Hydrogeology of Bermuda — Significance of an across-the-island variation in permeability

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The distribution of fresh groundwater in Bermuda reflects the lateral partitioning of the saturated zone into two sectors: a band of low-permeability limestones (Paget Formation) along one shoreline, and a band of older, more highly permeable limestones (Belmont Formation) along the opposite shoreline. The fresh groundwater occurs preferentially closer to the shoreline composed of Paget Formation.

The an-axisymmetric distribution of fresh groundwater results from the effects of the across-the-island variation of permeability on the interface-bounded lens (the theoretical Ghyben-Herzberg lens) and the transition zone, which, in Bermuda, makes up a sizable fraction of the interface-bounded lens. The variation in size of the interface-bounded lens can be modeled closely by application of Ghyben-Herzberg-Dupuit methodologies to an infinite-strip vertically stratified island — that is, an island partitioned into two strips, one of which is some 14 times as permeable as the other. The areal variation in thickness of the transition zone is similar to that of the amplitude of short-period water-table fluctuations that are generated by changes in ocean level (e.g., from astronomical tides and changes in atmospheric pressure). The amplitude of these water-table fluctuations diminish inland, more so in the Paget sector than in the more permeable, Belmont sector. Thus for a given distance inland of the shoreline, water-table fluctuations are greater and the transition zone is thicker in the Belmont sector than in the Paget sector.

In a larger perspective, the occurrence of fresh groundwater in Bermuda reflects the depositional and diagenetic history of the Pleistocene marginal-marine limestones that compose Bermuda. The depositional history involves a seaward accretion of bioclastic eolian ridges (eolianites) formed during successive sea-level highstands of Pleistocene interglacial stages. The diagenetic history includes large-scale erosion by CO2-enriched water; this chemical erosion involves solutional lowering of the landscape and phreatic solution peripheral to marshes. As a result of these processes operating during a period of large-scale sea-level fluctuations, older (intra-Belmont) inter-eolianite depressions have evolved through a marsh stage and are now interior sounds. The result is the across-the-island variation in permeability — solution-altered Belmont rocks adjoining the interior sounds and less permeable younger calcarenites of the Paget Formation along the external sea-facing shoreline.

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Journal of Hydrology, v. 39, issue 3-4, p. 207-226