A SEQUENCE-STRATIGRAPHIC BASED APPROACH FOR DEVELOPING A REAL-WORLD CONCEPTUAL HYDROGEOLOGIC MODEL OF THE KARSTIC BISCAYNE AQUIFER, SE FLORIDA, USA

A sequence-stratigraphic framework was used to map eogenetic Pleistocene karstic limestone solution features within the Biscayne aquifer; this framework provides a real-world conceptual model suitable for numerical simulation of ground-water flow in Miami-Dade County. A dual-porosity conceptual model characterizes this karst aquifer system composed of interlayered conduit, diffuse-carbonate, and leaky, low-permeability ground-water flow classes, which are correlated within the context of genetically related vertical lithofacies successions (VLSs) and high-frequency cycles (HFCs). VLSs form the fundamental building blocks of the Biscayne aquifer and HFCs are defined by single or bundles of two or more VLSs. Subtidal and paralic cycles are principal ideal cycles. Major conduit flow zones correspond to marine-shelf or lagoonal depositional environments and they are characterized by abundant touching vugs that coalesce to form tabular-shaped, stratiform bodies. The conduit flow class can comprise all of a subtidal cycle; but from base to top of a paralic cycle, permeability is represented by conduit, diffuse-carbonate, and leaky, low-permeabity flow classes.

Forced-gradient tracer tests using conservative and microbial tracers were used at a municipal wellfield to assess the efficacy of subsurface microbial filtration by the aquifer. The tracers, borehole fluid-temperature, and flowmeter measurements strongly suggest that a thin zone of touching-vug porosity at the base of a VLS is a major contributor to advective and particulate transport at the wellfield.

The same conceptual model is being used to examine the feasibility of controlling seepage beneath canal Levee 31N, using a vertical subsurface barrier to impede eastward movement of ground water from Everglades National Park (ENP). Integrated wet-season results from borehole flowmeter, fluid temperature, fluid conductivity, and water-level data suggest that (1) there is possible confinement or semiconfinement between the more permeable flow zones of the aquifer; and (2) warmer surface water of relatively lower salinity from the ENP wetlands may dominate the ground-water flow field in one part of the study area but not others. It is uncertain if the spatial variation in ground-water flow reflects natural or anthropogenic stresses.