2010 Volume 30 Issue 5 Pages 317-330
The freshwater in aquifers of coral atolls whose averages of elevation are a few meters is often distributed as lenses floating on salt water underneath the islands. The coastal aquifers are possibly salinized by the sea-level rise, the decrease of rainfall recharge rate associated with the climate change, the increase of storm overwash, extractions due to the overpopulation. There is high possibility that the salinization of the aquifers damages the important natural ambience for the people living habitat such as crops field and vegetation. In this study, the aquifer salinization was evaluated by electrical resistivity, hydrological and long-term meteorological data in three low-lying coral atolls ; Laura islet and Kolalin island of Majuro Atoll in Marshall islands and Fongafale islet of Funafuti atoll in Tuvalu. The hydrological surveys conducted in Kolalin island, where is relatively small and hardly anthropogenically-influenced, revealed the dynamics of the pore freshwater layer in Holocene aquifer. The horizontal migration, separation, and coalition of the pore freshwater were clearly forced by the tide, indicating that the freshwater lens cannot be steady in the small atoll islands. On the other hand, the freshwater lens in the relatively large atoll islands such as Laura islet, which is mainly influenced by global and anthropogenical factor, varied interannually. The interface between seawater and freshwater in Laura islet was shallowed in August 2009 as a result of the decadal decrease of the rainfall and/or the sea-level rise. The detailed structure of the freshwater lens indicated the saline water intrusion from the lagoon and the upcoming of saline water caused by over-pumping associated with the urbanization and modernization of the atoll. In Fongafale islet which is relatively large, however, the soil and groundwater salinization was mainly caused by the tidal forcing during spring tides, indicating that the vadose zone cannot retain the pore freshwater. Coastal aquifers beneath low-lying areas during flood tides were intruded by the brackish pore water mixed with the rain water and sea water in phase with the semi-diurnal tide. The aquifer allowing the intrusion can be formed by gravel retaining high porosity due to the past reclamation resulting from the airfield construction.