GRA CENTRAL COAST BRANCH MEETING: Seawater Intrusion Control in Orange County – Do We Need Another Barrier?

July 24, 2019 @ 6:00 pm – 8:00 pm
Traveler Cafe
1070 E. Front Street Ventura
CA 93001

Roy Herndon Chief Hydrogeologist Orange County Water District


Since the early 1900s seawater intrusion has represented the greatest water quality threat to the Orange County groundwater basin, which supplies 75 percent of the water demands of 2.5 million people and businesses in north-central Orange County.

As manager of the groundwater basin, the Orange County Water District (OCWD) has employed various methods to combat this threat. In the 1950s and 1960s, OCWD purchased and recharged large volumes of Colorado River water to fill the basin and reduce the inland gradient from the ocean. Subsequently, seawater intrusion barriers consisting of a series of freshwater injection wells were constructed at the Alamitos Gap (Long Beach/Seal Beach) and Talbert Gap (Fountain Valley/Huntington Beach) in 1965 and 1976, respectively. The Alamitos and Talbert seawater barriers have generally been successful in protecting the basin from seawater intrusion; however, both barriers required substantial expansions in recent years after investigations indicated that seawater was beginning to migrate inland.

Folding, faulting, and erosional unconformities have challenged groundwater scientists to accurately interpret and model the complex groundwater flow regime along the coast. After initial hints of a problem several years earlier, the loss in 2012 of a city of Huntington Beach production well due to seawater intrusion caught OCWD by surprise, because it occurred in an area known as the Sunset Gap, where it was unexpected and virtually no monitoring or production wells existed.

Two geophysical surveys, seven multi-depth monitoring wells, and hundreds of hours of data analysis later, OCWD has a much better understanding of the likely sources and extent of the seawater intrusion in an area covering approximately four square miles beneath the Naval Weapons Station Seal Beach. It is clear that measures are needed to contain and prevent further movement of the intrusion. To that end, a groundwater model is being developed to help OCWD evaluate different extraction/injection well configurations. Much work needs to be done, but a third seawater barrier may someday become a reality to protect Orange County’s vital groundwater resource.

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