Restoration of the wetland ecosystem of south Florida requires restoration of the natural hydrologic system. In this regard, land and water managers in south Florida are dependent on computer models that simulate both the natural hydrologic system and the added engineering structures. However, because insufficient hydrologic data exist for southwest Florida, current hydrologic models, such as the South Florida Water Management Model and the Natural System Model, do not cover the entire ecosystem area. The purpose of this project is to acquire subsurface geologic and hydrologic data in southwest Florida that will extend current ground-water models to the southwest coast, thereby expanding the utility of these models for land and water management (Weedman, 1996). We have completed coring and logging of seventeen holes through the surficial aquifer system in southwest Florida and can now begin to expand and refine our understanding of the geologic framework of the surficial aquifer system across south Florida.

We drilled seven continuous cores in FY96 in western Collier County (Weedman and others, 1997) and ten cores in FY 97 in the Big Cypress National Preserve; each core is approximately 60 m deep. The cores are being examined for mineralogy, composition, texture, structures, and fossils to determine lithology, alteration, porosity, and age of the aquifer rocks, and to construct a depositional model that will aid in the extrapolation of subsurface data from each site. Stratigraphic units are being correlated between coreholes and their lithologic and hydrologic properties estimated where core data are absent. Permeability tests have been run on selected lithified core samples and integrated with hydrologic and geophysical logs to estimate hydraulic conductivity of each lithofacies. Geophysical logs (natural gamma ray, caliper, induction, temperature, neutron, flowmeter, televiewer) are run soon after holes are drilled. While core analyses are in progress, several preliminary conclusions can be drawn and integrated with the geophysical data (Paillet and Weedman, 1996; Hite and others, 1997; Paillet, 1997).

The study area in southwestern Florida (Collier and Monroe Counties) can be divided roughly into two regions that fall to the east and west of State Route 29, respectively, and are bounded to the north by I-75 and to the south by the Tamiami Trail and the Loop Road. The greatest variability in both the surficial aquifer system and the subsurface sedimentology and diagenesis is from west to east, rather than north to south; however, there are regional depositional patterns that apply to the entire study area. In most cores there is a densely cemented "caprock" about 1 m thick near the surface, which is underlain, in the west, by a lithified carbonate unit and, in the east, by an unconsolidated carbonate unit, both of which contain increasing amounts of quartz sand with depth. Below the carbonate unit is an unconsolidated siliciclastic (that is, containing primarily quartz sand, silt, and clay) unit. In some cases the siliciclastic unit has a tightly cemented zone tens of centimeters thick at the lithologic boundary with the carbonate sediment, which may form a local confining zone where it is present. In the central and eastern part of Collier County, the siliciclastic unit is quite thick (>90 m). It is a poorly sorted (fine to pebble) unconsolidated, marine sand interbedded with an olive gray silty clay. Preliminary data indicate that this unit is not very permeable, despite its coarse component, especially when compared to the upper carbonate unit. Below the siliciclastic unit, in the western part of the study area, is a mottled dolomitized limestone; in the central part of the study area there is a dolomitized unconsolidated sand at about the same depth as the dolomite on the west; to the east, there is quartz sand and interbedded olive gray silty clay to the bottom of the cores, at depths of about 60 m.

The most porous and permeable zones of the surficial aquifer system, all across the study area, appear to be in the upper 15 m, with transmissivities that appear to be higher in the west than in the east. Those zones typically are the highly leached, but lithified, carbonate units, primarily found in the western part of the study area. Preliminary isotopic and paleontologic data, from the western part of the study area, indicate that the upper aquifer is primarily Pliocene, and that the Miocene-Pliocene boundary occurs within the siliciclastic unit. The occurrence of mixed-age fauna in the siliciclastic units suggests reworking of sediments at some horizons.

In the west, the surficial aquifer system is compartmentalized into aquifers that are successively more saline with depth, either by thin (< 1 m) horizons of tighly cemented carbonates and sandstone, or by thicker (> 3 m) zones of poorly sorted siliciclastics. In contrast, in eastern Collier County, virtually all ground water in the upper 60 m of the aquifer system is fresh, and lithification is rare in both the carbonates and the siliciclastics. While the base of the surficial aquifer system over the study area is assumed to be an olive gray, dolomitic silty clay, we did not encounter that lithofacies within the upper 60 m of the system in the western part of the study area; however, it was encountered within 12 m of the surface in the east, where it typically acts as a confining zone.

The next phase of this study is to develop a depositional model to correlate corehole sites, to install monitor wells in selected coreholes at depths of interest, and to design aquifer tests to quantify the transmissivities of the major rock and sediment types in the surficial aquifer system.

REFERENCES

Hite, L.R., Carlson, M.J., Wolfe, P.J., and Richard, B.H., 1997, Electromagnetics delineates aquifers between wells in southwest Florida: South Florida Ecosystem Meeting, Ft. Lauderdale, August 25-28, 1997.

Paillet, F.L., and Weedman, S.D., 1996, Using borehole logs and core to identify and characterize confining units in the surficial aquifer sytem of south Florida: GSA 1996 Annual Meeting, Denver, Colorado.

Paillet, F.L., 1997, Using geophysical measurements in boreholes to calibrate surface electromagnetic soundings in south Florida: South Florida Ecosystem Meeting, Ft. Lauderdale, August 25-28, 1997.

Weedman, S.D., 1996, Hydrogeology of the surficial aquifer system of southwest Florida: U.S. Geological Survey Fact Sheet FS-158-96.

Weedman, S.D., Paillet, F.L, Means, G.H., and Scott, T.M., 1997, Lithology and geophysics of the surficial aquifer system in western Collier County: U.S. Geological Survey Open-File Report 97-436, 18 figures, 4 tables, 180 p.

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