USING RESISTIVITY TO DETECT MOVEMENTS OF VARIABLE SALINITY FLUIDS IN THE BARRIER ISLAND SEDIMENTS OF PADRE ISLAND, TEXAS

The barrier island sediments of Padre Island, Texas represent a unique opportunity to use electrical resistivity methods to detect the locations and movements of variable salinity fluids in the subsurface. Mineralogical and sieve analyses of Padre Island sediments show a uniform mineralogy and grain size distribution at all sampled localities. Because of the uniformity of sediments within the study area, variations in electrical resistivity values are most likely the result of variations in groundwater salinities. This assertion is supported by fluid salinity and conductivity data from the study area. Groundwater salinities range from approximately one-half to twice that of normal seawater. We present a time-series of images of variable salinity fluids in the barrier island sediments of Padre Island, Texas produced using electrical resistivity methods. These resistivity images indicate a salinity inversion exists in the sediments beneath an evaporative basin near the Bird Island Basin road in the Padre Island National Seashore. The interpretation of a salinity inversion is supported by observations of fluid salinities and a vertical conductivity profile from within the evaporative basin. These images also suggest that salinity-driven free convection, caused by the evaporative concentration of near-surface groundwater, may be occurring in the sediments of Padre Island over a relatively short (<1 year) time interval. A simple application of the Ghyben-Herzberg principle predicts a lens of lower salinity groundwater beneath the barrier island. We present additional electrical resistivity images that strongly suggest the predicted groundwater lens is either absent or not in accordance with the predictions of the Ghyben-Herzberg principle.