Geohydrology of Enewetak Atoll islands and reefs
Extensive tidal studies in island wells and the lagoon at Enewetak Atoll have shown that island ground water dynamics are controlled by a layered aquifer system. The surface aquifer of unconsolidated Holocene material extends to a depth of approximately 15 m, and has a hydraulic conductivity K = 60 m/day. From 15 to 60 m (approximate lagoon depth) the reef structure consists of successive layers of altered Pleistocene materials, with bulk permeability substantially higher than that of the surface aquifer. Because of wave set-up over the windward reef and the limited pass area for outflow at the south end of the atoll, lagoon tides rise in phase with the ocean tides but fall later than the ocean water level. This results in a net lagoon-to-ocean head which can act as the driving force for outflow through the permeable Pleistocene aquifer. This model suggests that fresh water, nutrients or radioactive contaminants found in island ground water or reef interstitial water may be discharged primarily into the ocean rather than the lagoon. Atoll island fresh water resources are controlled by recharge, seawater dilution due to vertical tidal mixing between the surface and deeper aquifers, and by loss due to entrainment by the outflowing water in the deeper aquifers. Estimated lagoon-ot-ocean transit times through the deep aquifer are on the order of a few years, which corresponds well to the freshwater residence time estimates based on inventory and recharge. Islands in close proximity to reef channels have more fresh ground water than others, which is consistent with a locally reduced hydraulic gradient and slower flow through the Pleistocene aquifers.
- Research Organization:
- Lawrence Livermore National Lab., CA (USA)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 6500005
- Report Number(s):
- UCRL-85411; CONF-810547-1
- Country of Publication:
- United States
- Language:
- English
Similar Records
Circulation in Enewetak Atoll lagoon
Coral reef hydrogeology