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Innovative research technique used to find a new source of water in the area offshore of San Diego County.

USGS San Diego Hydrogeology project study area
Map illustrating the USGS San Diego Hydrogeology project study area, which extends offshore of San Diego. Sedimentary deposits are shown in yellow; hard rock is shown in pink; streams and water bodies are shown in blue; USGS monitoring-well sites are indicated by red dots. (Click to enlarge.)

In response to decreasing water supply, San Diego County is looking to develop additional local sources of freshwater. One such source comes from a desalination facility that purifies brackish groundwater from the coastal San Diego Formation (SDF). An article by USGS Research Hydrologist Wes Danskin and others discusses their findings that some fresh-to-brackish water in the SDF resides offshore, which was previously unknown. The article focuses on finding a new source of water in the offshore area and methods used in the research.

Results of the study show that a considerable volume of fresh-to-brackish groundwater is sequestered in the offshore part of the SDF. The water is located in continuous and isolated pockets that appear influenced by fault systems and shallow stratigraphy.

A new technique used was to conduct surface-towed electromagnetic surveys that mapped the pore-fluid salinity, a critical identification factor. Because controlled-source electromagnetic methods (CSEMs) are sensitive to changes in pore fluids, such as the transition from fresh to brackish groundwater, they are uniquely suited to detect offshore groundwater. The use of this new technique in the geologically complex San Diego area suggests that CSEM will become an important tool for better understanding coastal water resources.

Read the article >

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