Groundwater

Groundwater 101

Did you know that groundwater accounts for less than 2 percent of the total water on Earth? However, 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 only on groundwater. Groundwater can provide a necessary backup to local water supply, but only if we protect it.

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

  • Groundwater is replenished, or recharged, by rain, snowmelt, stream water, leaky underground pipes, and irrigation water.
  • Groundwater 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.

Groundwater Impacts

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 Management

Historically, in California groundwater basins have not been officially managed. However, due to the greater demand for groundwater use as a result of the drought, the state has initiated laws requiring groundwater management. The state enacted the Sustainable Groundwater Management Act in 2014 which requires sustainable management of all medium and high priority groundwater basins. A basin’s priority depends on several factors including groundwater reliance, population and projected growth, number of public and private wells, irrigated area, and current groundwater impacts such as subsidence and overdraft.

127 of California’s 515 groundwater basins and subbasins are classified as medium or high priority, and those 127 basins and subbasins account for 96% of the state’s groundwater use. Although no basins or subbasins in San Mateo County have been classified as medium or high priority, in the future their classifications could change.

Sources: Department of Water Resources Groundwater Report 2016; Water in the West- Sustainable Groundwater;  Oregon State University Understanding Groundwater; Bulletin 118, Chapter 6; USGS Water Science School

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 to learn more.

 Additional Resources

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 Office of Sustainability and County Environmental Health Services are jointly working on 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 is funded by Measure K (formerly Measure A), a countywide half-cent general sales tax passed by voters, and the Office of Sustainability.

Work on the assessment commenced in April 2016; the assessment is scheduled to be completed by June 2018. The development of the groundwater basin assessment will be a collaborative process. The Office of Sustainability and Environmental Health Services are holding a series of workshops to solicit public and stakeholder feedback.

Stakeholder Outreach Resources

DateLocationMaterials
July 12, 2018Redwood Shore Public Library, Community Room
399 Marine Parkway, Redwood City
Agenda
Presentation
April 17, 2018Sobrato Community Conference Center Redwood Shores
Shoreway Conference Room
350 Twin Dolphin Drive, Redwood City
Agenda
Presentation
February 2018N/AEmail Update
November 9, 2017Jennings Pavilion, Holbrook-Palmer Park
150 Watkins Avenue, Atherton
Agenda
Presentation
August 17, 2017Sobrato Community Conference Center Redwood Shores
Shoreway Conference Room
350 Twin Dolphin Drive, Redwood City
Agenda
Presentation
Discussion Feedback
May 2017N/AEmail Update
January 31, 2017Redwood Shore Public Library, Community Room
399 Marine Parkway, Redwood City
Agenda
Presentation
December 6, 2016Emergency Operations Center
Belmont City Hall, 2nd Floor
#1 Twin Pines Lane, Belmont
Agenda
Presentation
Discussion Feedback
November 21, 2016Menlo Park's Arrillaga Family Recreation Center
700 Alma Street, Menlo Park
Agenda
Presentation
September 7, 2016Wind Room, Foster City Community Center
1000 E. Hillsdale Ave, Foster City
Agenda
Presentation
May 17, 2016Oak Room, San Mateo County Library
55 W. 3rd Ave, San Mateo
Agenda
Presentation
Discussion Feedback

Project Results and Data

Additional Resources

Groundwater Reports

Groundwater Reports from Neighboring Cities

For more information on the San Mateo Plain Subbasin assessment, please contact Charles Ice at cice@smcgov.org

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.

Half Moon Bay Terrace Basin

The Half Moon Bay Terrace Groundwater Basin is located along the northern San Mateo coast about 20 miles south of San Francisco and 90 miles southwest of the Sacramento Valley. For additional basin details please check out California Department of Water Resources Bulletin 118: Half Moon Bay Terrace Basin.

Additional Resources

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.

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.

Additional Resources

Pescadero Valley Basin

The Pescadero Valley Groundwater Basin is located in southern San Mateo County along the Pacific Ocean about 30 miles south of San Francisco. For additional basin details please visit California Department of Water Resources Bulletin 118: Pescadero Valley Basin.

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.

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.

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.