cistern opening measured about 0.65 m¢ and was fed by means of galvanized metal sheeting used for catching rainwater (Figure 12). A little rain was reported on the afternoon of March 1, 1954 (Sh57). We assumed that the additional cistern catchment area did not contribute water or activity to the cistern. Results of the analysis for gross beta-~activity concentration in cistern water ranged from 1.8 x 10° to 2.0 x 108 Bq a} (0.005 to 0.054 uCi 1) with a mean of 5.0 x 105 Bq 27! (0. 027 UCi ml!) at 50 hours post- detonation (0C68). The fallout from Castle series coral surface bursts including BRAVO was reported as being barely soluble in water (Ka66). BRAVO fallout, which was collected with mixtures of rain and sea spray, lost only 20 to 50% of the iodine activity to the Liquid phase (Ka66). The serviceman at Rongerik Atoll examined the terrestrial fallout under a microscope and reported that the sand-like granules were not soluble in water on the microscope slide (Sh57). Therefore, most BRAVO activity and a good portion of the radioiodine activity probably remained with the fallout particles at the bottom of a cistern. We extrapolated the 50-hour post~detonation beta-activity concentrations back to 0.5 day post-detonation using decay characteristics for Bikini ash components. The range for gross betaractivity concentrations was 1.0 x | to 1.2 x 107 Bg, 2-1 (9.027 to 0.31 UCi ml7!) and the mean was 2.9 x 108 Bq & (0.078 uci mi~!). At 0.5 day, a total of 3.7 x 1019 Bq m(1 Ci m2) of Bikini ash gross beta ieiviey was estimated by us as the sum of the activity of all nu- clides listed in Table 13. Given the area of a cistern opening, we calculated that the average cistern contained 8.3 m> of water if all the Bikini ash activity was in the liquid phase. This water volume would be necessary to dilute the activity to-the level reported by the radsafe team. We estimate that this was about 55 times greater than the observed water volume of the cisterns as previously indicated by Sharp (Sh57). Assuming only 20% of the iodine activity in Figure 12. A cistern at Rongelap Island. -41-