The Bernal Subbasin is an aquifer located in the southwestern corner of Livermore Valley Groundwater Basin, Alameda County, California, USA. All of the groundwater in Livermore Valley moves toward the Bernal Subbasin, which is bounded on the east by Pleasanton Fault, on the north by the Park Fault, and on the west by the Calaveras Fault.(Earth Metrics, 1989) All the streams draining the Livermore Valley merge above the Bernal formation and exit this subbasin and Livermore Amador Valley via the Arroyo de la Laguna.
Groundwater occurs throughout the valley floor portion of the Bernal Subbasin under conditions ranging from unconfined to confined. There is no inflow of groundwater across the Pleasanton Fault south of the City of Pleasanton, because any movement of groundwater in the southern section is essentially parallel to the fault.(Bonnie, 2000)
Groundwater in the Bernal Subbasin typically occurs occurs at an approximate depths of 12 to 30 feet (3.7 to 9.1 meters) below ground surface and flows from east to west.