A study of seawater intrusion using direct-current soundings in the southeastern part of the Oxnard Plain, California

On the basis of ground-water quality data, previous investigations have estimated that more than 60 square kilometers of the Oxnard aquifer have been intruded by seawater to distances reaching several kilometers inland from the shoreline of the Pacific Ocean. A direct current resistivity survey was made to investigate whether the increased salinity in the inland wells is caused by lateral invasion of seawater or by downward leakage of saline water from near surface formations through some wells. The resistivity survey, which consisted of 94 Schlumberger soundings, helped solve the problem by mapping the three dimensional geometry of subsurface materials having different resistivities down to depths of about 500 m. The results of the resistivity survey showed that over most of the study area, near-surface layers (down to a depth of about 10 m) often have very-low to low resistivities (<1 to 4.5 ohm-m); but at depths greater than 30 meters, where the Oxnard aquifer is found, the very-low resistivity layers (seawater-saturated layers) do not extend inland as far as previously suspected. The resistivity survey showed that it is possible to detect fresh-water aquifers beneath saline-water aquifers, and that the fresh-water aquifers in the study area mostly are characterized by resistivities in the 30 to 70 ohm-m range.