Saltwater Intrusion in Everglades National Park, Florida Measured by Airborne Electromagnetic Surveys

Saltwater intrusion is an ideal target for mapping with airborne electromagnetic techniques because of the high electrical conductivity of saline water and its contrast with that of freshwater. Using electromagnetic sensors to measure the electromagnetic response of the ground at several frequencies, information from various depths is obtained. The electromagnetic response is then converted to resistivity-depth models by a nonlinear parameter estimation technique called inversion. To rapidly cover large areas of the Florida Everglades where ground access is often very difficult, the electromagnetic sensors are suspended below a helicopter which is flown back and forth across the survey area. A measurement is made every 10 to 15 m along flight lines that are typically spaced 400 m apart. The high sampling density provides a detailed picture of aquifer resistivity that is not readily obtainable by any other means. The extent of saltwater intrusion can be estimated from interpreted resistivity models. In addition to airborne electromagnetic measurements, borehole geophysical measurements provide information on the aquifer formation resistivity and pore water specific conductance (SC). The relationship between these two properties offers a means of indirectly estimating porewater SC from the airborne geophysical estimate of formation resistivity. Specific conductance can then be converted to chloride concentration using an established relationship for the aquifer. The resulting product is a three-dimensional estimate of aquifer water quality. Surveys of Everglades National Park in south Florida have been made over an area of about 1000 sq. km. These maps show in detail the extent of saltwater intrusion and the influence of natural processes and human activities. The data are being used to develop ground-water flow models which incorporate salinity. The resulting flow models will be used to plan for restoration of the South Florida Ecosystem. One advantage of the airborne electromagnetic resistivity mapping technique is the speed with which it can survey large areas where ground access is difficult or impossible. The technique works very well in the Everglades because of the sub-horizontal, slowly varying geology and absence of clay. However, it should be of use in many coastal aquifers, even where the geology is more complicated, because of the tendency of saltwater intrusion to overprint geologic boundaries and to predominate the electromagnetic response. INTRODUCTION One of the principal characteristics of seawater is its high concentration of dissolved ions—approximately 35,000 mg/l [Freeze and Cherry, 1979]. These dissolved ions enhance the ability of seawater to conduct electricity. Seawater, which is a very good conductor, has a specific conductance (SC)of 50 mS/cm [Hem, 1970]. Freshwater suitable for domestic purposes 2 has much lower dissolved ion concentrations (0 to 1000 mg/l) and lower SC in the range of 50 to 500 μS/cm [Hem, 1970; Freeze and Cherry, 1979; Hearst et al., 2000]. This large difference in the electrical properties of fresh and saline water means that the presence of saline water in an aquifer can be detected by various electromagnetic geophysical measurements either in boreholes, on the surface, or in the air. In this paper we discuss how helicopter electromagnetic (HEM) measurements can be used to measure the electrical resistivity of aquifers to produce a three-dimensional view of the subsurface. Using correlations between water quality, as measured by SC and chloride content, and formation resistivity measured in boreholes, we produce a three-dimensional model of estimated water quality. Such information is of great use in developing a ground-water model which incorporates solute transport. As an example of the HEM method we show the results of a survey flown over Everglades National Park in south Florida (Fig. 1). This survey, which is part of a larger study of the South Florida Ecosystem being conducted by Department of the Interior agencies, is aimed at understanding the impact on the ecosystem of development over the past 100 years and establishing the best course of action for restoring the system to a revitalized state [Gerould and Higer, 1999]. EVERGLADES HYDROLOGY The Florida Everglades is unique in many ways, not the least of which is the ubiquitous presence of surface water ranging from 10 cm to 2 m in depth. Surface water, coupled with sawgrass marshes and mangrove swamps, makes ground access difficult. Consequently, the traditional method of obtaining subsurface hydrologic information from boreholes and groundwater sampling is limited to existing roads or the use of portable drill rigs at remote locations. Previous hydrologic work in Everglades National Park by Fish and Stewart [1991] was based upon a dozen wells covering an area of about 2000 km. Their work provides a very useful regional framework which defines the major hydrologic units. The hydrogeology is characterized by three distinct zones, which, from the surface to depth, are the surficial aquifer system, the intermediate confining unit, and the Floridan aquifer system. The surficial aquifer system is composed, from top to bottom, of the Biscayne aquifer, a semiconfining unit, the gray limestone aquifer, and the lower clastic unit of the Tamiami Formation. The intermediate confining unit consists of a 167to 243-m thick sequence of green clay, silt, limestone, and fine sand [Parker et al., 1955, p. 189]. These sediments have relatively low permeability and produce little water. The Floridan aquifer system is not of importance to our work because of its great depth (290 to 305 m) in Miami-Dade County [Miller, 1986]. In terms of ground-water supply, the most important unit is the Biscayne aquifer. The Biscayne aquifer contains high permeability limestone and calcareous sand units. Fish and Stewart [1991] require that there be at least a 3-m section of greater than 305 m/d horizontal permeability for these units to be considered part of the aquifer. The base of the Biscayne aquifer is defined as the depth where the subjacent sands and clayey sands fail to meet this permeability criterion. In the study area the Biscayne aquifer ranges from 0 to 30 m thick; its thickness increases toward the east. Below the Biscayne, a second aquifer composed of a gray limestone unit of the Tamiami Formation is found at depths of 21 to 49 m in western Miami-Dade County [Fish, 1988; Fish and Stewart, 1991]. While less permeable than the Biscayne aquifer, the gray limestone aquifer is