Jonathan Martin, PhD , University of Florida
To determine the impact of tides on exchange and on the development of secondary porosity in modern carbonate platforms, we measured water elevations at high temporal resolution in the ocean, blue holes (the local name for sinkholes) and wells on San Salvador Island and Rum Cay, Bahamas. The lag and dampened amplitude of the tidal variations provide information about the diffusivity values (transmissivity/storativity) at the wells. Diffusivity was found to average around 1.3 x 106 m2/day for the wells and around 76.9 x 106 m2/day at blue holes, assuming dampening results only from head loss. These diffusivity values were used to estimate hydraulic conductivity that averages around 6 x 104 m/day for the wells and 2 x 107 for the blue holes based on an assumed aquifer thickness of 10 m and a storativity of 0.3. We assume values obtained from wells represent hydraulic conductivity of flow that occurs primarily in matrix permeability and the values from the blue holes represent flow that occurs primarily in conduit permeability. These differences in permeability cause hydraulic heads in the aquifer to lag heads in the conduits and blue holes through a tidal cycle. The resulting differences in head would create gradients between the two components of the aquifer and could drive exchange of water at tidal frequencies. We made observations of tidal exchange over 4 days at Ink Well Blue Hole on San Salvador Island. Assuming the observed tidal dampening is caused by exchange, we estimate the average exchange over the 4 days of observation to be about 0.9 m3 of water per half tidal cycle, or about 1% of the complete tidal change in volume of water in the blue hole. Water in the blue holes commonly has distinct chemical compositions from aquifer water because of input of terrestrial and autochthonous organic carbon. Oxidation of the organic carbon forms carbonic acid, driving the water to undersaturation with respect to the carbonate minerals. The tidally driven exchange could thus cause dissolution within the aquifer matrix.
Exchange of water between the blue holes and the matrix porosity is reflected in variations between high and low tide in depth profiles of specific conductivity and pH that were measured using a logging submersible field meter. The pH profiles suggest the water has reacted with the surrounding aquifer material and then flowed to the blue hole on the ebbing tide, but we have no corresponding water samples to compliment these field measurements with calculations of saturation state of the water. We have since returned to the field area to repeat the tidal pumping and logging measurements and to simultaneously collect profiles of water samples to compare with the logger data. These data analyses are on-going but should be useful in quantifying the extent of dissolution of the matrix rocks resulting from the exchange of water between the blue hole, conduit, and aquifer matrix. Although the estimated volume of the exchange is small, the exchange occurs twice daily, and thus the cumulative amount of dissolution could be large. On a platform-wide scale, tidal exchange should decrease away from the coast so that tidally driven alteration will be enhanced at the rims over interior of carbonate platforms.