As surface water, such as rain, seeps into the ground, it passes between soil particles and collects in underground reservoirs called aquifers. An aquifer is made up of sand, gravel, and fractured rock that readily yields water to springs or wells.
Groundwater is an essential part of the state and national water supply. Eighty percent of Californians depend on it for all or part of their drinking water supply, and have been doing so safely for generations.
Our Groundwater Program includes two projects, currently in progress, plus a stewardship and management program:
Together, these projects will increase the reliability of our local and regional water supply, diversify our water supply portfolio and make us less vulnerable to disrupted service from drought and natural disasters such as earthquakes.
Our groundwater supply comes from the 40-square-mile Westside Basin, which contains a stratified aquifer system that extends from Golden Gate Park in San Francisco southward through Millbrae. The basin is a vital local resource for San Francisco and neighboring communities in San Mateo County.
In April, 2017, we began pumping groundwater from the Westside Groundwater Basin, an aquifer that extends to depths of approximately 300-600 feet below ground in San Francisco. The groundwater is treated and blended with our regional drinking water supplies before it is delivered to customers in San Francisco. Over the next few years we will continue adding groundwater in order to reach our goal of blending 4 million gallons a day (mgd) of treated groundwater with our regional water supplies.