Los Osos Groundwater Basin and Sewer

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A watershed-related issue examined by the ENVS 560/L Watershed Systems class at CSUMB.


Los Osos, on the central coast of California, has experienced water supply issues over several decades due to over-pumping of the local groundwater basin and nitrate pollution from agriculture. This unincorporated area relies solely on local groundwater for potable water. Overdraft of the local lower aquifer has caused saltwater intrusion, further limiting fresh water supplies [1].

Los Osos residents have relied on individual septic tanks for wastewater collection since the 1970s, causing health concerns due to the shallow water table. Several wastewater collection and treatment projects have been proposed and litigated, ultimately failing due to a lack of community support [1]. Under the authority of San Luis Obispo County, construction of The Los Osos Wastewater Collection System began in 2014 and now provides sewer service to residents [2].
Los Osos Groundwater Basin, San Luis Obispo County, California. Image: Cleath-Harris Geologists 2016


Los Osos is an unincorporated community south of Morro Bay in San Luis Obispo County, California with a 2010 population of 14,276 [3]. The town's water supply is locally sourced from the Los Osos Groundwater Basin, which is managed by three water providers: Los Osos Community Services District, Golden State Water Company, and S&T Mutual Water Company [4].

Los Osos Groundwater Basin

The Los Osos Groundwater Basin provides residential, commercial and agricultural water for Los Osos. The basin covers about 10 square miles with a maximum depth of 1,000 feet. The basin is lined with unconsolidated deposits such as alluvium, ancient sand dunes, mostly-impermeable Paso Robles Formation and basement Franciscan Complex [5]. Water is extracted from the multi-level Paso Robles Formation, but the underlying Franciscan Complex does not bear water and confines the basin to the west [1].

Rainfall and runoff percolation account for the majority of aquifer recharge. Additional sources of basin recharge are from irrigation and septic system return flow, as the town has strictly used septic systems for wastewater treatment until recently [6].

Saltwater Intrusion

Saltwater Intrusion was first recorded in the Los Osos Groundwater Basin in 1972 by the Department of Water Resources. Saline water has subsequently increased on the western edge of the basin where the water table is shallow, impacting water purveyors' access to fresh water. Overdraft of the lower aquifer in response to nitrate pollution of the upper aquifer has increased saltwater intrusion [6]. The annual advance of saltwater intrusion from 2005 to 2015 was approximately 190 feet [7].
Seawater Intrusion. Retrieved from: California Waterboards[3]

2015 Basin Plan for the Los Osos Groundwater Basin

The Los Osos Community Services District filed a lawsuit in 2004 against the three water purveyors, Golden State Water Co., the county of San Luis Obispo and S&T Water Co. for over water consumption and water rights.

That litigation led to the drafting of the Los Osos Basin Plan, which establishes a process and timeframes to reach the goals of reversing seawater intrusion, promoting conservation and ensuring the basin will provide a reliable, sustainable water supply that benefits the Los Osos community. [8] The program and infrastructure will cost about $34 million and will be equitably distributed among water users in the basin.

  • $11 million to build a nitrate removal plant
  • $5.5 million for conservation efforts
  • $18 million to build a system to inject treated wastewater into the aquifer

The plan also includes a monitoring program to analyze groundwater quality and levels of 73 well, metering and infrastructure to shift well production to upper and inland aquifers, and conservation caps on customer water use.

The basin plan was issued and approved by the court in October 2015 and can be found here.

Immediate Goals of the Plan [9]

  • Halt or to the extent possible, reverse seawater intrusion in the Basin
  • Provide sustainable water supplies
  • Promote water conservation

Sewer Lateral Connections and Septic System Decommissioning

Los Osos residents have used septic systems as wastewater treatment for more than 40 years. This sanitary system has caused concern since 1971 due to the shallow depth of groundwater and the town's reliance on the local aquifer for potable water. The Regional Water Quality Control Board (RWQCB)-Central Coast Region determined in 1983 that septic systems had contaminated the upper aquifer. Major construction or expansion in that area was halted until the water pollution was addressed and reduced [1].

