APPLICATION OF CARBONATE CYCLE STRATIGRAPHY TO DELINEATE POROSITY AND PREFERENTIAL FLOW AND TO ASSESS ADVECTIVE TRANSPORT IN THE KARST LIMESTONE OF THE BISCAYNE AQUIFER, SE FLORIDA, USA

Ground-water flow zones are being characterized in the eogenetic karst limestone of the Biscayne aquifer in the 230 km2 Lake Belt Area, Miami-Dade County, SE Florida, using cyclostratigraphy, surface geophysics, digital borehole image logs, cores, hydraulic tests, and conservative tracer tests. Descriptions of rock-fabric facies and cyclostratigraphy were linked to the geophysical and hydrologic interpretations to provide an accurate hydrogeologic framework and to assess preferential ground-water flow and advective transport.

A dual-porosity conceptual model was developed for this karst aquifer system with a series interlayered carbonate diffuse and conduit flow zones that are vertically arranged within the context of high-frequency cycle (HFC) stratigraphy. Porosity and permeability of the Biscayne aquifer is highly heterogeneous and anisotropic, and mostly related to secondary porosity that overprints vertically stacked rock-fabric facies within HFCs. Throughout much of the aquifer, conduit flow zones are characterized by touching-vugs that have a sheet-like geometry and occur just above flooding surfaces within HFCs. Carbonate diffuse flow occurs in stratal units containing rock-fabric facies that are generally devoid of touching vug porosity, and where flow is principally through a small-scale network of interparticle porosity.

Hydraulic measurements and a forced-gradient, convergent tracer test using Rhodamine WT and deuterated water were conducted to characterize aquifer transmissivity and advective transport near a high-capacity municipal production well. The apparent mean velocity of advective flow (366 m/day) greatly exceeds a simulated velocity of 8 m/d in this area. Fluid-conductivity measurements collected in an observation well and the high rate of advective flow suggest advective transport is greatest within a thin zone of touching-vug porosity located at the base of a HFC.