Los Osos Groundwater Basin and Sewer
A watershed-related issue examined by the ENVS 560/L Watershed Systems class at CSUMB.
Summary
Los Osos, on the central coast of CA,
Location
Los Osos is an unincorporated community south of Morro Bay in San Luis Obispo County, CA with a population of 14,276 in 2010 [1]. 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 (LOCSD), Golden State Water Company, and S&T Mutual Water Company [2].
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 approximately 1,000 feet. The basin is underlain by unconsolidated deposits such as alluvium, ancient sand dunes and mostly impermeable Paso Robles Formation and basement Franciscan Complex [3]. Water is extracted from the multi-level Paso Robles Formation, underneath which lies the Franciscan Complex. The Franciscan Complex does not bear water and confines the basin to the west [4].
Saltwater intrusion was first recorded in the Los Osos Groundwater Basin in 1972 by the Department of Water Resources. Saltwater has subsequently increased on the western edge of the basin where the water table is shallow, impacting water purveyor's access to fresh water. Overdraft of the lower aquifer in response to nitrate pollution of the upper aquifer has increased sea water intrusion [5]. The annual advance of saltwater intrusion from 2005-2015 was approximately 190 feet [6].
Rainfall and runoff percolation accounts 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 [5].
Sewer
Los Osos residents have used septic systems as wastewater treatment for over 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 Boards(RWQCB)-Central Coast Region determined in 1983 that septic systems had contaminated the upper aquifer based on State standards. Major construction or expansion was halted until the water pollution was addressed and reduced [4].
Many attempts have been made to rectify the water pollution problem following this action by RWQCB, ultimately becoming a divisive issue in the community. The first wastewater treatment facility was proposed in 1987. An Environmental Impact Report was written for the project and revised when the site location was changed. With little community support, the new facility was not completed [4]. Community members were concerned due to the:
- project cost
- use of percolation ponds
- 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 and produced an EIR for it in 2001. Members of LOCSD were recalled in 2005, and the project EIR was revoked and LOCSD filed for bankruptcy in 2006 [4].
AB 2701 was passed in 2007 to transfer jurisdiction over the wastewater treatment project to the County of San Luis Obispo. Proposition 218, passed in 2007, authorized funding the wastewater collection and treatment project through property taxes [4].
The Los Osos Wastewater Project construction began in 2014 with pipeline installation and road resurfacing. The Los Osos Wastewater Treatment Plant was completed and individual residence sewer hookups beginning in March, 2016 [7].
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