Groundwater flow beneath a hypersaline pond, Cluett Key, Florida Bay, Florida

Florida Bay carbonate mud islands are known to be sites of Holocene diagenesis, including dolomitization, and the hydrology is an essential component of this process. On Cluett Key, a typical mud island in western Florida Bay, groundwater salinities range to 145‰ beneath a shallow ephemeral pond which occupies 70% of the island. A sharp hydrostratigraphic boundary separates low-permeability unconsolidated carbonate mud from underlying highly permeable Pleistocene limestone. We report the results of a 2 year project aimed at determining the rates, patterns, and driving force of groundwater flow beneath the island. Water level measurements are used to demonstrate the presence of a large hydraulic head drive caused by the difference in elevations of the pond and Florida Bay, and enhanced by the higher density of groundwaters in the carbonate mud compared with groundwater in the limestone. The hydraulic head drive is essentially vertical because Florida Bay water levels are transmitted with little attenuation to the limestone underlying the island. Distributions of groundwater density and pore pressures are consistent with vertical groundwater flow. Based on an estimated vertical hydraulic conductivity of approximately 5×10−3 m day−1, vertical interstitial velocities are on the order of 25 cm year−1 with a residence time in the carbonate mud of approximately 15 years. This velocity is very similar to that calculated independently from tritium concentrations in pore waters. Both horizontal and vertical density gradients exist in the carbonate mud. These density variations induce circulations owing to vorticity and may lead to the formation of instability plumes (reflux), but dynamical scaling suggests that these motions are much slower than those induced by the dominant hydraulic drive. Buoyancy effects may, however, be dominant on other lower islands in Florida Bay where the hydraulic head drive is much smaller than on Cluett Key. Diffusion may blur sharp gradients in salt, Ca2+, or Mg2+, but is not an important vehicle for wholesale movement of these species through the island. Cluett Key shares some similarities with Holocene carbonate atoll islands: the two-layer hydrostratigraphy, and transmission of tidal signals under the island. In contrast to atoll islands, however, groundwater flow on Cluett Key is governed by transmission of the mean sea level to the base of the Holocene. Basal tidal fluctuations induce little mixing in the low-permeability Holocene sediments. In addition, the groundwater on Cluett Key is dominantly oceanic, not meteoric, and is consequently denser than underlying groundwater. © 1997 Elsevier Science B.V.