Skip to main content

Using Translation

The County of San Mateo has added a translation feature developed by Google Translate to assist web visitors in understanding information on this website in a variety of foreign languages. Please be aware that Google Translate, a free third party service which the County does not control, provides automated computer translations that may not give you an exact translation. The County cannot guarantee the accuracy of translations through Google Translate so translations should not be considered exact and only used as a rough guide. Anyone relying on information obtained from Google Translate does so at his or her own risk. The County disclaims and will not accept any liability for damages or losses of any kind caused by the use of the Google Translate feature.

groundwater in well
GROUNDWATER
Conserving our water resources.

Groundwater

Did you know that groundwater accounts for less than 2 percent of the total water on Earth? Even so, groundwater is a crucial resource: in California, it provides about 40 percent of our total water. In years of drought, that number jumps up to 60 percent. Some communities rely on groundwater as their only source of water. Groundwater can provide a necessary backup to local water supply, but only if we protect it.

Groundwater 101

Groundwater fills spaces in soil and cracks in rocks, and the amount of groundwater is constantly changing:

  • It is replenished, or recharged, by rain, snowmelt, stream water, leaky underground pipes, and irrigation water.
  • It is depleted through natural processes like flow to rivers, lakes, and oceans, evaporation and transpiration, and by human use.

Groundwater does not follow the boundaries of cities, counties, or states. Instead, we think about groundwater in terms of basins and subbasins. A groundwater basin is an area made up of at least one aquifer (area that contains groundwater). And a groundwater subbasin is a portion of a groundwater basin, oftentimes created for ease of management and studying.

Impacts to Groundwater

The recent drought in California has caused a greater reliance on groundwater for drinking water and irrigation. However, major problems arise when we use groundwater faster than it is recharged, called groundwater overdraft. In 2016, there were 21 basins in the state that were experiencing overdraft.

Groundwater overdraft causes many problems, including:

  • The need to drill existing wells deeper or drill new wells to access deeper aquifers
  • Land subsidence, or sinking of the land surface due to the compaction of clays
  • Salt water intrusion into coastal groundwater basins
  • Groundwater pollution

Groundwater in San Mateo County

The County of San Mateo is dedicated to protecting the quality and quantity of groundwater resources within its borders.

There are nine groundwater basins with boundaries—either partial or whole—within San Mateo County. Click on each basin region on the map below to learn more.

San Mateo Plain Subbasin

The San Mateo Plain Subbasin underlies the bayside of San Mateo County from approximately the City of San Mateo on the north, to approximately the County boundary at San Francisquito Creek on the south. Currently, there is no entity actively managing the basin.

The Sustainability Department and the Environmental Health Services division of San Mateo County Health jointly completed a groundwater basin assessment of the San Mateo Plain Subbasin to assess the groundwater resources and current condition of the subbasin; and identify potential groundwater management strategies. The project was funded by Measure K (formerly Measure A), a countywide half-cent general sales tax passed by voters, and the Sustainability Department.

Work on the assessment commenced in April 2016; the assessment was completed in August 2018. The development of the groundwater basin assessment was a collaborative process. The Sustainability Department and Environmental Health Services held a series of workshops to solicit public and stakeholder feedback.

Project Report and Presentations

Project Data

Stakeholder Outreach Resources

DateLocationMaterials
July 12, 2018

Redwood Shore Public Library, Community Room
399 Marine Parkway, Redwood City

April 17, 2018

Sobrato Community Conference Center Redwood Shores
Shoreway Conference Room
350 Twin Dolphin Drive, Redwood City

February 2018
N/A

November 9, 2017

Jennings Pavilion, Holbrook-Palmer Park
150 Watkins Avenue, Atherton

August 17, 2017

Sobrato Community Conference Center Redwood Shores
Shoreway Conference Room
350 Twin Dolphin Drive, Redwood City

May 2017

N/A

January 31, 2017

Redwood Shore Public Library, Community Room
399 Marine Parkway, Redwood City

December 6, 2016

Emergency Operations Center
Belmont City Hall, 2nd Floor
#1 Twin Pines Lane, Belmont

November 21, 2016

Menlo Park’s Arrillaga Family Recreation Center
700 Alma Street, Menlo Park

September 7, 2016

Wind Room, Foster City Community Center
1000 E. Hillsdale Ave, Foster City

May 17, 2016

Oak Room, San Mateo County Library
55 W. 3rd Ave, San Mateo

Visitacion Valley Basin

The Visitacion Valley Groundwater Basin is a roughly triangular shaped basin in the San Francisco Bay Hydrologic Region within the City of San Francisco. For additional basin details please check out California Department of Water Resources Bulletin 118: Visitacion Valley Basin (PDF).

Islais Valley Basin

The Islais Valley Groundwater Basin, within the city of San Francisco, is in the San Francisco Bay Hydrologic Region. For additional basin details please check out California Department of Water Resources Bulletin 118: Islais Valley Basin (PDF).

Westside Basin

The Westside Basin underlies both northern San Mateo County as well as the City and County of San Francisco. The basin is jointly managed by City of Daly City, City of San Bruno, City of Burlingame, Town of Colma, City of South Francisco, City of Millbrae, California Water Services Company, and San Francisco Public Utilities Commission.

The managing entities are currently working on a joint Regional Groundwater Storage & Recover (GSR) project that includes the construction of up to 16 new recovery wells in northern San Mateo County and facilities consisting of chemical treatment equipment, tanks, pumping systems, and associated pipelines. As part of the GSR project, surface water will be used instead of groundwater in wet years, allowing the groundwater to recharge through rainfall and decreased pumping. This will create a savings account of up to 20 billion gallons of groundwater that will be stored in the basin.

For additional basin details please check out California Department of Water Resources Bulletin 118: Westside Basin (PDF).

Additional Resources

San Gregorio Valley Basin

The San Gregorio Valley Groundwater Basin is located in southern San Mateo County along the Pacific Ocean about 25 miles south of San Francisco. For additional basin details please visit California Department of Water Resources Bulletin 118: San Gregorio Valley Basin (PDF).

Additional Resources

San Pedro Valley Basin

The San Pedro Groundwater Basin is located in northern San Mateo County along the Pacific Ocean. The basin is about 10 miles south of San Francisco, and 80 miles southeast of the Sacramento Valley. For additional basin details please visit California Department of Water Resources Bulletin 118: San Pedro Valley Basin (PDF).

Año Nuevo Area Basin

The Año Nuevo Area Groundwater Basin is located in southern San Mateo County along the Pacific Ocean fifty-five miles south of San Francisco. For additional basin details please visit California Department of Water Resources Bulletin 118: Año Nuevo Area Basin (PDF).

Frequently Asked Questions

The Latest News & Events

Fixit Clinic @ John Daly Library
Jun
7

Fixit Clinics are fun community-based workshops where neighbors, friends, and families work collectively to learn how to repair broken items.

Keep All Plastic Out of the Green Bin Everyone in San Mateo County is required to compost all food scraps and yard…
Getting food to people who need it while also reducing food waste requires many hands. As one food recovery non-profit…