54 



RISTVET 



1981; Buddemcier, 1981). An additional study was spon- 

 sored by the DNA (Buddemeier and Jansen, 1976) to 

 investigate the groundwater potential for use in the 

 Enewetak Radiological Cleanup. 



Atkinson et al. (1981) investigated the water budget 

 and circulation of water in the Enewetak Lagoon and 

 found that essentially all the water input to the lagoonal 

 system comes from wind-driven transport across the wind- 

 ward reef. Since the windward reef crest is typically near 

 mean sea level, waves drive water from the ocean into the 

 lagoon at nearly all times. The windward reef blocks any 

 return flow. Atkinson et al. (1981) determined that nearly 

 all of the outflow occurs through the South Channel. The 

 Deep Channel had a balanced inflow and outflow. Other 

 input/output pathways, i.e., transport over the leeward 

 reef was insignificant in comparison to input over the wind- 

 ward reef and output through the South Channel. Atkinson 

 et al. (1981) calculated a mean residence time for lagoon 

 waters of 1 month with a maximum of 4 months for water 

 in the northeast section of the lagoon. Although water lev- 

 els were not directly observed, the circulation pattern 

 requires the existence of a net lagoon to ocean gradient 

 (Buddemeier, 1981). 



Koopman (1973) first noted that, for the islands of 

 Enewetak, a significant discrepancy existed between the 

 calculated thicknesses of a fresh water Gyben-Herzberg 

 lens and that observed in trenches and borings in the field. 

 Koopman (1973) observed that the islands of Engebi 

 (Enjebi) and Aomon had only thin brackish water lenses 

 approximately one-tenth as thick as would be predicted 

 using conservative calculations. Buddemeier and Holladay 

 (1977) measured tidal lags in wells on Engebi (Enjebi) 

 Island and noted that there was a sharp discontinuity in 

 the plot of tidal lag time versus depth between 10 and 

 20 m subsurface depth. They hypothesized that the effect 

 might be due to a more p)ermeable aquifer below the first 

 unconformity of Ristvet et al. (1977). Wheatcraft and Bud- 

 demeier (1981) demonstrated, using tidal data from Engebi 

 (Enjebi) Island, that the classical Gyben-Herzberg lens 

 model does not describe the hydrologic system observed, 

 which is controlled by vertical transmission of tidal signals 

 from deeper and more permeable Pleistocene aquifer(s). 



Buddemeier (1981) noted that total fresh water content 

 of island groundwater was essentially independent of island 

 area and radius and that the southern islands have approx- 

 imately 50% more fresh water volume than the northern 

 islands. In addition to this difference in gross fresh water 

 inventory, Buddemeier (1981) noted the northern islands 

 have thinner layers of p>otable water and more extensive 

 brackish water transition zones than do the southern 

 islands. 



Buddemeier (1981) made additional tidal measure- 

 ments on Japtan, Biken (Rigili), Enewetak, Aomon, and 

 Engebi (Enjebi) Islands and concluded that significant differ- 

 ences were present between the amplitudes of reef and 

 lagoon tide stations on the falling tide resulting in a net 

 lagoon to ocean head. Buddemeier (1981) concluded that 

 this net head of water will tend to set up a lagoon to 



ocean flow of water through the permeable Pleistocene 

 aquifer and that the amount and quality of fresh island 

 groundwater is controlled by the rate of lagoon to ocean 

 flow through the Pleistocene aquifer. The estimated lagoon 

 to ocean transit times are on the order of 3 to 6 years, 

 which corresponds well to the fresh water residence time 

 estimates of the islands based on inventory and recharge. 

 The rate of flow from lagoon to ocean dep>endency 

 explains why islands in close proximity to reef channels, 

 such as the southern islands, have greater volumes of fresh 

 water than others. 



ACKNOWLEDGMENTS 



I wish to express much appreciation to Edward Tremba 

 for critically reviewing this manuscript and many hours of 

 stimulating discussion. I wish to acknowledge the fine 

 technical support provided by J. MacCornack and L. D. 

 Evans in preparing this manuscript. I express my apprecia- 

 tion to the many personnel of the Defense Nuclear 

 Agency, Department of Energy, Mid-Pacific Research 

 Laboratory, University of Hawaii, Air Force Weapons 

 Laboratory, U. S. Geological Survey, and Holmes and 

 Narver, Inc., who have participated and supported the geo- 

 logic investigations of Enewetak Atoll for the last 40 years. 

 Funding for this effort was provided by the Defense 

 Nuclear Agency. 



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