THE WESTSIDE GROUNDWATER BASIN AND ITS HYDROSTRATIGRAPHIC UNITS, SAN FRANCISCO AND SAN MATEO COUNTIES, CALIFORNIA

The Westside Groundwater Basin is a coastal aquifer system located on the San Francisco Peninsula between Golden Gate Park and Burlingame. Since the beginning of the 20th century groundwater from the Basin has been used for drinking water and irrigation purposes. Unfortunately, the Basin wide potentiometric surface has gradually declined; consequently, a better understanding of the hydrostratigraphy of the basin is needed. Previous consulting investigations and studies conducted by the US Geological Survey have looked at groundwater movement within the Basin assuming horizontal layering of the hydrostratigraphic units. These studies did not take into consideration the tectonic deformation and intense folding that has occurred in the Basin along the San Andreas and Serra faults. In this study, we delineated the hydrostratigraphic units in the Westside Basin by using lithologic well logs, water quality, isotopic, and water level data. The upper part of the Merced Formation (sequences P through Z of Clifton and Hunter (1991, 1999)) contains the major hydrostratigraphic units where groundwater is extracted. Eleven hydrostratigraphic units are proposed for the upper part of the basin. These include two thick clay layers, observed in well logs and identified in cross sections that were tentatively correlated with sequences W and S2.