-17- when the ship was three and one-quarter miles due west of target zero. As the ship continued northward and then circled back (1542) to parallel the course taken earlier, radiation readings of 2 to 20 mr/hr were observed. At the northernmost point of the survey the readings were 2mr/hr, and at the westernmost point they were 1 mr/hr. This line represents the approximate detectable downwind limits of contamination at this time. At 1618 the initial survey was completed when the ship was approximately 6, 500 yards downwind (WSW)from target zero. The level of radioactivity in the target area during the next fortyeight-hour period decreased to the pre~Wahoo level. Water The gross beta radioactivity in the pre and post test ‘samples is given in Table 1. The radioactivity in the samples collected prior to the Wahoo detonation was limited to the top 39 meters and for the most part was ina soluble form. nation, Five and one-half hours after deto- radioactivity was present at all depths sampled and the amounts of radioactivity were much higher than those found one and three-quarter miles southwest of target zero. The particulate matter suspended in the water accounted for 6 to 40 per cent of the gross beta activity. Twenty-seven hours after detonation, four miles due west of target zero, the measured amounts of beta radioactivity in the water were approximately the same. At Wt 48, at target zero, the greatest