Geoelectrical and Hydrogeological Modeling of the Fresh Water / Saline Water Interface in the Lower Florida Keys.

Abstract

Salt water intrusion is a critical challenge for the management of fresh water resources on small oceanic islands like those in the lower Florida Keys, which have extensive coastal development including construction of canals. We present the use of the Direct Current Resistivity (DCR) method for investigating the location of the fresh water, saline water (FW/SW) interface. A Super-Sting R1 28-electrode resistivity imaging system was used to investigate the subsurface geology and the FW/SW interface on Big Pine Key. the survey consisted 54m long profile in the center of the island and a 250m long roll along profile extending inland from the western shore. in the center of the island, resistivities range from 40 – 250 ohm-m near the surface and correspond to the freshwater lens which extends to a depth of 6 m below sea level. Below this depth, values of 20-30 ohm-m indicate brackish groundwater. On the western shore of the island, resistivities of 1-3 ohm-m corresponding to sea water are found at the surface and extend 25 m inland. Farther inland, resistivities increase to 50 ohm-m indicating a 2-3 m thick fresh water lens. the FW/SW interface slopes landward and 250 m inland from the shore, brackish resistivity of ~20 ohm-m extend to 4 m below sea level and saline found at 8 m depth. Observed geoelectrical data were compared with a 2-D salinity profile simulated from the USGS SEAWAT model .Analysis of the data show that the largest depth to FW/SW interface occurs at 8m depth in the middle of the island with a gradual decrease in the FW towards the saline zone on the shore of the island and this is generally consistent with the geophysics. Ultimately, both geophysical methods and hydrogeological model Tools like SEAWAT can predict island sea water intrusion with reasonable accuracy.