Groundwater

Groundwater 101

Look down at your feet. Now imagine you suddenly are gifted with X-ray vision and can see through the ground. Keep looking down, down, down… and there you will find groundwater. Groundwater is water under the ground that fills spaces in soil and cracks in rocks. Some other key definitions:

  • An aquifer is an underground area that contains or could contain a large amount of groundwater in those spaces.
  • A groundwater basin is made up of one or a series of stacked aquifers.
  • A groundwater subbasin is a subset of a groundwater basin, oftentimes created for ease of management and studying.

Just like rain or snow, groundwater is part of the hydrologic cycle, and the amount of groundwater is constantly changing. Groundwater is replenished, or recharged, by rain, snowmelt, stream water, leaky underground pipes, irrigation water, and in some cases through managed recharge. Groundwater is depleted through natural processes like flow to rivers, lakes, and oceans, evaporation and transpiration, and by human use. Groundwater accounts for less than 2 percent of the total water on Earth.

The Importance of Groundwater

Out of sight does not mean out of mind. Groundwater is a crucial resource, providing half the population of the United States with drinking water. In California, groundwater provides approximately 40 percent of our total water on average. In years of drought when there is less water in our reservoirs, we rely on groundwater sources to supply up to 60 percent or more of our total water. Some communities rely solely on groundwater. In times of drought or other extreme weather events, groundwater can provide a necessary backup to local water supply, but it will only be available to us if we protect it.

Groundwater Impacts

The recent drought in California has caused a greater reliance on groundwater for drinking water and irrigation. However, groundwater is a limited resource. Major problems arise when groundwater basins are pumped out faster than they are recharged, which is called groundwater overdraft. In 2016, the California Department of Water Resources identified 21 basins that were experiencing critical groundwater overdraft.

Groundwater overdraft leads to a decline in the level of groundwater in a basin. 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 of California 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. Three subbasins in the San Francisco Bay Area are classified as medium priority. Local agencies and stakeholders in those areas are required to form Groundwater Sustainability Agencies to manage those subbasins.

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 Sub-Basin

San Mateo Plain Subbasin

The San Mateo Plain groundwater 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 groundwater sub-basin to assess the groundwater resources and current condition of the sub-basin; and identify potential groundwater management strategies. The project is funded by Measure A, a countywide half-cent general sales tax passed by voters in 2013.

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

Stakeholder Meeting Resources

Workshop DateLocationWorkshop Materials
August 17, 2017Sobrato Community Conference Center Redwood Shores
Shoreway Conference Room
350 Twin Dolphin Drive, Redwood City
Agenda
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

Final Phase I Report

Data will soon be made available on Open San Mateo County.

Additional Resources

Groundwater Reports

Visitacion Valley Basin

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

Islais Valley Basin

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

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

San Gregorio Valley Basin

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

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

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.