In southwest Florida, the surficial aquifer system is a primary source of fresh water. These water-bearing sedimentary rocks are the result of periodic sea level fluctuations that resulted in the deposition of alternating sands, silts, and clays and marine carbonate sediments. The surficial aquifer system is predominantly unconfined, receives recharge directly from precipitation and is composed of materials of Miocene to recent age (Knapp and others, 1986).

The surficial aquifer system is characterized by a near-surface, water-table aquifer containing freshwater and a lower brackish to saline aquifer termed the lower Tamiami aquifer (Bennett, 1992; Jacob, 1980). The lower Tamiami aquifer has variable formation conductivity values of several thousand microsiemens per centimeter due to the upward movement of waters with high chloride ion content. An aquitard separates these aquifers of distinct water quality. Accurate models of aquifer geometry and estimates of aquifer parameters are needed in planning the restoration and management of natural ecosystems in the parklands of Collier County.

This study developed after it was determined that surface geophysical mapping was needed to correlate data from individual boreholes within Collier County, Florida. Time-domain electromagnetics (TDEM) was chosen as the surface geophysical tool based on agreement between core descriptions, water sample data, geophysical logs, and TDEM soundings near coreholes. Surface geophysics would help provide a better understanding of the lateral continuity of the aquifers and aquitard(s) between borehole locations. The ultimate goals of this study are as follows: (1) to create electrical conductivity profiles from the results of modeling 64 soundings along transacts of the surficial aquifer system and (2) to determine where encroachment by the lower Tamiami aquifer may exist and therefore infer where the aquitard may be discontinuous.

Time-domain electromagnetic soundings measure the apparent conductivity of the layers below the transmitter loop by measuring the strength of the signal returned to the receiver as a function of time when the equipment is operated in a pulsed mode (Fitterman and Stewart, 1986). The equipment response in millivolts as a function of time is used to generate models of conductivity versus depth. A Geonics Protem TEM 47/P system was used for data collection. In central Collier County west of Big Cypress National Preserve, 33 TDEM soundings were completed on a north-south 17 kilometer (10. 5 mile) transect along Miller Boulevard. Twenty-six soundings were completed along Stewart Boulevard and Janes Memorial Scenic Drive covering approximately 28 km (17.5 mi) to determine the east-west variations in the aquifer system. The computer program TEMIX (Stoyer, 1988) was used to model the results.

The TDEM models were interpreted in relation to the geology and the hydrology evidenced in the boreholes. Whether using a 2, 3, 4, or 5 layer model the prominent feature was a lower conductivity layer on a higher conductivity half space. We interpret this as the water-table aquifer ranging in thickness from 7 to 50 m overlying the brackish-to-saline lower Tamiami aquifer. Soundings that were made in the vicinity of boreholes were found to agree closely with the profiles of subsurface conductivity given by the induction logs.

The north-south profile along Miller Boulevard shows a gentle southward dip of the interface from approximately 25 m at the north end to about 50 meters at a location 5,000 m north of State Route 4l. This is interpreted to be the result of the aquitard dipping to the south. At the southern end of the profile, near State Route 41, the depth to the conductivity increase is at approximately 7 m. This is interpreted as the edge of the water-table aquifer because south of State Route 41, the surface water is saline. The shallow depth to the interface in the south is a direct result of lateral sea water intrusion above the aquitard, as this portion of the profile is nearest the coast. The east-west transact shows a fairly constant interface at a depth of approximately 35 meters. Localized variation in the conductivity of the underlying formations along the profiles may be due to upward movement of saline water into underlying aquifers, lateral variation in lithology and permeability, or undetected interference with electrically conductive debris.

This study has shown that TDEM soundings can be related to well logs and provide an efficient method to interpolate the information from a small number of boreholes over a relatively large area. This combination of TDEM soundings with logged boreholes is effective for studying the hydrostratigraphy of the surficial aquifer system of south Florida.

REFERENCES

Bennett, M.W., 1992, A three-dimensional finite-difference groundwater flow model of western Collier County, Florida: West Palm Beach, Florida, South Florida Water Management District, Technical Publication 92-04, 358 p.

Fitterman, D.V., and Stewart, M.T., 1986, Transient electromagnetic sounding for groundwater: Geophysics, v. 51, no. 4, p. 995-1005.

Jacob, P.G., 1980, Some aspects of the hydrogeology of coastal Collier County, Florida, in P.J. Gleason, ed., Water, Oil, and Geology of Collier, Lee, and Hendry Counties: Miami Geological Society, 1980 Field Experience Guidebook, p. 21-26.

Knapp, Michael, Burns, W.S., and Sharp, T.S., 1986, Preliminary assessment of the groundwater resources of western Collier County, Florida: West Palm Beach, Florida, South Florida Water Management District, Technical Publication 86-1, 142 p.

Stoyer, C.H, 1988, TEMIX-G-User's Manual: Interpex Limited, Golden, CO.

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