An inspection of the isodose contours for the two underwater shots indicates that gamma doses in excess of 100 r occur within the first 15 minutes at downwind distances less than 16,000 feet from Wahoo surface zero and 14,000 feet from Umbrella surface zero. Since the airborne radioactive material is greatly influenced by the surface wind, the direction of closest approach is from upwind; however, because of the rapid and energetic transport mechanisms that appear tc be operative and the possible existence of radioactive remnants trailing behind the receding surge, close approaches should be made with extreme caution. According to the isodose contours, an upwind approach as close as 4,000 feet is possible on either Wahoo or Umbrella without exceeding a 100-r dose. To assure a total free-field dose of less than 25 r, minimum downwind and upwind distances of 30,000 and 5,000 feet for Wahoo and 24,000 and 4,500 feet for Umbrella would have to be maintained. Because of the existence of pronounced lobes, particularly on the Umbrella isodose contours, it may be possible to approach to much closer downwind distances; however, since the exact location of the areas between such lobes of higher dosage cannot be predicted, the larger distances have been quoted. 4.2 RECOMMENDATIONS Although considerable information concerning the radiological environment associated with underwater nuclear detonations has been obtained, a large number of questions remain unanswered. If any nuclear devices are fired underwater in the future, another attempt to document the radiation fields should be made with instruments specifically designed to obtain more infor- mation on the mechanism of base surge formation and its relationship to the observed radiation fields. The differences between Wahoo and Umbrella indicate that depth of burst has a pronounced influence on the radiation fields produced. The similarity between Wigwam and Wahoo Suggests that, however, the most extensive changes occur as the depth of burst approaches zero, probably because of differences in bubble stage upon breaking the water surface. Therefore, a Series of underwater detonations commencing at the surface and gradually increasing in depth should be fully documented. Sucha series could probably consist of fractional-kiloton devices. On all future tests, the density of stations documenting the radiological event should be two to three times that employed by this project. There should be much greater use of film packs, both fixed and floating. The feasibility of radar spotting of film packs and large elements has been demonstrated and should be used more extensively. Lack of accurate position information was one of the project’s greatest difficulties. A second great difficulty is lack of micrometeorological information over the entire area traversed by the base surge. The use of surface wind information provided by a station some 6 miles from surface zero is probably not justified. The problem of complete 345