Many attempts have been made to rectify the water pollution problem following RWQCB's action, ultimately becoming a divisive issue in the community. The first wastewater treatment facility was proposed in 1987. An Environmental Impact Report (EIR) was written for the project and revised when the site location was changed. With little community support, the new facility was not completed [1]. Community members were concerned due to the: project costs, use of percolation ponds and flooding potential.

In 1998, the community voted to create a wastewater community services district known as The Los Osos Community Services District (LOCSD). LOCSD created a new wastewater collection and treatment project, preparing its EIR in 2001. After members of LOCSD were recalled in 2005, the project EIR was revoked. LOCSD filed for bankruptcy in 2006 [1].

Jurisdiction over the wastewater treatment project was transferred to the County of San Luis Obispo with the passage of AB 2701 in 2007. Proposition 218, also passed in 2007, authorized funding of the wastewater collection and treatment project through local property taxes [1].

Construction of the Los Osos Wastewater Project began in 2014 with pipeline installation and road resurfacing. The Los Osos Wastewater Treatment Plant was completed in March 2016 [10], and individual residents have been scheduled to connect to the sewer laterally by a specified date [11].


Sewer system cost and location has postponed its implementation since groundwater pollution concerns began decades earlier. Community members voted to take on costs of the Los Osos Wastewater Project with Prop. 218, though grants and financial assistance are available. Homeowners are responsible for the cost of hooking up their sewer line to the street, which varies between $2,000 and $10,000 per household. Project construction costs are added to homeowners' property taxes - $165 per month on average or $1,980 per year [12]. The project has received $21.5 million in grants to date [13]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Michael Brandman Associates. 2008. Draft Environmental Impact Report County of San Luis Obispo: Los Osos Wastewater Project. Irvine, California. [1]
  2. Waddell J. 2016. Los Osos Wastewater Project: Project Construction Update January-March, 2016. Available from: http://www.slocounty.ca.gov/Assets/PW/LOWWP/PM+Monthly+Update+2016+Q1.pdf
  3. U.S. Census Bureau. Quick Facts: Los Osos CDP, CA. https://www.census.gov/quickfacts/table/PST045216/0644182,00
  4. [CSLO]County of San Luis Obispo. 2013. 2010-2012 Resource summary report: San Luis Obispo County general plan. http://www.slocounty.ca.gov/Assets/PL/RMS/2010-2012_RMS.pdf
  5. Yates and Wiese. 1988. Hydrogeology and water resources of the Los Osos Valley ground-water basin, San Luis Obispo County, California. U.S. Geological Survey. Water-Resources Investigations Report 88-4081. [2]
  6. 6.0 6.1 Cleath and Associates. 2005. Draft Final Report: Seawater intrusion assessment and lower aquifer source investigation of the Los Osos Valley Ground Water Basin San Luis Obispo, California. Prepared for: Los Osos Community Services District
  7. Cleath-Harris Geologists. 2016. Los Osos Basin Plan: Groundwater Monitoring Program 2015 Annual Monitoring Report.
  8. http://www.sanluisobispo.com/news/local/article40426140.html
  9. http://www.waterboards.ca.gov/centralcoast/board_info/agendas/2015/november/item14/item14_presentation_lobmp.pdf
  10. Wilson N. 2016. Los Osos sewer line connections start March 28th, 2016. The Tribune. Available from: http://www.sanluisobispo.com/news/local/article66988712.html
  11. San Luis Obispo County. 2017. Sewer lateral connections and septic system decommissioning. Available from: http://www.slocounty.ca.gov/PW/LOWWP/Sewer_Lateral_Connections_and_Septic_System_Decommissioning.htm
  12. Wilson N. 2016. Workshop on Los Osos sewer hookups planned as system nears completion. The Tribune. Available from: http://www.sanluisobispo.com/news/local/article55733220.html
  13. San Luis Obispo County. Date unknown. Project costs and financial assistance. Available from: http://www.slocounty.ca.gov/PW/LOWWP/Project_Costs_and_Financial_Assistance.htm



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