Integrating Surface and Borehole Geophysics in the Characterization of Salinity in a Coastal Aquifer

A combination of geologic descriptions of cores, borehole geophysics, surfacegeophysical soundings and water-sample analyses was used to characterize flow in the surficial aquifer in the vicinity of Big Cypress National Preserve, Collier County, south Florida. The geophysical analysis augmented by aquifer tests and piezometer measurements showed that the surficial aquifer is under unconfined conditions, is in hydraulic connection with surface water bodies, and is underlain by a series of confining units and confined aquifers. Sea-water intrusion appears as a simple wedge of saline water underneath the coastal region. The pattern of salinity in the underlying aquifers is variable and shows no relation to distance from the coast or from the surficial wedge of sea-water intrusion. Borehole flow-log data document the existence of a strong vertical hydraulic-head gradient throughout the study area. The subsurface distribution of salinity in the study area apparently is determined by variations in the amount of seepage of saline water from underlying aquifers and not by lateral sea-water intrusion except near the wedge of sea water imaged by the surface geophysics.