Groundwater Cleanup Project

The City of San Luis Obispo is leading the Groundwater Cleanup Project, an initiative to ensure responsible use of groundwater in the San Luis Obispo Valley Groundwater Basin (Basin). A Characterization Study completed in 2022 helped located and measure a small plume of tetrachloroethylene (PCE), a chemical associated with dry cleaning and industrial operations, that was detected in the Basin.  

While activities associated with the contamination no longer appear to be active, the presence of PCE limits the use of groundwater for drinking water. In an effort to expand its local water resources, the City will build two new groundwater supply wells, which are expected to be fully operational in 2026. Then, the City will spend the next decade or more pumping out the groundwater, treating it to remove any contamination to ensure that it meets drinking water standards, and adding it to the City’s drinking water system. This work is being funded by a $6.6 million grant from the California State Water Resources Control Board.   

Project Benefits

What is Groundwater?

Groundwater is water that fills the porous and fractured space beneath the ground surface. It is

 primarily found within rock layers called aquifers where the sediment characteristics of an aquifer, as well as topography, determine the rate and direction of groundwater movement– typically at a rate of feet per month. A groundwater basin is one, or a series of aquifers, with relatively defined boundaries, and a subbasin is a smaller unit within a groundwater basin that is defined by either geologic or hydrologic barriers such as fault zones.

The City of San Luis Obispo overlies the San Luis Valley Groundwater Basin that extends eastward to Edna Valley and contains three significant aquifers – Alluvial, Paso Robles Formation, and Pismo Formation.

Groundwater can be seen at the ground surface through natural springs or extracted through wells that are drilled down to the water and pumped to the surface for domestic or municipal uses.

Figure from the SLO Valley GSP

Although we transitioned away from using groundwater as a potable source in 2015, the City is working to secure groundwater as a potable source once again to provide a redundant, secondary source of water for the community. Currently, the City operates three non-potable production wells used for irrigation near the City’s Corporation Yard and Laguna Lake Golf Course. Additional drinking water wells remain in operable condition, and after additional treatment, the wells could be used as part of the municipal water supply for the future.

Groundwater levels are measured in various production wells throughout the City to monitor storage capacity of the basin. Groundwater is considered over drafted when the amount that is extracted exceeds the amount being replenished over an extended period; however, levels may exhibit natural fluctuations with variations in seasonal precipitation.

The City is investigating opportunities to replenish the groundwater supplies with recycled water in the event that groundwater is utilized as part of the municipal supply. To learn more about the practice of recharging the groundwater basin with highly treated recycled water, commonly referred to as Indirect Potable Reuse, and our recycled water supply, visit our recycled water page.

The City has invested in a robust, diverse water supply so the community can remain water-secure in the future. Groundwater development projects will continue to ensure the resiliency of our City’s water supply.

SGMA is a State regulation requiring local governmental agencies to form Groundwater Sustainability Agencies (GSAs) and create Groundwater Sustainability Plans (GSPs) for designated medium-high priority and critically over drafted groundwater basins. 

The San Luis Obispo Valley Groundwater Basin is classified as a high priority basin due to declining groundwater levels throughout the Edna Valley subbasin. The goal of SGMA is to create and implement sustainable groundwater practices to ensure adequate supply is available for our communities, allow sufficient time for aquifer replenishment, and to maintain environmental health. 

The City is one of two GSAs that manage the SLO Valley Basin resources and future implementation projects. The City and County GSAs work in conjunction with a diverse group of local water purveyors and the Groundwater Sustainability Commission (GSC) to develop a GSP that is both technically compliant and representative of the values of our community. 

The portion of the SLO Valley Basin beneath the City is largely distinct from Edna Valley due to a geologic divide that significantly restricts groundwater flow between the Edna Valley and the City. Additionally, each side of the basin experiences drastically different land use, recharge, and extraction rates. While the City’s subarea has a surplus of approximately 700 acre-feet of groundwater annually, the Edna Valley subarea is estimated to be overdrafted by approximately 1,100 acre-feet per year.

For more information about the San Luis Valley Groundwater Sustainability Plan, and how the City is working to ensure sustainable groundwater supplies for future generations, please visit https://www.slowaterbasin.com/.