DIFFUSE VERSUS CONDUIT FLOW IN COASTAL KARST AQUIFERS: THE CONSEQUENCES OF ISLAND AREA TO PERIMETER RELATIONSHIPS

The majority of limestone islands are made of eogenetic carbonate rock, with intrinsic high porosity and permeability. The fresh-water 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, and diffuse flow becomes inefficient; conduit flow develops to produce traditional epigenic cave systems that discharge the fresh-water 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 as such, the diffuse to conduit flow transition is demonstrated both with field observations and modeling at an island radius of ~20km.