EFFECTIVE POROSITY, STORAGE, AND GROUND-WATER SIMULATION IN THE TRIPLE-POROSITY, KARSTIC BISCAYNE AQUIFER, SE FLORIDA, USA

A fundamental problem in simulating ground-water flow and solute transport in karst aquifers is how best to represent aquifer heterogeneity, including the spatial distribution of its porosity, permeability, and storage. This problem extends, however, to other porous media types including fractured-rock aquifers. Combined analyses of cyclostratigraphy and borehole geophysical logs have improved the conceptualization of porosity, permeability, and storage within the triple-porosity, karstic Biscayne aquifer, which underlies an approximately 230 km2 study area in north-central Miami-Dade County, southeastern Florida. The triple porosity system principally consists of (1) matrix of interparticle and separate-vug porosity, providing much of the storage and, under dynamic conditions, diffuse-carbonate flow; (2) touching-vug porosity, creating stratiform ground-water flow passageways; and (3) less common conduit porosity composed mainly of bedding plane vugs, thin solution pipes, and cavernous vugs.

Equivalent continuum, single-porosity models can provide reasonable volume-averaged approximations of large-scale karstic ground-water flow, but concentrated, near-field flow within preferred stratiform pathways is poorly represented. The outcome of a tracer test at the Miami-Dade County Northwest well field shows that effective porosity between the point of injection and a production well ranges between 2-4%, an order of magnitude less than estimates based on specific yield equivalency assumptions. An existing well-field protection model assumes 20% effective porosity, whereas estimates of matrix total helium porosity are as much as 50% based on measurements of 3.4- and 4-inch diameter cores.

The U.S. Geological Survey is developing a Conduit Flow Process (CFP) for MODFLOW-2000 based on pipe-flow theory. Laminar and turbulent ground-water flow will be explicitly represented within a conduit network of tortuous pipes having internal roughness. The conduit network also will exchange ground water with a continuum porous-media matrix that provides most of the storage. The CFP will allow users to simulate multi-pore flow systems, such as the triple-porosity Biscayne aquifer, more realistically than is presently possible.