Use of D-C resistivity to map saline ground water

It has been estimated in previous studies that 23 square miles of the Oxnard aquifer, a member of a multi-layered aquifer system beneath the Oxnard plain in Ventura County, California, has been contaminated as a result of seawater intrusion. To investigate this and other potential sources of saline water, a direct-current resistivity survey was made as an alternative to the costly and time-consuming method of well drilling in the part of the Oxnard plain where ground water is believed to be most affected by seawater. Findings from this survey and water-quality data collected from wells as part of this study suggest that the extent of seawater intrusion is much less than reported. A field inventory of the current monitoring-well network utilized by managing agencies suggests that the integrity of most of the well casings is questionable. Leakage of saline water from an unconfined `perched zone' through these and other failed or corroded well casings is a possible source of increasing chloride concentration in the underlying Oxnard aquifer. Saline water also may be present in fine-grained deposits along the eastern limit of the Oxnard aquifer. Pumping near this area could induce the lateral migration of saline water from these deposits.