Groundwater

Santa Clara County’s groundwater basins are an important but unseen part of our water supply picture. In fact, these basins hold more water than all 10 of our surface reservoirs combined.

As the groundwater management agency for the Santa Clara County, the district works to protect and augment groundwater supplies. The 2016 Groundwater Management Plan, which was adopted by the district Board of Directors in November 2016, describes the district's groundwater sustainability goals and the strategies, programs, and activities that support those goals.

• Download the 2016 Groundwater Management Plan (PDF 45 MB)

Importance of groundwater
Groundwater is water that flows beneath the surface through small pores and cracks in the rock and soil. Throughout the world, the majority of available fresh water is in the form of groundwater. In Santa Clara County, nearly half of all water used comes from groundwater. The county's groundwater basins have vast storage capacity, estimated to be two times the capacity of all the district's 10 surface reservoirs combined.

Ensuring a reliable supply
Groundwater basins are naturally replenished by rainfall, water percolating through stream beds, and other sources. As natural recharge is not sufficient to replenish the amount of groundwater pumped, the district releases local and imported surface water through recharge facilities, which include streams and recharge ponds. This coordinated use of groundwater and surface water is critical for ensuring a reliable water supply, storing water for use during droughts and shortages, and preventing land subsidence, which is very costly to the community.

Ensuring a safe supply
Numerous sources can pollute groundwater, making it costly to treat or even unusable.  Since the restoration of contaminated groundwater can take years, decades, or longer, the district works to protect the groundwater basins from contamination and the threat of contamination. The district's groundwater protection programs allow us to assess regional groundwater quality, identify and evaluate threats, and help prevent or mitigate contamination.     

Groundwater Subbasins
Within Santa Clara County, the district manages two groundwater subbasins that transmit, filter, and store water: the Santa Clara and the Llagas Subbasins. These subbasins cover approximately 325 square miles and are bordered by the Santa Cruz Mountains to the west and the Diablo Range to the east. The aquifers that comprise the subbasins are made up gravel, sand, and silty sand deposits. In the Santa Clara and Llagas Subbasins, aquifers extend to depths of over 1000 feet in places. The Coyote Valley region of the Santa Clara Subbasin is fairly shallow, extending to a maximum depth of approximately 500 feet.

Both the Santa Clara and Llagas Subbasins contain confined zones and recharge areas. In the confined zones, lower permeability clay and silt deposits restrict the downward flow of groundwater and separate shallow and deep aquifer zones. These low permeability deposits also provide some natural protection to deeper aquifers as they restrict the movement of contaminants. Groundwater in the Santa Clara Subbasin generally flows to the northwest toward San Francsico Bay while groundwater in the Llagas Subbasin generally flows to the southeast toward San Benito County.