Helicopter electromagnetic (HEM), transient electromagnetic (TEM), and borehole measurements have been used to develop a detailed three-dimensional picture of the extent of saltwater intrusion in Everglades National Park (ENP). These results are of value in assessing the effect of past water management practices on the Everglades. The influence of manmade structures on surfacewater and ground-water flows is seen in detail. These data serve as a baseline for mapping changes in saltwater intrusion that result from restoration activities.

Helicopter electromagnetic surveys and transient electromagnetic soundings have been used to obtain formation resistivity throughout the study area. The HEM surveys provide closely spaced samples (10 m along flight line with 400-m flight line spacing). The TEM soundings provide greater depth of exploration and better model resolution than the HEM data; however, the sampling interval is much coarser (one sounding per 25 km²). The HEM data are interpreted as layered earth models at each measurement point and displayed as formation resistivity at selected depths. The TEM data are also interpreted as layered earth models.

The geophysical models can be used to estimate water quality. Water quality can be evaluated in many ways, ranging from visual observations of clarity to detailed chemical analyses of surface and well samples. Electrical conductivity lies in the middle of this analysis spectrum, providing a rapid technique when the primary concern is the concentration of dissolved ions. Electrical conductivity is a superb indicator of whether seawater is present in the surficial aquifer under the Everglades because of the large difference in chloride ion concentration between the fresh, surface water in the Everglades (40 mg/L) and the saltwater of Florida Bay, the Gulf of Mexico, and Biscayne Bay (15,000-35,000 mg/L). Geologic formations saturated with these two different fluids have formation resistivities that differ by factors of 50 to 100. Through the analysis of borehole induction logs from wells in and near ENP, the relation between measured formation resistivity and specific conductance (SC) of water samples has been established, allowing SC to be estimated from interpreted formation resistivities. An empirical relation between SC and salinity, which is valid for surface water in Dade County, is then used to estimate salinity.

The HEM data show a clear transition from freshwater to saltwater saturated regimes. This transition occurs from 8 to 20 km inland from the coast. The nature of this transition changes if tidal rivers are present. For example, to the west of Taylor Slough, where many tidal rivers are found, the landward extent of the transition is near the terminus of rivers, resulting in a jagged boundary. In Taylor Slough and eastward across the region draining toward Florida Bay and Barnes Sound, the interface is smooth because of large volumes of overland water flow and the absence of tidal drainages cutting deeply into the area. The lack of significant drainages to the east of Taylor Slough is due to the bedrock ridge that parallels the coast and forms a barrier to large stream formation. In addition to the freshwater/saltwater interface, the HEM data show other significant features: (1) a deep resistive zone in the middle of Taylor Slough where freshwater flow recharges the underlying aquifer, (2) variations in resistivity near raised roadways reflecting their influence on surface-water flow and aquifer recharge, (3) freshwater zones associated with infiltration from canals due to control structures and flow through cuts in the canals, and (4) historic saline water transport along a canal, formerly open to Florida Bay, adjacent to old Ingraham Highway.

The TEM soundings also locate the transition from freshwater to saltwater saturated zones. Interpreted formation resistivities from the TEM data fall into two clusters: 1) a freshwater saturated zone with resistivities in the range of 18 to 300 ohm-m, and 2) a saltwater saturated zone with resistivities of 2 to 7 ohm-m. On the basis of these resistivity values, the location of the freshwater/saltwater interface is mapped. The result agrees well with the HEM results but lacks the spatial detail of the HEM data because of the much coarser sampling interval.

An issue of great concern to ecosystem restoration is whether freshwater is flowing into Florida Bay. Although surface-water flows can be determined from measurements of streams leading to the bay, ground-water flows are more difficult to evaluate. Our HEM and TEM data sets can shed some light on possible ground-water flows. In the region from Florida Bay landward for distances of 5 to 10 km the HEM resistivity depth-slice maps show a uniformly low resistivity (1-2 ohm-m) zone from the surface down to a depth of at least 24 m. The base of the Biscayne aquifer, as mapped by drilling, is less than 24 m deep at locations where geophysical data are available. The low observed resistivities are indicative of saltwater saturation of the Biscayne aquifer. (Freshwater saturated zones would have resistivities of 30 ohm-m or more.) Similarly, the TEM results do not show the presence of a high resistivity zone in the Biscayne. While the geophysical models do not indicate the presence of a freshwater zone, thin resistive zones (1-2 m thick), that are not detectable and do not degrade the fit of the model to the data, could be embedded in the models. The likelihood of such zones existing over extended distances and being isolated hydrologically from the surrounding saltwater intruded aquifer is not considered to be of significance. Therefore, we conclude that there is no evidence of fresh ground water flowing to Florida Bay from the surficial aquifer.

The results of this project are of immediate value to managers who are responsible for restoration decisions because the results provide a detailed picture of saltwater intrusion and the effects of human activity on the hydrologic regime. Furthermore, this work provides a baseline for long-term monitoring of changes in the ground-water regime. Future geophysical surveys can be used to look for changes in subsurface conditions associated with planned modification of water deliveries to Everglades National Park. Airborne electromagnetic surveys are currently the only way to obtain rapidly a detailed snapshot of saltwater intrusion over this large, inaccessible area.

Through the course of this study, there has been ongoing collaboration with personnel from Everglades National Park, the South Florida Water Management District, the University of Miami, the U.S. Army Corps of Engineers, and the Florida Geological Survey. Our data have been used by these organizations to understand various aspects of the regional hydrology, to locate wells, and to plan hydrologic sampling activities.

(This abstract was taken from the Proceedings of the South Florida Restoration Science Forum Open File Report)