Comparative Hydrogeology of Fresh-Water Lenses of Bermuda and Great Exuma Island, Bahamas

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The islands of Bermuda and Great Exuma are similar in that they are both composed of Quaternary marginal-marine, calcarenitic limestones. These limestones are predominantly eolianite. Topographic depressions between eolian ridges are locally lower than present sea level; consequently, the ground water is coupled to inland marshes and ponds. The islands differ climatically. Rainfall in Bermuda exceeds potential evapotranspiration by about 0.1 m/yr, while in Great Exuma, potential evapotranspiration exceeds rainfall by some 0.5 m/yr. As a result, the inland ponds of Great Exuma are brackish to hypersaline, and fresh-water lenses are localized beneath the topographic highs between the ponds and the shoreline. In contrast, the marshes of Bermuda are not strong discharge sites, and the surrounding fresh-water lenses are geologically rather than topographically controlled. Dupuit-Ghyben-Herzberg modeling indicates that if the climates of Bermuda and Great Exuma were reversed, the characteristics of their fresh-water lenses would be reversed as well. This comparison indicates that climate, as reflected in the water budget, is one of the important controlling variables in the occurrence and geometry of fresh-water lenses in low-lying subtropical islands.

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Groundwater, v. 30, issue 1, p. 15-20

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