SEEKING THE TRUTH ABOUT THE HYDRAULIC CONDUCTIVITY OF THE BISCAYNE AQUIFER, MIAMI FL

The Biscayne Aquifer of southeastern Florida is a Pleistocene eogenetic carbonate karst unconfined aquifer characterized by meter-thick strata of dense networks of 2-cm burrow ichnofossil touching vugs and bedding plane conduits. The porosity of the burrowed zones can be 50% or more. These features result in extremely high hydraulic conductivities (Ks) that have proven challenging to quantify and the true K of the aquifer remains unknown. Four separate lines of investigation are being followed to address this and some consistent results are emerging, though one technique yields unexpectedly low Ks.

1) Careful aquifer tests published in the 1950s and 1990s yielded minimum estimates of the high end of the K range of the Biscayne Aquifer of 0.1 to 0.2 m/s.

2) The K of the intensely burrowed zones has been shown to be ~15 m/s through laboratory measurements on a 0.1 m diameter 3-D printed epoxy core based on computed tomography data and corroborating Lattice Boltzmann flow modeling. Other Lattice Boltzmann results on smaller samples yield K estimates approaching 50 m/s.

3) Lattice Boltzmann modeling of flow in a 0.4 x 0.4 x 17-m geostatistically-simulated block of the aquifer based on borehole imaging indicated a K of 50 m/s.

4) A series of slug tests on three closely-spaced wells of 6, 12, and 25 m depths showed oscillatory responses indicative of inertial flow conditions. The results of these tests were highly uniform at 0.01 m/s.

These disparate results demand explanation. The agreement of laboratory and Lattice Boltzmann results at the 0.1 m scale suggests that the K of the burrowed zones is well constrained in the 10-50 m/s range. Simple thickness-weighted averaging of these Ks for a number of 1 and 2 m thick burrow zones observed in a 16 m thick portion of the aquifer suggests that the overall K would be about 5-25 m/s. The older aquifer tests are inconclusive and are prohibitive if not impossible to conduct conclusively. The slug test analyses yield unexpectedly low Ks; further examination of the standard approaches for analysis of oscillatory slug tests seems needed.