FIRST 3D LITHOLOGIC MODEL OF THE SAN DIEGO COASTAL AQUIFER, SOUTHERN CALIFORNIA, USA

A three-dimensional (3D) lithologic model of the San Diego Coastal Aquifer was created as part of the United States Geological Survey (USGS) hydrogeologic study of the San Diego–Tijuana area, USA and Mexico. The coastal aquifer provides a modest groundwater resource for the San Diego metropolitan area, primarily from the Plio-Pleistocene San Diego Formation and the overlying Quaternary deposits. Detailed mapping of subsurface lithology of the aquifer provides a greater understanding of the hydrogeologic system and will aid in future groundwater management. Lithologic data from 13 wells, including two deep USGS monitoring-well sites, were used to construct the 3D lithologic model in a 20 km² area south of downtown San Diego. Downhole lithologic descriptions from drillers’ logs were catalogued in a database and verified with borehole sediment cuttings and geophysical logs from the USGS well sites. Two lithologic classification schemes were used to construct the model: a simple scheme using only primary lithologic descriptors (e.g., clay, sand, gravel), and a more detailed scheme using primary and secondary descriptors (e.g, sandy clay, gravelly sand). Preliminary results indicate that lithologic trends are controlled by local geomorphology and regional changes in sea level and tectonics. A 1-km-wide lens of sand and gravel is interpreted to be the paleo-footprint of the modern day Sweetwater River. Increasing thickness of lithologic packages from east to west is consistent with down-drop of the structural San Diego Basin. Sediment deposition during different transgressive and regressive sea-level cycles may have influenced vertical and horizontal variations in lithology, such as a series of lithologic layers that appear to inhibit vertical groundwater flow, as indicated by hydraulic-head differences at the USGS well sites. Evaluation of depositional patterns also may identify sedimentary layers that are preferential conduits for groundwater flow; such highly permeable layers could be targets for future groundwater extraction but also could serve as pathways for seawater intrusion.