Island cistern water at 50 hours post detonation. The activity concentration in Sifo Island cistern water was assumed to be 12% the values given at Rongelap Island. This was based on a ratio of mean exposure rate at both islands at one point in time and the ratio of the fractions of total granule activity on the surface of the mean granule size in each cistern. The cisterns and wells at Utirik Island were observed to be covered as reported by the evacuation team aboard the USS RENSHAW (0C68). Interestingly, the range of cistern water activity at Utirik Island was 7,200 to 33,000 dm! as computed for 0930 on March 3, 1954 based on 2 different cistern water samples taken on March 9 (0C68). The roof over each cistern apparently was not effective in preventing some contamination. The mean Utirik cistern water activity was about 1/3 the mean cistern water activity reported for Rongelap Island at this same time. On the basis that activity in the liquid phase in two cisterns would be directly proportional to the ratio of exposure rates near each cistern and, directly proportional to the ratio of the fraction of total activity on the surface of the mean granule size in each cistern, one would expect roughly equal activity in the liquid phase in cisterns at Rongelap and Utirik Islands at the same point in time. Thus, covered cisterns at Utirik provided some degree of activity reduction in drinking water. On the basis of the observed average gross beta activity ratio of Utirik to Rongelap cistern water on March 3, 1954, the estimates of radioiodine activity in Utirik Island cistern water were made (see Table 6). The instantaneous activity concentra- tions given in Table 6 were modified by the activity deposition rate indicated by the slope of Figs. 2, 4 or 6. 25