Abstract
The majority of limestone islands are made of eogenetic carbonate rock, with intrinsic high porosity and permeability. The freshwater lenses of small islands are dominated by diffuse flow regimes as the island perimeter is everywhere close to the meteoric catchment of the island interior. This flow regime produces flank margin caves at the lens margin, where dissolution is enhanced by mixing corrosion, superposition of organic decay horizons and higher flow velocities as the lens thins. The lens interior develops touching-vug flow systems that result in enhanced permeability and lens thinning over time. As islands become larger, the area (meteoric catchment) goes up by the square, but the island perimeter (discharge zone) goes up linearly; diffuse flow becomes inefficient; conduit flow develops to produce traditional epigenic cave systems that discharge the freshwater lens by specific turbulent flow routes, which in turn are fed by diffuse flow in the island interior. Locally, diffuse flow to the island perimeter continues in coastal proximal areas between major conduit flow routes to produce flank margin caves. The Bahamian Archipelago represents a case history in which tectonics is limited, the rocks are entirely eogenetic and the diffuse to conduit flow transition is demonstrated.
Highlights
Coastal karst aquifers are a unique type of aquifer, involving the interplay of dissolutional processes, freshwater lens chemistry and dynamics and sea-level change
Karst aquifers can be viewed as a three-part aquifer, with water flow occurring as matrix flow, fracture flow and conduit flow [2,3]
The field evidence from the Bahamian Archipelago indicates that conduit flow systems of an epigenic nature have developed only on the large Bahama Bank platforms
Summary
Coastal karst aquifers are a unique type of aquifer, involving the interplay of dissolutional processes, freshwater lens chemistry and dynamics and sea-level change. The vast majority of coastal karst aquifers in the world are made up of young carbonates that have not yet undergone burial diagenesis, termed eogenetic by Choquette and Pray [1]. Most karst studies from the interior of continents have been done on diagenetically-mature carbonates, which Choquette and Pray [1] termed telogenetic. Karst aquifers can be viewed as a three-part aquifer, with water flow occurring as matrix flow, fracture flow and conduit flow [2,3]. Matrix flow is minimal as the pore space is small in size and amount and poorly connected; flow is usually through bedding planes and fractures, which serve as the origin points for the water transmission, which subsequently evolves into turbulent conduit flow. Telogenetic coastal karst aquifers exist, such as Gotland Island off the coast of Sweden or much of the coast of the Adriatic Sea (e.g., [4,5])
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