SUMMARY

The stratigraphy of the Biscayne aquifer at the Levee 31N study area is characterized by vertical lithofacies successions (VLSs) that occur individually or bundled within high-frequency cycles (HFCs). The VLSs are the smallest set of genetically related lithofacies and represent the fundamental building blocks of the Biscayne aquifer. The HFCs are defined on the occurrence of regional-scale bounding upper and lower exposure surfaces and their likely relation to fifth-order, 10- to 100-kiloyear cyclic sea-level events. The VLSs are related to a single rise and fall in relative sea level in the less than 10- to 100-kiloyear duration range (fifth order or higher), suggesting an association with sub-Milankovitch frequencies observed in patterns of cyclic deposition within the Fort Thompson Formation.

Two types of ideal cycles have been defined for the Pleistocene limestone of the Biscayne aquifer: paralic and subtidal cycles. Paralic cycles are typically composed of shallow-marine shelf facies that shallow upward and are capped by freshwater deposits. Incomplete paralic cycles are capped by brackish or restricted marine deposits. The thicknesses of paralic cycles largely mimic the accommodation (plus decompaction) that occurs during the deposition of each cycle. Subtidal cycles are typical of the Miami Limestone and represented by HFC4 and HFC5.

A dual-porosity conceptual model is used to characterize the eogenetic karst Biscayne aquifer. This system contains vertically stacked inter-layered conduit and diffuse-carbonate flow zones and leaky, low-permeability zones, which are placed within the context of a nested arrangement of VLSs and HFCs. The porosity and permeability of the Biscayne aquifer are highly heterogeneous and anisotropic, and mostly related to secondary porosity that overprints vertically stacked lithofacies within VLSs.

Three ground-water flow classes were defined for the Biscayne aquifer based on subdividing the range of types of ground-water flow that occurs in the aquifer according to unique categories of lithologies and kinds of pore systems that are contained in each flow class: (1) horizontal conduit; (2) diffuse carbonate; and (3) leaky, low permeability. Throughout much of the aquifer, horizontal conduit flow zones are characterized by touching-vugs that have coalesced to form a tabular shape. These porous zones typically occur directly above flooding surfaces. Ground-water flow for the conduit flow class is envisioned as a system in which ground-water movement occurs from vug to vug, rather than a system of pipes or subsurface streams; water moves along a tabular-shaped passage formed by touching vugs that function as a major route for ground-water flow. Diffuse-carbonate flow is envisioned within lithofacies devoid of touching-vug porosity; the movement of ground water occurs within a small-scale network of interparticle, intraparticle, and separate-vug porosity as flow through vug-to-matrix-to-vug connections. Ground-water flow within the leaky, low-permeability flow zones probably occurs at a relatively small scale within semivertical vugs and irregular pores that transect a very low permeability matrix, except where bedding plane vugs are present. Bedding plane vugs, which have a sheet-like geometry, could represent significant conduits for moving large volumes of ground water.

Multiple lines of evidence, including water-level data, flowmeter measurements, fluid conductivity, fluid temperature, and mapped zones of stratigraphically controlled high relative porosity strongly suggest that the limestone of the Biscayne aquifer is a mosaic of semiconfining units and preferential flow zones. The geologic attributes of each VLS and HFC and their combined vertical arrangement was critical to correlation of flow zones in the Levee 31N study area. As shown herein, sequence stratigraphy is useful in predicting the spatial distribution of ground-water flow zones within an eogenetic karst carbonate aquifer.

Comparison of hydrographs (during a dry season period in 2004) at the two monitoring well clusters along Levee 31N and surface-water stage in ENP indicates there was good hydraulic connection between the wetlands and the aquifer. Precipitation-driven changes in surface-water stage produced a rapid increase in ground-water levels. For a wet-season period in 2003, borehole flowmeter, fluid-conductivity, and fluid-temperature data suggest that: (1) the Biscayne aquifer beneath Levee 31N was mostly recharged by relatively low-salinity, warm surface water from the ENP wetlands along the canal reach spanning wells G-3782 and G-3788; and (2) there was possible confinement or semiconfinement between the more permeable flow zones of the Biscayne aquifer. Mapped patterns of borehole-fluid conductivity and temperature suggest that relatively higher salinity, cooler Biscayne aquifer ground water may have dominated the flow field west of Levee 31N, and there was more limited surface-water recharge in the most southern and northern parts of the study area.

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Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88), except for figure 2 where the vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the vertical datum.