This report describes a helicopter electromagnetic survey flown over the Model Land Area in southeastern Miami-Dade County, Florida, to map saltwater intrusion in the Biscayne aquifer. The survey, which is located south and east of Florida City, Florida, covers an area of 115 square kilometers with a flight-line spacing of 400 meters. A five-frequency, horizontal, coplanar bird with frequencies ranging from 400 to 100,000 Hertz was used. The data were interpreted using differential resistivity analysis and inversion to produce cross sections and resistivity depth-slice maps. The depth of investigation is as deep as 100 meters in freshwater-saturated portions of the Biscayne aquifer and the depth diminishes to about 50 meters in areas that are intruded by saltwater. The results compare favorably with ground-based, time-domain electromagnetic soundings and induction logs from observation wells in the area. The base of a high-resistivity, freshwater-saturated zone mapped in the northern 2 kilometers of the survey area corresponds quite well with the base of the surficial aquifer that has been determined by drilling. In general, saltwater in the survey area extends 9 to 12 kilometers inland from the coast; however, there is a long nose of saltwater centered along the Card Sound Road Canal that extends 15 kilometers inland. The cause of this preferential intrusion is likely due to uncontrolled surface flow along the canal and subsequent leakage of saltwater into the aquifer. Saltwater also extends farther inland in the area between U.S. Highway 1 and Card Sound Road than it does to the west of this area. Until 1944, a railroad grade occupied the current location of U.S. Highway 1. Borrow ditches associated with the railroad grade connected to Barnes Sound and allowed saltwater to flow during droughts and storm surges to within a few kilometers of Florida City. Relicts of this saltwater that settled to the bottom of the Biscayne aquifer can be seen in the helicopter electromagnetic data. The area to the west of U.S. Highway 1 is more resistive in the upper 10 meters than the area to the east of the road; this reflects the influence of surface-water flows that are blocked by U.S. Highway 1. Between Card Sound Road and U.S. Highway 1, resistivities are slightly lower compared to adjacent areas. In the southern portion of the survey area, the surficial aquifer underlying the Biscayne aquifer is more resistive; this indicates that it contains fresher water than that found at the base of the Biscayne aquifer.
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