TIDAL PUMPING AND FRESH WATER – SALTWATER MIXING IN BAHAMIAN BLUE HOLES

Blue holes are connected to the ocean through zones of elevated permeability (e.g., conduits or preferential flow paths) embedded in porous limestone that make up karstic aquifers of modern carbonate banks. The rate and magnitude of water exchange between the ocean, aquifer, and blue holes is controlled by hydraulic conductivity and head gradients that reverse with tidal periodicity. This exchange was assessed at two blue holes: Ink Well Blue Hole on San Salvador, Island and Mermaid Pond, Rum Cay, Bahamas by monitoring water elevations and chemical compositions through eight tidal cycles. Tidal periodicities in the blue holes lagged ocean tides by less than 10 minutes and tidal efficiency (amplitude of blue hole tide/amplitude of ocean tide) ranged from 0.67 to 0.89. These efficiencies indicate average hydraulic diffusivity (transmissivity/storativity) and hydraulic conductivity are 19 to 47 m²/day and 57 to 141 m/day, respectively, assuming aquifer thickness of 10 m and storativity of 0.3. We used the measured tidal amplitudes to estimate hydraulic gradients and calculated wetted surface area in Ink Well Blue Hole by assuming a cylindrical shape. These values indicate that fluxes of water from the aquifer into and out of the blue hole averaged around 1.3 m³/tidal cycle, which is about 2% of the change in total volume of water in the blue hole between low to high tide. This small fraction of exchanged water suggests that the surface of the blue hole oscillates at nearly the same rate as the water table and most water is exchange through conduits. Despite the small fraction, specific conductivity profiles through the water column were fresher by about 10,000 μS/cm at low tide than at high tide. This large change in composition indicates vertical mixing between the fresh water lens and underlying salt water is extensive, regardless of small horizontal gradients and could be important for magnitudes and location of speleogenesis (dissolution and cementation) in the aquifers.