APPENDIX A GEOLOGY, OCEANOGRAPHY, AND HYDROLOGY OF BIKINI ATOLL Physical Setting and Climate Bikini Atoll, located in the northwestern part of the Marshall Islands, is an oval-shaped coral reef atoll approximately 40 km long and 25 km wide (see Figure 1). It comprises 23 separate coral islands which have a total land area of 8.8 km2, Bikini Island, the largest island in the atoll is approximately 4 km long and 0.8 km wide, and Eneu Island, the next largest is approximately 3 km long and 0.6 km wide. Together they comprise about half of the total land area in Bikini Atoll]. These two main islands also are higher than the other islands, with an average elevation of about 3 m above msl, and a maximum on Bikini of about 5m. about 1-2 m above msl. The average elevation of the other 21 islands is only The climate of Bikini Atoll is tropical, and the mean monthly temperature is quite uniform throughout the year, ranging between 81° and 83°F. The prevailing winds are the northeast trades which blow most persistently during the winter months, from December through March, when they have an average velocity of nearly 20 knots. During the rest of the year the winds are some- what lighter and more variable in direction. Hurricanes are infrequent, and usually occur during the summer and fall months and come from the southeast. Rainfall in the Bikini Atoll has been measured only since 1980 at Eneu Island by Lawrence Livermore Laboratory. During this time rainfall has averaged about 135 cm a year. Rainfall. is heaviest during the months of August to November and lightest during the months of December to March. Over a long-term basis, intense tropical storms contribute much of the total rainfall. Geo logy The geology of Bikini Atoll was described extensively by Emery, Tracey, and Ladd (1954). The atoll is of geologic structure typical of deep oceanic atolls, and consists of a basaltic volcanic core overlain by approximately 800 m of essentially unconsolidated calcareous materials capped by a shallow wave resistant reef platform enclosing a slightly deeper oval-shaped lagoon. The atoll] was formed when the original volcanic land mass subsided beneath the ocean surface, leaving exposed only a narrow band of a living reef which continued to grow upward to keep pace with subsidence. The reef platform is very shallow (at approximately ms1) and continuous around the perimeter of the atoll except where passes cut through and deepen the connection between the lagoon and ocean waters. Two deep passes cut through the reef rim or platform, one near Enidrik and the other near Adrikan Islands. Other narrow passes of intermediate depth occur off Bokdrolul, Bokaetoktok, Oroken, and Jalete Islands, and a wider shallow passage occurs between Lukoj and Enidrik Islands. By far the largest passage is the 16 km wide pass between Eneu and Aerokojlol Islands at the southeast corner of the atoll. Although the pass is relatively shallow (averaging some 15 m depth), it is the major connection between the waters of the ocean and lagoon. S00c0b2 at