Evaluation of Groundwater Resources in the Adelaida Area of San Luis Obispo County, California

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Stakeholders in San Luis Obispo County are concerned that the increased demand for water use has, and will continue to, affect groundwater levels and availability in the Adelaida area. To address stakeholder concerns, the County of San Luis Obispo Board of Supervisors has asked the USGS to conduct a comprehensive evaluation of groundwater resources of the Adelaida area. 


aerial view of Paso Robles, CA

View of Paso Robles and the Adelaida area near the central California coast as taken from an airplane flying overhead. (Credit: Geoff Cromwell, USGS. Public domain.)

Groundwater is the primary source of water in the Adelaida area and local stakeholders use private domestic and commercial wells to pump groundwater from the aquifer system. There is concern among stakeholders that the increasing demand for water is negatively affecting groundwater levels and groundwater availability. In response to these concerns, the County of San Luis Obispo Board of Supervisors (County Board) seeks to evaluate groundwater management solutions in the Adelaida area and has asked the United States Geological Survey (USGS), in cooperation with the San Luis Obispo County Flood Control and Water Conservation District (SLOFC&WCD), to provide a comprehensive evaluation of historic and current groundwater resources in the area. 

The USGS will gain an increased scientific understanding of the hydrologic system in the Adelaida area by compiling and collecting hydrogeologic and hydraulic data in the defined study area. The California Department of Water Resources (DWR) has not defined the highlands of the Adelaida area as a groundwater basin, therefore the area is not subject to California Sustainable Groundwater Management Act (SGMA) regulations (CADWR; 2016). The eastern border of the Adelaida area is outside of of the Salinas Valley-Paso Robles Area (Paso Robles sub-basin) and Salinas Valley-Atascadero Area (Atascadero sub-basin) groundwater basins (CADWR, 2016; fig. 1). Publicly available hydrogeologic and hydraulic data (such as aquifer yield, water quality, or water availability) are limited throughout the Adelaida area, and to date, no investigation of the groundwater resources or water-bearing units of the aquifer system of the Adelaida area has been completed. The Adelaida Study (Study) will extend to the eastern area of the Paso Robles sub-basin and Atascadero sub-basin to better understand the groundwater connectivity between the sub-basin boundaries and the Adelaida areas.

This Study will benefit water managers, stakeholders and potential future interested entities and studies by providing: 

  • an improved understanding of the aquifer system, including identifying the water-bearing units 
  • evaluating the historic and current impacts of groundwater use in the Adelaida area 
  • quantifying the groundwater resources in an undefined local aquifer system 
  • advancing monitoring networks in the County for groundwater and water-quality data 
  • providing a more thorough understanding of geological controls on water availability 


Water Supply 

The communities of Paso Robles, Templeton, and Atascadero supply water through their respective water districts, which is sourced from a combination of groundwater from the Paso Robles and Atascadero sub-basins, underflow from the Salinas River, recycled water, and surface water from Nacimiento Reservoir (Carollo, 2012b). These communities primarily supply water to residents in the more populated urban and suburban zones in the eastern border of the Adelaida area and vicinity. In the northern part of the Adelaida area, the unincorporated community of Heritage Ranch sources its water downstream of Nacimiento Reservoir through its own water district; the service area of this water district is limited in its extent in the Adelaida area. Stakeholders outside these four communities likely rely exclusively on groundwater and utilize private domestic and commercial wells to meet their water demand. 

Land and Water Use 

Stakeholders in San Luis Obispo County have a specific need to evaluate and understand historic and current hydrologic conditions in the Adelaida area to consider the current and future population and agricultural water needs. In the highlands of the Adelaida area, groundwater is pumped for rural use and for agriculture, primarily vineyards.  As the world-wide popularity of wine has increased, land use in the highlands has changed from undeveloped rural lands, and other agricultural crop types, to include 5,669 acres of vineyards; this and other crops rely solely on groundwater from wells for irrigation.  

The Adelaida area is predominantly rural and undeveloped. In 2014, urban land use comprised 16 percent (18,853 acres) (LandIQ, 2017) of the Study area, more than double the acreage in 1996 (7,822 acres) (CADWR, 2000). Urban centers include the cities of Paso Robles, Templeton and Atascadero in the eastern part of the Adelaida area, and the unincorporated community of Heritage Ranch in the northern part. In 2014, agricultural land use comprised 12 percent (13,712 acres) of the Study area. Vineyards were the most common crop type in 2014, comprising 41 percent (5,669 acres) of agricultural lands, up from 8 percent (2,000 acres) in 1996. Vineyards were the only crop type to substantially increase acreage from 1996 to 2014 (pasture lands also increased from 446 to 669 acres). 

Groundwater Flow 

The direction of groundwater flow in the Adelaida area highlands is unknown at present, but likely flows from west to east, following the topographic gradient from the highlands towards the Paso Robles and Atascadero subbasins. Groundwater in the Paso Robles and Atascadero subbasins flows north and east towards the Salinas River (Todd Engineers, 2007). Folds, faults, and fractures in the basement and consolidated sedimentary rocks may provide conduits for groundwater flow, may compartmentalize the aquifer system, and (or) may impede groundwater flow.  

The study will be completed over a five-year period using staff from the USGS California Water Science Center, while drawing on expertise of local stakeholders, including the SLOFC&WCD. 



California Department of Water Resources, 2000, 1996 San Luis Obispo County Land Use Survey, accessed January 3, 2020 at https://gis.water.ca.gov/app/CADWRLandUseViewer/.

California Department of Water Resources, 2016, Bulletin 118-Interim Update 2016, California's Groundwater, Working Toward Sustainability: California Department of Water Resources, 226 p., accessed November 10, 2018 at https://water.ca.gov/Programs/Groundwater-Management/Bulletin-118

LandIQ, 2017, i15_Crop_Mapping_2014_Final, Geospatial Dataset prepared for California Department of Water Resources, accessed January 3, 2020 at https://gis.water.ca.gov/app/CADWRLandUseViewer/.

Todd Engineers, 2007, Update for the Paso Robles Groundwater Basin, San Luis Obispo, California, accessed January 6, 2020 at https://www.slocounty.ca.gov/Departments/Public-Works/Forms-Documents.aspx 

Carollo, 2012b, Master Water Report – Volume II of III: San Luis Obispo County Flood Control and Water Conservation District, San Luis Obispo, California, 347 p., accessed October 1, 2019 at https://www.slocounty.ca.gov/Departments/Public-Works/Forms-Documents/Water-Resources/Master-Water-Report.aspx