INSIGHTS TO THE HYDROGEOLOGY OF THE UPPER FLORIDAN AQUIFER FROM DETAILED STUDY OF ITS MATRIX PERMEABILITY

The Upper Floridan Aquifer (UFA) is a multi-porosity aquifer: double porosity (fractured porous aquifer) downdip where it is confined; triple porosity (karstic, fractured porous aquifer) in the updip, unconfined region. Minipermeameter measurements at 30-cm intervals in 1210 m of core from 8 observation wells in the confined region indicate high and highly variable matrix permeability (<1E-14.4 to 1E-11.1 m^2). The distribution of matrix permeability is stratiform: it correlates strongly with depositional texture. Some grainstones and sucrosic dolomites can compete with fractures for flow. Comparison of cumulated matrix permeability with the results of an APT in a Suwannee Formation observation well (ROMP-20, UFA confined region) allows calculation of the average permeability of the thin, strataform layers of secondary permeability (i.e, where core recovery is <100% and the flow log indicates flow is concentrated). The permeability of these paleokarst layers and fractured lithostromes adds two orders of magnitude to the range of permeability. Simulation (MST-3D) of solute transport in a hypothetical aquifer storage and recovery (ASR) well with layered heterogeneity patterned after ROMP 20 indicates that the geometry of the ASR plume is much more appropriately thought of as a “bottle brush” than the traditional “bubble.” Caves and springs characterize the UFA’s unconfined region. Cave mapping shows the caves to be laterally extensive and tabular in shape. The stratiform distribution of matrix permeability opens the hypothesis of stratigraphic control. Spring hydrographs do not have spiky responses to rainfalls typical of karst springs draining Paleozoic limestones, where matrix permeability is orders of magnitude less. The UFA’s matrix permeability apparently provides access to storage in the huge interparticle pore volume beneath Florida spring basins. We conclude that facies-controlled matrix permeability may play a significant role in Cenozoic carbonate aquifers.