Use of electrical and electromagnetic techniques to map seawater intrusionnear the Cross-Florida Barge Canal

A sea-level canal, originally dredged to form the westernmost portion of th e proposed Cross-Florida Barge Canal, extends approximately 13 km inland fr om the Gulf coast of Florida. Previous regional geophysical studies and wat er quality data showed high ground conductivities associated with seawater intrusion (increased chloride concentrations) and other processes near the canal. New electromagnetic and resistivity surveys demonstrate the shallow high-conductivity zone associated with seawater mixing extends 150-200 m pe rpendicular to the canal along the westernmost 7 km of the canal. Ground co nductivities show no significant deviations from background values along th e eastern 6 km of the canal. Values measured within 100 m of the canal show a general decrease from >100 mS/m near the Gulf coast to a background valu e of similar to 10 mS/m 7 km inland. Superimposed on this general trend are considerable local variations which probably reflect spatial variability i n the fracturing of carbonate rocks and the impact of gravel mining operati ons. Comparison of vertical electrical soundings made in 1994 and in 1986 ( Hagemeyer and Stewart, 1991) suggests that the saline water zone around the canal was stable during this period. Numerical models simulating resistivi ty surveys near such a high-conductivity zone indicate that one-dimensional interpretation of resistivity soundings yields only slight overestimates o f the width of the high-conductivity zone.