Over the past six years a series of geophysical investigations have been carried out to obtain information needed to construct ground-water models of the southern portions of Everglades National Park and adjacent areas (fig. 1). Because of the inaccessible nature of the region and the extreme difficulty of drilling except on established roads,

Figure 1. Location showing the HEM survey, TEM soundings, and observations wells in and near Everglades National Park used in this study. Click for larger image.

geophysical measurements from the air, and on the ground, are the only way of obtaining information on geologic and hydrologic boundaries needed for model development. Hydrologic models are a very important tool for resource management and ecosystem restoration planning. Inadequate or insufficient data with which to construct these models reduces their reliability. Geophysical data have provided information on three factors critical to ground-water model construction: (1) the extent of saltwater intrusion in the surficial aquifer; (2) the depth to the base of the Biscayne aquifer; and (3) evidence refuting the existence of fresh ground-water flows to Florida Bay.

Helicopter electromagnetic (HEM) surveys and transient electromagnetic (TEM) soundings have been used to estimate formation resistivity throughout the study area. Formation resistivity is influenced by the resistivity of pore water in the formation and the formation porosity. If the formation porosity and geology vary gradually, then the formation resistivity is a direct measure of pore-water quality. Borehole measurements of formation resistivity and pore-water specific conductance (SC) are used to establish a relationship between these parameters, allowing SC to be estimated from the formation resistivity derived from the HEM and TEM data. Finally, an empirical relationship between SC and salinity, which has been established for the region, is used to estimate aquifer salinity. The result is a three-dimensional estimate of water quality.

The HEM surveys provide closely spaced samples (10 m along flight line with 400 m flight line spacing). The HEM data are interpreted as layered earth models at each measurement point and displayed as formation-resistivity maps at selected depths. 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 TEM data are also interpreted as layered earth models. Comparison of the results of these methods show good agreement.

The HEM data show a clear transition from freshwater to saltwater saturated regimes, which occurs from 8 to 20 km inland from the coast (fig. 2).

The presence of tidal rivers, as found to the west of Taylor Slough, results in a jagged transition boundary whose landward extent corresponds to the terminus of the rivers. In Taylor Slough and eastward across the region draining toward Florida Bay and Barnes Sound, the interface is smooth because of the absence of tidal drainages 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. Other features visible in the HEM data include: (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. TEM interpreted formation resistivities fall into two groups: (1) a freshwater saturated zone with resistivities in the range of 18-300 ohm-m, and (2) a

Figure 3. Location of the FWSWI in Everglades National Park based on well, TEM, and HEM data. The HEM data is from the 10-m depth slice. The color bar shows the estimated chloride concentration. Click for larger image.

saltwater saturated zone with resistivities of 2-7 ohm-m. On this basis the location of the freshwater/saltwater interface is mapped (fig. 3). The result agrees well with the HEM results, but lacks the spatial detail of the HEM data because of the much coarser sampling interval.

The geophysical data also provide some insight on the issue of whether fresh ground-water is flowing to Florida Bay. If flows are present, we would expect to see a high resistivity zone leading to Florida Bay. Five to ten kilometers landward of Florida Bay 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. 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, groundwater 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 they provide information about the impact of natural and human activities on saltwater intrusion and the hydrologic regime. This work also 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.

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(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report (PDF, 8.7 MB))

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