Groundwater

Groundwater 101

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 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

Keeping Our Water Clean

Groundwater Protection

The goal of the Groundwater Protection Program is to protect underground water supplies and waterways from chemical pollution. Inspection staff oversee clean-up of pollution caused by leaking underground tanks and chemical spills. They work with other agencies, such as the Environmental Protection Agency (EPA) and the Water Quality Control Boards, to make sure the clean-up process follows State and local laws. Groundwater Protection Program staff also issue permits for installing wells and soil borings for geotechnical or environmental investigations.

Website: Learn more about the Groundwater Protection Program here.

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

Frequently Asked Questions

How many wells are in my area and where are they located? 

View the California Department of Water Resources (DWR) well logs inventory. A well log is required to be submitted to the state for every well drilled.  You can filter the city field by the location of interest or planned use/future use field by the use of interest.  Please note, these are the number of wells ever installed; each of these wells’ current status (e.g. active, temporarily inactive, destroyed) is not tracked by DWR. To view the actual driller’s well log, click on the link in the final column.  You can view the well types map to view where the wells are located. This data was collected as part of the recent San Mateo Plain subbasin assessment, which included data collection on well information for areas outside of the San Mateo Plain subbasin.

What is groundwater used for in my area? 

The well types map is color coded in the legend to show all public water supply, private water supply, cathodic protection, irrigation, environmental monitoring, test, and other wells.  If you are interested in wells that usually are associated with groundwater extraction then you would focus on the public water supply, private water supply, and irrigation wells.

San Mateo County does not have a water management agency like our neighboring counties so what the groundwater is used for (and more specifically how much is used) is not exactly known.  As implied by the well types, some wells could be used for drinking water supply (most likely for public and private emergency backup supply but could be for private domestic use) while most are likely for irrigation purposes.  There may be other purposes such as commercial/industrial processes.

Note: This data was collected as part of the recent San Mateo Plain subbasin assessment, which included data collection on well information for areas outside of the San Mateo Plain subbasin.  The final assessment report (page 207) does have annual average estimates for extractions within the San Mateo Plain subbasin.

How deep is groundwater? 

The groundwater levels map shows locations of wells only where we were able to also find at least one depth to water measurement in the source of the data. Select a well to view the specific groundwater depth information associated with it.  This map is also color coded to show which depths the wells are specifically targeted (i.e. where the wells screen is located below the ground surface).

What is the County doing related to the Sustainable Groundwater Management Act (SGMA)? 

Currently all 9 basins within San Mateo County are designated as Very Low Priority and are not required to comply with SGMA. If you would like to discuss the County’s efforts related to SGMA further, please contact Charles Ice at 650-399-6911.

What is the County’s involvement in the Westside Basin?

The County is not facilitating any activities within the Westside Basin. There is a voluntary Groundwater Management Plan for the southern part in San Mateo County.  You can contact SFPUC, Daly City, San Bruno, or CalWater directly for more information on efforts in that basin.

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 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 Office of Sustainability.

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 Office of Sustainability and Environmental Health Services held a series of workshops to solicit public and stakeholder feedback.

Project Reports and Presentations

Project Data

  • All GIS data can be viewed on San Mateo County GIS Open Data
  • Numerical groundwater model files (784 MB compressed), including historical basin conditions and the four modeled scenarios. Visit United States Geologic Survey’s website to download MODFLOW for free to use with these files
  • Project Database (42 MB compressed), of well data
  • Maps:
    • Well types: locations of wells, color coded by type
    • Well construction: location of wells, color coded by era of construction date
    • Water levels: location of wells, with water level measurements color coded by depth
    • Water quality: location of wells, with water quality measurements of 10 parameters shaded by concentration
    • Searchable map: location of wells with user selected water quality parameters with user defined types and depths
  • Bay Mud/tidal study measurements (1.75 MB compressed), in Excel. Specific subset of Project Database
  • Well logs from California Department of Water Resources, as of April 2016

Stakeholder Outreach Resources

DateLocationMaterials
July 12, 2018Redwood Shore Public Library, Community Room
399 Marine Parkway, Redwood City
Agenda
Presentation
Discussion Feedback
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

Additional Resources

Groundwater Reports

Groundwater Information from 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.