HYDROGEOLOGIC FRAMEWORK OF THE SANTA CLARA VALLEY, CALIFORNIA

The hydrogeologic framework of the Santa Clara Valley in northern California was redefined using data collected from recently constructed multiple-well monitoring sites. Correlation of geophysical logs indicates that there are six major aquifer layers in the valley. The aquifer layers are relatively flat lying and range from 10 to 200 feet in thickness. The aquifer layers are separated by thin, low-permeability, fine-grained layers that result in as much as 10 feet of vertical head differences between aquifer layers. The effective base of the ground-water flow system ranges from about 500 to 900 feet below land surface. Faults subdivide the aquifer system into three subbasins.

Depth-dependent chemical and isotopic data collected from the monitoring sites and long-screened production wells indicate that artificially recharged water is contained within the upper 500 feet of the aquifer system. Uncorrected carbon-14 data indicates that the ground water in the upper 500 feet generally is less than 2,000 years old, and ground water in the deeper aquifer layers generally ranges from 16,700 to 39,900 years old. Depth-dependent sampling indicates that wellbores are the main path for vertical flow between aquifer layers. Isotopic data indicate as much as 60 percent of water pumped from production wells originated as artificial recharge.

A ground-water flow model was developed using the new hydrogeologic data. The model demonstrates the importance of the aquifer layering, faults, and stream channels in relation to ground-water flow and infiltration of recharge. The faults partially restrict ground-water flow from recharge areas and affect land subsidence driven by ground-water pumpage. The shallow coarse-grained stream-channel deposits are the main conduits for natural and artificial recharge that occurs along the margins of the valley to reach the center of the valley. Model results indicate that wellbore flow in wells screened over multiple aquifers is the main component of vertical flow between aquifers and is the main conduit for recharge to the lower aquifers.