Gaining the necessary geologic, hydrologic, and geochemical understanding for additional brackish groundwater development, coastal San Diego, California, USA

Local water agencies and the United States Geological Survey are using a
combination of techniques to better understand the scant freshwater resources and the much
more abundant brackish resources in coastal San Diego, California, USA. Techniques include
installation of multiple-depth monitoring well sites; geologic and paleontological analysis of
drill cuttings; geophysical logging to identify formations and possible seawater intrusion;
sampling of pore-water obtained from cores; analysis of chemical constituents including trace
elements and isotopes; and use of scoping models including a three-dimensional geologic
framework model, rainfall-runoff model, regional groundwater flow model, and coastal
density-dependent groundwater flow model. Results show that most fresh groundwater was
recharged during the last glacial period and that the coastal aquifer has had recurring
intrusions of fresh and saline water. These intrusions disguise the source, flowpaths, and
history of ground water near the coast. The flow system includes a freshwater lens resting on
brackish water; a 100-meter-thick flowtube of freshwater discharging under brackish
estuarine water and above highly saline water; and broad areas of fine-grained coastal
sediment filled with fairly uniform brackish water. Stable isotopes of hydrogen and oxygen
indicate the recharged water flows through many kilometers of fractured crystalline rock
before entering the narrow coastal aquifer.