MATRIX PERMEABILITY OF THE UNCONFINED FLORIDAN AQUIFER

The Floridan aquifer of peninsular Florida is unconfined in west-central Florida and confined farther to the south. Shallow-water, high-energy grainstones are more numerous in the limestones of the up-dip unconfined region: 59% of the thickness as compared to 30% in the confined region. Nevertheless, our 12,000 measurements of matrix permeability (k) in cores of the confined Floridan Aquifer and 2,000 measurements of matrix k in cores and cave-wall samples in the unconfined Floridan Aquifer indicate that matrix k is similar in the two regions. Although there are differences between the two areas in the details of the frequency distribution of log matrix k, both sets of measurements indicate a comparable facies-dependent heterogeneity spanning three order of magnitude (10^-11.4 m² – 10^-14.6 m²) and a mean matrix k of about 10^-13 m².

The unconfined region is the location of the Floridan aquifer's legendary high transmissivity and is the site of eogenetic karst. This study clarifies relationships between the large and highly variable matrix k, which characterizes eogenetic karst, and the cave-scale porosity, which accounts for the aquifer's high transmissivity. Although levels of passages in caves within the unconfined Floridan Aquifer do not follow geologic structure at the regional scale (and do tend to correlate with ancient sea levels as marked by local terraces), matrix k impacts the passage morphology at the scale of individual caves. For example, passages tend to be wider and laterally continuous where matrix k is larger; in at least one cave, a laterally continuous zone of dissolution is inclined relative to the modern water table by more than a degree. Also, the presence of caves affects the matrix k. Measurements on cores from cave walls suggest that dissolution is not limited to the immediate surface of the cave. Matrix k decreases with distance from the surface; at one location, the matrix k in the outer 2 cm is 14 times the matrix k deeper within the wall.

In triple-porosity systems of eogenetic karst, all three elements (cavernous, fracture, and intergranular porosity) are capable of transmitting water in significant quantities. The case of the unconfined Floridan aquifer illustrates feedbacks between the elements that can occur in the evolution of such systems.