The general pattern seen on the depth-slice maps is a low resistivity zone (<10 ohm-m) near the coast which transitions into a high resistivity region (>50 ohm-m) in the landward direction. This transition is interpreted as being due to a change in formation water from saltwater to freshwater. Near Taylor Slough (see Feature 1 in Figure 4) the transition is fairly abrupt, occurring over a distance of 500 m. Moving parallel the transition, the distance landward of the transition does not vary greatly, i.e. the boundary has a smooth appearance. Taylor Slough in the vicinity of the freshwater-saltwater transition (FWSWT) has no streams flowing into Florida Bay; water flow is overland. In other regions, such as the western part of the ENP survey (Feature 2) or the Big Cypress survey (Feature 3), the FWSWT is wider and more irregular than in Taylor Slough. The many tidal rivers and streams that course deeply inland from the Gulf of Mexico strongly influence the location of the FWSWT because their presence lowers the hydraulic head. The result is that the FWSWT is much further landward in drained than undrained areas.

Variations of the FWSWT with depth can be seen by comparing the various depth-slice maps. In the Big Cypress coastal area the transition moves landward with depth as expected by the Ghyben-Herzberg model. (See the circles associated with Features 4a, 4b, and 4c that locate the transition.) This regular and expected behavior is not seen in all locations. In the eastern portion of the ENP survey (Feature 5) the 20-m depth slice is more conductive (evidenced by the darker orange color) than shallower or deeper depth slices. This behavior was seen in well logs in the area (Fish and Stewart, 1991) and is attributed to a very permeable zone at the base of the Biscayne aquifer where sea water is currently intruded. Below the Biscayne aquifer in the semiconfining unit the formation is more resistive due to lower porosity and relict seawater that has been diluted by mixing with surface water during a time of lower sea level.

Figure 4. Resistivity depth-slice map at 10 m. The color bar is also valid for Figures 5 and 6. Circled numbers designate features that are discussed in the text. [larger image]

Man-made features have influenced the FWSWT in the region. For example, where control structure S-18C blocks canal C-111 (Feature 6), fresh water backs up behind the control structure and infiltrates through the unlined canal walls. As a result the FWSWT is fixed at the control structure, but in the surrounding areas it is further seaward. This results in the cusp in the resistivity maps (Features 6a, 6b, and 6c).

Another interesting low resistivity zone (Feature 7) is seen running westward from the large bend in the old Ingraham Highway. This highway was built between 1915 and 1919 by digging a canal and using the removed material to form the roadway. Near Feature 7 a large cavern was discovered during excavation of the canal. The cavern was about 10 m across and over 10 m deep (Stewart et al., 2002). If the cavern communicated with the semiconfining unit below the Biscayne it could serve as a source of saline water and could produce the low resistivity zone along the canal. Another possible cause would be seawater from Florida Bay that flowed up the Ingraham Canal beyond this location. The canal was filled in 1951 from a point 10 km west of Feature 7 all the way to the coast, thereby stopping the infiltration of saltwater. It is hard to imagine that a saltwater signature associated with the canal still exists more than 50 years later.

Most of the study area is covered with water ranging in depths from 10 cm to 2 m. Roads are usually built on elevated areas that stand 1 to 2 m above the water. As a result, roads alter the flow of surface water and influence the location of recharge areas. This fact is reflected in the resistivity patterns, which often correspond to the location of roads. For example, in Big Cypress National Preserve, the area inside the Loop Road (Feature 8) is about twice as conductive on the 10-m depth slice map as the area outside the loop and to the south and west. This is due to the fact that overland water flow is diverted around this area by the raised roadway and canals. Another example is seen along a north-south section of the main park road (SR 9663) near UTM E 520 000 m, where the interpreted resistivity changes by a factor of four from the east to west side of the road because the roadbed blocks the westward flow of freshwater out of the Taylor Slough region. (This feature is difficult to see with the color scale chosen for this regional data display.)

Canals also affect the location of the FWSWT along the western end of the Tamiami Trail. Here the landward extent of saltwater intrusion is controlled by the canal alongside the road (right side of Feature 9). The canal aligned perpendicular to the coast (left side of Feature 9) brings saltwater further landward in much the same way as tidal rivers.

Geologic control also plays an important role in the patterns seen in the depth-slice maps. In Taylor Slough a resistive zone is seen extending to great depth (Feature 10). A great deal of water is pumped into Taylor Slough resulting in recharge of the aquifer. The location of the slough is probably controlled by channel eroded during a period of low sea level and later filled, possibly with lower porosity material than the surrounding formation making the zone resistive.

Parallel northwest trending linear features (Feature 11) are seen in the 10- and 20-m depth slices. The cause of these is uncertain, however, they strike in the same direction as a nearby rock reef that manifests itself as a slight rise in topography. This coincidence suggests that these linear boundaries are caused by structural features that overprint the hydrologic regime.