Groundwater

We protect, store, and utilize our local groundwater, which is a critical resource for our city. Groundwater is an essential part of our state and national water supply. In fact, 80 percent of Californians depend on groundwater for all or part of their drinking water, and this has been true for generations. 

Where Does Groundwater Come From?

While surface water supplies are visible in reservoirs, you can’t see groundwater. As rainfall and surface water slowly move downward beneath the ground surface, it collects deep underground in the spaces found between rocks, gravel, and sand. Geologic formations of rocks, gravel, and sand that are able to store water are called aquifers. Water stored in aquifers is called groundwater.

There are 7 small groundwater basins in San Francisco. Our local groundwater supply comes from the 45-square-mile Westside Basin, a series of aquifers extending from Golden Gate Park in San Francisco southward through San Bruno. In 2017, after collecting water data for about a decade, we began pumping groundwater from the Westside Groundwater Basin aquifer from depths of approximately 400 feet below the surface. We treat and blend this groundwater with our surface water supplies before it is delivered to our customers. Over the next few years, we plan to gradually increase pumping of our local groundwater in order to reach our goal of blending 4 million gallons a day of treated groundwater with our regional water supplies.

Our Groundwater Program utilizes groundwater found in our local aquifers to make our water supply more diverse and reliable, which makes us less vulnerable to disrupted service from drought and natural disasters such as earthquakes.

Monitoring and Data Management

The Westside Basin Groundwater Monitoring Program provides information summarizing basin-wide groundwater pumping, groundwater levels and quality in the different aquifers of the basin, and surface water conditions, most notably in Lake Merced. The program also monitors physical and chemical changes in the groundwater system that may result from current pumping at the San Francisco Zoo, and Golden Gate Park, the San Francisco Groundwater Supply Project wells, and Regional Groundwater Storage and Recovery Project wells. 

• Groundwater Fact Sheet
• Groundwater Blend Map
• Annual Groundwater Monitoring Report
• Groundwater Blend Report

Regional Groundwater Storage and Recovery Project