movement of radioactive foam (Section 3.3.2). Complete analysis of the waterborne record requires consideration of both the 0- to 15-m:nute and the 0- to 6-hour records. Most of this analysis is presented as part of the free-field dose rate discussion (Section 3.3.2), since certain spikes in dose rate presumed due to radioactive foam might be mistaken for radiation from the base surge. The arrival and departure of both foam and white water, computed for a number of speeds and directions, are presented in Table 3.25. Drifting coracles are assumed to move with the wind at speeds determined for a dragging mooring cable unless there is good photographic evidence of free drift (Figure 2.16). Foam is assumed to move in directions and at speeds determined by the combined effects of ocean currents, sea conditions, anc the surface wind. For Wahoo, directions ranging from 250° T (average direction of coracle drift) to 302° T (Reference 92) and speeds of 1, 2, and 6 knots are calculated. For Umbrella, the same range of speeds is used, but the direction of movement 1s limited to that of the surface wind. As shown in Table 3.11, the best agreement between the gamma records and these assumed foam movements is obtained for a set and drift of 270° T at 6 knots after Wahoo and 230° T at 2 knots after Umbrella. Although all evidence for foam is at best circumstantial, the assumed foam movement after Wahoo is compatible with the official surface wind direction and the reported ocean currents (References 53 and 93, and project observations). The slower rate of foam movement after Umbrella is presumed to be a consequence of smoother water conditions inside the lagoon, which would reduce stripping action by the wind. For Wahoo, white water arrival and cessation times calculated on the basis of a set and drift of 270° T and 1 knot are also in reasonable agreement with observed dose rate values, although sets of 250° T and 302° T give equally good or slightly better comparisons. For Um- brella, the assumed radial expansion at 0.5 knot gives best general agreement with both the gamma records and the observed early white water expansion. This radial expansion cannot of course be used for calculation of cessation times. Since the limited current data available for lagoons (Reference 94) indicates little surface current, the assumed radial expansion is at least reasonable, although such continued expansion is not indicated by other late-time observations currently available to the project. At 5 or 6 hours after Umbrella, the white water patch was still in the neighborhood of surface zero but was transected at least at the surface by a channel of clear water roughly parallel to the surface wind direction (observation by project personnel aboard the USS Munsee). For Umbrella, white water is assumed to contain a suspension of pulverized coral from the lagoon bottom. A significant fraction of the residual radioactivity is probably associated with this suspension. The closer stations located within or near the white water boundary show a steady decline in dose rate due to a combination of decay and sinking of the radioactive material below the surface. Laboratory tests of Umbrella crater material indicate that its sinking rate in sea water is about 0.96 m/hr, a rate about a third that observed during Operation Redwing (Section 1.3.1 and Reference 33). This difference is probably due to the finely pulverized con- dition of the bottom material. Using the total linear attenuation coefficient for 1-Mev gammas in water, this would indicate a decrease in dose rate of 1 decade per 21 minutes due to sinking alone. Such slopes are observed in the normalized rate curves (Section 3.3.2). An indication of the combined sinking and dilution rate for water sources is obtained by comparing the observed and the standard decay rates as shown in Figures 3.141 and 3.142 (the Standard decay curve is described in Section B.2). In these figures the average dose rates for the later Wahoo records and the decay rates for a number of close-in Umbrella stations are plotted as a function of time. The decrease in average dose rate shown by the later Wahoo records is probably due to sinking or mixing with deeper water, since the white water area at the surface remains unchanged. For Umbrella, the decrease in dose rate after 30 minutes agrees closely with the standard decay curve, which could mean that a fine radioactive suspension persists after the sinking of the coarser material. Such agreement also implies no dilution by expansion or by surface currents. A survey meter 15 feet above the center of the white water at H+4 hours read 200 252 (Text continued on Page 276)