BIOGENIC POROSITY AND GROUND-WATER FLOW IN THE KARSTIC BISCAYNE AQUIFER, SE FLORIDA, USA

Numerous ground-water karst studies have documented dissolutional structures that function as principal ground-water flow paths in carbonate aquifers. Typical examples include solution-enlarged conduits and fractures that cut across stratigraphic boundaries. An alternative to this karst-carbonate paradigm is proposed for a 345-km² study area that extends from the Everglades wetlands to coastal Biscayne Bay and includes the karst-carbonate Biscayne aquifer of SE Florida. Specifically, it is suggested that much of the ground-water flow in this area is related to biogenic touching-vug porosity that forms tabular-shaped stratiform flow zones. The biogenic porosity typically manifests as one or more of the following: inter- and intra-burrow porosity, inter- and intra-root (commonly mangrove) porosity, and fossil moldic porosity. Biogenic porosity and its hydrologic importance were determined by examining outcrops, cores, and borehole geophysical data that include digital optical image logs and flowmeter, fluid conductivity, and fluid temperature measurements. Digital image logs and 72 of 85 geophysical measurements (85%) in 21 boreholes indicate that biogenic porosity is the principal pore type in ground-water flow zones. All the remaining borehole geophysical measurements across ground-water flow zones detected inflow or outflow from bedding plane vugs, with the exception of a single cavern-sized vug. Intraburrow pores (probably Ophiomorpha and/or Thalassinoides) typically are cm-scale-wide and either unfilled or partly filled with sediments. Computer-aided tomographic 3-D modeling indicates complex flow paths within interwoven burrows.

It is suggested that ground-water flow related to biogenic porosity is of primary importance in the study area and traditional karst-conduit dominated flow paths, which are problematic to physically map, may be of secondary importance. Biogenic porosity in the Biscayne aquifer is related to predictable vertical positions in stratigraphic cycles and much of it can be mapped reliably. The relative magnitude of ground-water flow associated with unfilled intraburrow porosity (primary porosity) as compared to dissolution-produced intraburrow porosity (karstic secondary porosity) remains to be determined, but is being investigated.