CONCEPTUAL HYDROGEOLOGIC FRAMEWORK OF THE CALIFORNIA COAST BASINS PRINCIPAL AQUIFER

The U.S. Geological Survey has undertaken the development of a National-Extent Hydrogeologic Framework (NEHF) model of the continental United States (CONUS) in support of nationally-consistent assessments of water supplies for human and ecological needs. The objective of the NEHF is to develop a nationally-consistent three-dimensional digital representation of the subsurface hydrogeology of CONUS. The NEHF is designed to be a living framework that is capable of being useful at multiple scales and for multiple purposes, and can be both a source of information for hydrogeologic investigations and a repository for updated information. The NEHF focuses, in part, on previously mapped principal aquifers (PAs) and secondary hydrogeologic regions. One of these PAs is the California Coastal Basins (CACB), which consists of multiple basins in proximity to the California coast. California’s largest population centers are in the CACB and groundwater from the PA is used for irrigation and public and domestic supplies. The CACB is also a source of oil and gas production. The CACB consists primarily of alluvial deposits, marine sediment, and volcanic rocks that occupy structural depressions caused by deformation of the Earth’s crust. CACB aquifer materials were evaluated with respect to spatial and stratigraphic relationships within individual basins, age, thickness, lithology or rock type, and previously defined hydrogeologic provinces. Data for this evaluation were derived from hydrogeologic framework models, groundwater-flow models, basin characterization reports, and a general understanding of the hydrogeology of the CACB. The aquifer materials were then split into five hydrostratigraphic layers, each representing different ages of deposition or emplacement. For each basin within the CACB, the hydrostratigraphic layers were attributed by lithology or rock type, thickness, and relative productivity as described in basin characterization reports. This conceptual approach allows for integration of this highly complex principal aquifer into the NEHF and provides a quantitative framework for use in future hydrogeologic investigations in California.