Tidal pumping of water between Bahamian blue holes, aquifers, and the ocean

摘要

Exchange of water between conduits and aquifers occurs in many continental karst settings because allogenic recharge from confined catchments causes hydraulic heads in conduits to increase faster than in the aquifer. Most modern carbonate platforms lack allogenic catchments, allowing rainfall to recharge the aquifer uniformly without sufficiently altering head gradients to drive exchange between conduits and aquifers. Some modern carbonate platforms experience tidal variations which could lead to head gradients that drive exchange. To determine the impact of tides on exchange, we measured water elevations at high temporal resolution in the ocean, blue holes and wells on San Salvador Island and Rum Cay, Bahamas. Dampened tidal amplitudes inland indicate diffusivity values (transmissivity/storativity) at wells were around 1.3×10 ~6m ~2/day and at blue holes were around 76.9×10 ~6m ~2/day, assuming dampening results only from head loss. These diffusivity values were used to estimate hydraulic conductivity values of around 4.0-294×10 ~4m/day although they may be lower if the aquifer thickness is greater than the estimated 10m. We assume wells provide values representing greater influence of matrix permeability than values from blue holes, which represent a greater influence of conduit permeability. Differences in permeability drive exchange because hydraulic head in the aquifer lags the head in the conduits and blue hole through a tidal cycle. If negligible head loss occurs with flow through conduits, as reflected in lag times less than 10min, then differences in elevation at the blue holes and the ocean may represent exchange of water between the blue hole and matrix porosity. With this assumption, about 0.9m ~3 of water is exchanged per half tidal cycle, or about 1% of the complete change in volume of water in the blue hole per half tidal cycle. This volume represents an average penetration into the aquifer of 6-8mm although it could be further in zones with elevated permeability. Exchange is reflected in systematic changes in specific conductivity and pH between high and low tide and the pH changes reflect reaction with the surrounding aquifer material. Since exchange occurs twice daily, cumulative alteration of aquifer porosity could be large. Tidal exchange should decrease away from the coast on large carbonate platforms so that tidally driven alteration will be enhanced at the rims over interior of carbonate platforms.