the dose rate vs time curve that contaminants were in the water near the ship both et early times and at 6 hr after shot. However, during the period when the ship was enveloped by the base surge, the peak underwater dose rate registered was only 0.19 r/hr at 8 min after burst. Following this period, the uriderwater dose rates were very low until they again rose to the same peak rate at 6.4 hr afte: burst. The early peak was attributed to contaminants depositing in the water from the base surge, and possibly to some contaminants washed off the ship, which had washdown in operation. The late increase of underwater dose rate is attributed to a patch of contaminated water (detonation debris originally upwelling at surface zero) that drifted down on the ship. A few early-time surface-water and shallow underwater activity records from the coracles are also available for shots Umbrella and Wahoo, along with a comprehensive discussion of the significance of the records.22 Seven early-time underwater GITR records were obtained for Wahoo, and four for Umbrella. The instruments were so mounted on the edges of the coracles that the passage of the shock wave triggered a mechanism to drop them into the water. It was planned that, after release, they would be suspended at approximately 6 ft below the water surface. Similarity of the underwater records to the above-watcr standard GITR records of corresponding coracles indicoted that a number of the detectors may have been closer to the surface than the planned 6-ft depth. Nevertheless, the close-in station records are of value, and show evidence of radiation due both to water directly contaminated by the bomb and to patches of radioactive foam. The closest-in record obtained was that of the under- water GITR (calculated to be almost at the surface) located at 1760 ft _ BEST AVAILABLE COPY