RESTRICTED 



1. SUMIARY 



The atonic blast will contaminate directly a volxrae of 

 water which is sioall compared to the total volurae of the la- 

 goon. This small water ip-ass will increase in size, with cor- 

 responding decrease in the concentration of contaii^inant , by 

 current transport and by the processes of horizontal and ver- 

 tical diffusion. The water raass contaminated internally by the 

 blast will be spread the full length of the lagoon within about 

 tvro days. Radioactive materials deposited with the plume or by 

 convective rains following the blast will be spread more widely 

 and v/ill reach the edges of the lagoon sooner, but their concen- 

 tration probably will be relatively low. 



The current system is particularly important in predicting 

 events subsequent to the blast. The description presented here 

 is the result of a survey conducted during March and April, fur- 

 ther studies will be made in June to determine whether conditions 

 have changed significantly. The system consists priuiarily of a 

 v<rind driven surface CTirrent flowing in a V/SV/ direction vvith an 

 average speed of 0,3 knot (varying slightly with wind velocity), 

 extending to a depth of about 40 feet where it gives way to a 

 thicker and slower (0.1 knot) ENE bottom current. These tv/o 

 currents form a continuous, rotary circulation, with bottom wa- 

 ter upwelling at the eastern end of the lagoon to Join the sur- 

 face flow and surface water sinking at the western end. 



Oceanic water flows into the lagoon continuously over the 

 eastern and northern reefs. The total voliome of flow is about 

 three percent of the voluiuo of the lagoon per day. Continuous 

 outflow occurs through the western part of Enyu Channel, i'lse- 

 where, channels, passes, and ths v/estern reef , the current re- 

 verses v/ith the tide. The tidal flow is strongest through the 

 southwestern passes, but the tidal interchange is relatively in- 

 effective in flushing the lagoon. , It is estimated that only 40^^ 

 of the water leaving the lagoon on the ebb tide is true lagoon 

 water. The remainder is oceanic v/ater that has coi-e into tlie 

 lagoon on the preceding flood tide. Not much nore than 10'^j of 

 the water entering on the flood tide becoi::es thoroughly nixed 

 with lagoon water and carried into the general lagoon circula- 

 tion. 



By far the larger part of the water in the central part of 

 the lagoon has therefore come in over the eastern and northern 

 reefs. As this water flows in, it is absorbed into the rotary 

 circulation of the lagoon, thus gradually renewing the lagoon 

 water, while at the sau.e tiiue the latter is being flushed out 

 of the southwdS'^ern passes at a rate of 3«2'^t per day. At this 

 rate of flushing, any given mass of water in the lagoon will on 



