Introduction

Water quality in coastal areas of South Florida such as Everglades National Park (ENP) and the discharge of fresh water into Florida Bay are closely tied to water use and water management policies. Determination and monitoring of water quality is essential to restoration of the South Florida ecosystem (SFE). Increased domestic water use, drainage of land to allow farming, increased farming and subsequent nutrient loading of runoff, and changes in water management practices over the years have had a profound effect on the SFE. Monitoring of these effects is made difficult by the inaccessibility of much of this area. Airborne geophysical methods provide a means of rapidly and economically monitoring large areas where access is difficult.

Project Objective and Scope

This project addresses the question of determining the location of the fresh-water/salt-water interface (FWSWI) in the coastal regions of southern Dade and Monroe Counties, synoptic monitoring of changes in water quality associated with changes in water management practices, and looking for geophysical evidence of subsurface discharges of fresh water to Florida Bay.

This project covers a 1036-square-kilometer region of the Everglades located in Everglades National Park and surrounding areas. The study area is bounded on the east by U.S. Highway 1, on the south by Florida Bay and Whitewater Sound, and on the far west by the mouths of the Harney River and Shark River Slough. From these boundaries the study area extends inland from 14 to 22 km.

Summary of Methods

This project relies upon the fact that changes in water salinity produce changes in specific conductance (SC) or water resistivity. As pore fluid resistivity strongly influences the bulk resistivity of geologic materials, geophysical methods which measure rock resistivity can be used to obtain information on ground water quality.

Airborne electromagnetic geophysical surveys are used to collect resistivity data. The interpreted data provide information on geologic and hydrologic conditions including locations of geologic boundaries and spatial changes in water quality, which are of use to ground-water modelers. Interpretation relies heavily upon the use of borehole geophysical logs, namely induction logs from monitoring wells and water quality data. Surface geophysical measurements are used to refine the interpretation. Resistivity maps and their interpretations will be of use to agencies managing water levels in South Florida and assessing their impact on the SFE.

The primary tool used in this study is helicopter electromagnetic (HEM) surveys. A large (9-m-long) cigar-shaped instrument package called a "bird" is slung 30 m below a helicopter. Electrical current flowing in transmitter coils in the bird induces current in the ground. The intensity of the induced currents increases as the ground conductivity increases. The magnetic fields generated by the induced currents are recorded by receiver coils in the bird. The transmitter coils are excited at five frequencies to obtain different depths on investigation. Flying with the bird 30 m above the ground, measurements are made every 0.1 second along flight lines. Flight lines are nominally spaced 400 m apart. Analysis of these data produces apparent resistivity maps. Using multi-frequency data sets, the data are inverted to obtain resistivity-depth information along flight lines. Resistivity-depth results are used to generate cross sections and interpreted resistivity maps, such as depth to geologic or hydrologic interfaces.

Water quality measurements from monitoring wells will be used along with borehole resistivity logs to establish the relationship between water quality and formation resistivity. Laboratory measurements of cores from wells in the area will be used to refine the water-quality-formation-resistivity correlation. This correlation is needed to convert interpreted resistivity maps into water quality maps.

Summary of Results to Date

HEM apparent resistivity data collected in December 1994 show a resistivity transition becoming more conductive in the direction of Florida Bay. This feature is interpreted to be the fresh-water/salt-water interface (FWSWI). The transition is narrowest where water from Taylor Slough forces the transition seaward and becomes more dispersed to the east of Taylor Slough and between Taylor Slough and Shark River Slough. These sloughs show up as resistive features due to the fresh ground-water flows associated with them. There are several features on the maps which are attributed to the effect of man-made structures on ground-water flow. These include: a conductive feature along the old Ingraham Highway caused by the road bed blocking fresh-water flow southward which would wash away more saline water, 2) discontinuities in the resistivity values across the Flamingo road suggesting that the road bed inhibits water flow, and 3) resistive features near the S18C control structure on the C-111 canal suggesting that water impounded by the control structure flows into the surrounding aquifer.

In the region west of the Flamingo road toward the area of Tarpon Bay, the resistivity maps are dominated by the influence of tidal flow of saline to brackish water in the streams draining west and southwestward to the coast. The resulting resistivity transition attributed to the FWSWI is just inland of the upper reaches of most of these drainages. Coastward of Tarpon Bay, the area is uniformly conductive except for a small resistive feature thought to be associated with a fresh-water lens sitting under a small topographic high. The region is marked by vegetation changes indicative of higher ground and less saline ground water than the surrounding terrain which is covered by mangrove.

Interpretation of the FWSWI in the airborne resistivity data is confirmed by borehole geophysical logs and water quality data. We have established a correlation between formation resistivity and water specific conductance in the eastern part of the study area. Wells drilled farther to the west will provide information indicating if this correlation is valid over a wider region.

Helicopter electromagnetic (HEM) surveys were flown in April 1994 and December 1994 at the end of the dry and wet seasons respectively providing information at extremes of the hydrologic cycle. Comparison of the dry season (April 1994) and wet season (December 1994) HEM surveys shows that there is an increase in apparent resistivity of nearly a factor of 2 along the main portion of the FWSWI. This is attributed to increased fresh-water flow in the surface and near-surface portions of the aquifer. There is a very pronounced increase in resistivity in Long Sound and Madeira Bay. We interpret this as being caused by these water bodies becoming fresh due to increased fresh-water discharges from the Everglades during the wet season. Conductivity monitoring in Long Sound by the National Park Service confirms this hypothesis as well as conductivity surveys conducted in Florida Bay by USGS. The resistivity changes are very encouraging as they suggest that HEM surveys can be used to monitor the long term effect of changes of water flows in the Everglades. Repeat HEM surveys are planned over the next four years to monitor temporal resistivity changes.

Borehole geophysical data including induction logs and water specific conductivity measurements were collected starting in September 1994. To date a total of 16 wells have been logged. Additional wells are planned in and near ENP. These wells will be logged on a regular basis to monitor changes in resistivity associated with changes in surface and ground-water flows. Further analysis is needed to determine required frequency of repeat logging.

Time-domain electromagnetic soundings were collected during August 1995 at 35 locations in Everglades National Park. These soundings give very detailed information about the resistivity-depth structure from the surface to a depth of about 80 m. These data are being used to calibrate the HEM survey results.

(This abstract was taken from the Florida Bay Science Conference Proceedings, 1995)

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