T194 Karst Hydrology and Geomorphology in North America Over the Past Half Century (Posters): In Honor of Derek Ford and William White

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

Paper number: T194-11
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

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.