33-2 Three-dimensional geologic framework of the San Diego–Tijuana area, USA and Mexico

Session: Borderless Geoscience: Linking Baja and Southern California

Presenting Author:

Authors:

Abstract:

The San Diego area in southwestern California lacks adequate local water supplies and historically has imported as much as 90 percent of the demand from northern California or the Colorado River. Beginning in 2016, this need for imported water was reduced to about 75 percent of demand by construction of two local desalination facilities. The city of San Diego and surrounding municipalities are seeking to expand the use of groundwater. The first step to better understanding potential groundwater supply is gaining an improved understanding of subsurface geology of the regional aquifer. The regional aquifer includes parts of the United States (USA) and Mexico, encompassing the cities of San Diego and Tijuana in what is referred to as the San Diego–Tijuana area, USA and Mexico. Previous geologic studies focused on localized site investigations, individual rock formations, or earthquake hazards, but no comprehensive, quantitative geologic model had been developed for the entire area. This study presents the first quantitative, three-dimensional, geologic framework of the San Diego–Tijuana area. The framework can be used to evaluate subsurface geology and structure, fault systems, earthquake hazards, groundwater flow, seawater intrusion, and other hydrogeologic conditions. Subsurface geologic interpretations were made from downhole lithologic, geophysical, and paleontological information from U.S. Geological Survey multiple-depth monitoring-well sites, water supply wells, and oil exploration wells. Additional subsurface information were compiled from onshore gravity-derived depth-to-basement data and offshore seismic data. The surface and subsurface geometry and extent of nine simplified geologic units were interpolated using geologic modeling software. Groundwater historically has been extracted from three  of these units: the upper and lower members of the Plio-Pleistocene San Diego Formation, and the overlying sedimentary deposits. This study mapped the geometry and extent of these three units throughout the study area for the first time. This quantitative geologic framework supports and enhances existing geologic concepts, including the depositional and structural history of the San Diego–Tijuana area, and provides the foundation for development of a regional hydrologic model.

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.