Water quality in Everglades National Park (ENP) and the discharge of freshwater into Florida Bay are influenced by water use and management policies in South Florida. The flow of freshwater through the Everglades into Florida Bay is critical to the well being of the South Florida Ecosystem (SFE). Restoration activities by Federal agencies are aimed at mitigating the effect of increased demand for water, farming, and flood control practices in South Florida on the SFE. Assessing the effectiveness of restoration efforts is difficult because of inaccessibility of much of this area.

This project employs a variety of electromagnetic geophysical techniques to map the location of the freshwater/saltwater interface (FWSWI). Of special note is the use of airborne electromagnetic measurements to rapidly and economically survey large areas where ground access is difficult.

Electrical conductivity is the physical property describing how easily a material conducts electricity. The conductivity of water is controlled by the concentration of dissolved ions. Freshwater typically found in ENP has a conductivity of 0.450 mS/cm and a chloride ion concentration of 40 mg/L. In contrast, the saline water of Florida Bay has a conductivity of 20 to 50 mS/cm and chloride levels of 15,000 to 35,000 mg/L. Geologic materials saturated with these waters will have resistivities (the reciprocal of conductivity) which vary by a factor of 40 to 100, with the more saline waters producing lower rock resistivities.

In our study of the Everglades we employ a variety of electromagnetic (EM) geophysical methods to measure rock resistivity. These techniques make use of a transmitter, which induces electrical current flow in the ground, and a receiver, which measures the electromagnetic field produced by these induced currents. By analyzing the electromagnetic fields, variations of electrical conductivity with depth below the Earth's surface from and location to location can be determined. From these results, we can map the freshwater/saltwater interface.

We employ three EM techniques: 1) helicopter electromagnetic (HEM) resistivity mapping to produce regional resistivity maps, 2) transient electromagnetic (TEM) soundings to provide additional information on resistivity-depth variations and to calibrate the HEM results, and 3) borehole induction logging to calibrate the HEM results and to determine the relationship between rock resistivity and pore water conductivity.

HEM resistivity mapping uses a 10-meter-long instrument pod slung below a helicopter which flies back and forth over the survey area along parallel lines about 400 m (1/4 mi) apart, making a measurement approximately every 8 m along the flight line. The instrument pod contains five pairs of transmitter and receiver coils which measure the electromagnetic response of the ground at different frequencies to obtain information from different depths. The raw data are processed to produce apparent resistivity maps which show several interesting features, including a prominent transition from higher resistivities in the landward direction to lower resistivities toward the shore, which is interpreted as the freshwater/saltwater interface (FWSWI). The FWSWI interface is very sharp in the region of Taylor Slough where there is a relatively high flow of water. In contrast, the FWSWI in Shark River Slough follows the terminus of rivers that have tidal flow. The influence of manmade features is apparent where the FWSWI crosses the C-111 canal. Downstream of the FWSWI, the canal recharges freshwater into the saltwater saturated surface aquifer. A very distinctive resistivity low associated with portions of the now blocked canal adjacent to the Old Ingraham Highway has been mapped. Saltwater formerly flowed through the canal from Florida Bay near Flamingo toward Royal Palm. The modern road through the ENP to Flamingo also influences the hydrology, as revealed in the HEM maps. It inhibits the westward flow of freshwater coming from Taylor Slough producing a four-fold change in resistivity across the road near Nine Mile Pond.

To date, three HEM surveys have been flown. The first was flown in April 1994 and covered 362 km2 centered on Taylor Slough. The second survey, flown in December 1994, reflew this area and extended coverage west beyond Tarpon Bay for a total survey area of 1,036 km2. The third survey (November 1996) covered the core Taylor Slough area and expanded coverage (1,010 km2) north of the main Park Road toward the headwaters of Taylor Slough. Comparison of the first two surveys reveals resistivity variations associated with seasonal hydrologic changes. Data collected in the wet season (December 1994) shows an increase in apparent resistivity at the location of the FWSWI, reflecting increased freshwater in the system, compared to results from the dry season survey (April 1994). The ability of the HEM surveys to detect seasonal resistivity variations suggests the potential for use of this technique in monitoring long-term changes in the ground-water system.

While the HEM apparent resistivity maps show interesting and significant features, they provide no depth information. We have developed a method of inverting the HEM data to obtain resistivity-depth information at each measurement point. To accomplish this task, we analyzed the errors in the HEM data produced by the standard calibration procedures. A technique to remove these errors from the data was developed. This procedure uses transient electromagnetic soundings to determine the resistivity-depth function at about fifty points scattered over the survey area. We then compute the HEM electromagnetic response produced by these resistivity-depth models. Finally, we determine correction factors needed to bring the computed and measured HEM responses into agreement. The correction factors are applied to all the survey data. By using this procedure we are able to compute stable resistivity-depth estimates for the entire HEM survey. The inversion models are presented as resistivity-depth slices at selected depths. Interpreted depths to the saltwater saturated zone range from less than 1 m seaward of the FWSWI deepening to 15 to 20 m or more landward of the interface.

Because of their extensive coverage in areas which are otherwise difficult to impossible to access from the ground, HEM surveys provide a means of assessing regional ground-water quality in the Everglades. From well log data, we have established a relationship between rock resistivity and pore fluid conductivity, which allows the interpreted resistivity-depth models to be converted into estimated water quality models. This provides a basis for developing ground-water flow models which incorporate solute transport.

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