Sunset Gap Geophysical Survey for Orange County Water District

Client: Orange County Water District

Location: Fountain Valley, CA

Summary: In 2010, ENTRIX conducted a geophysical investigation to map salt water intrusion for the Orange County Water District (OCWD) across the Sunset Gap. Sunset Gap is a coastal region of the groundwater basin where salt water intrusion of the aquifer was believed to be a problem. The area lies beneath the Seal Beach Naval Base, which placed limitations on site access and restricted drilling locations. ENTRIX designed a geophysical investigation to map the position of brackish to salty water in the four major aquifers and provide OCWD with three-dimensional information on the distribution of saline water, the integrity of the confining units between the aquifers, and zones of faulting that may breach the confining units and allow migration of salty water between aquifers.

The project included electrical resistivity and Time Domain Electromagnetic induction (TEM) surveys on the Naval Base which consisted of approximately 7 miles of electrical resistivity data acquisition along four profile lines and 10 TEM soundings along two profile lines. The survey mapped brackish and saline water plumes within the layered aquifer system to depths of approximately 1,200 feet. The results of the survey will be used to evaluate the extent of sea water intrusion and evaluate mitigation options.