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Hydrogeologic Exploitation of a Raw Water Resource from an Unconfined Aquifer at Coast Guard Beach, San Nicolas Island, Offshore California

J. M. Saenz1, G. Gong1, K. Zaiger1, D. Vann1, and T. J. O'Neil2
1US Department of the Navy, Naval Facilities Engineering Service Center, EV 411, Port Hueneme, CA, [email protected], [email protected], [email protected], [email protected]
2Oxnard College, Oxnard, CA, [email protected]

The US Navy established an integrated technical assessment process to site seven new saltwater intake wells adjacent to a jetty on the shoreline at Coast Guard Beach, San Nicolas Island, offshore California. The three technical approaches used for the well siting process included a hydrogeologic survey, a geographic information system (GIS) shoreline time-series analysis, and an oceanographic assessment of coastal currents and sediment transport. Results integrated from these three different technical approaches were synthesized to accurately site and install seven new wells at Coast Guard Beach.

The detailed hydrogeologic survey identified a dynamic and coastal unconfined aquifer. This aquifer is composed of fine-grained beach sand with a clay-silt layer encountered at depth. The aquifer has direct hydraulic connection with the Pacific Ocean, and responds rapidly to tidal fluctuations. Survey results were supplemented with mathematical computations used for specifying well spacing and depth. GIS analysis of satellite images provided isolines representing shoreline changes over time. The oceanographic assessment and sediment transport analysis showed potential impacts on the shoreline by long shore currents and predicted future shoreward migration of the shore isoline. Analysis of erosion and deposition in the vicinity of the jetty allowed for accurate saltwater intake well siting and installation on this dynamic shoreline.

In December 2008, the existing saltwater intake well field was upgraded by installing seven new wells in addition to the three existing wells that provides raw saltwater supply to a Reverse Osmosis Unit (ROU). This upgrade increased the total raw water supply rate to 250 gpm. Using a unique well design, the wells were installed near the shoreline to about 25 feet below ground surface. Hydrogeologic materials provide a native filter for the raw seawater supply prior to transport to the ROU, where the seawater is converted into potable water to support the Island population.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009