Interceptor Bypass Project
This map shows the location of the interceptor sewer pipelines (beneath Lake Nacimiento) for the CSA 7A wastewater collection system and Lift Station No. 3
Project Purpose
The CSA 7A Oak Shores - Interceptor Bypass Project is intended to reduce vulnerability of the existing CSA 7A wastewater collection and treatment system to damage and leakage and to reduce the associated potential risk to public health and safety from potential sewer spills that could impact Lake Nacimiento, a regional drinking water source and recreation body of water. This project focuses on bypassing a majority of the sewer pipeline located below the high-water line of the lake. The risk of a sewer spill has significantly increased because soil erosion has exposed much of the pipeline along the lake, impacting the supporting structure for the pipes and leaving it unprotected to hazards such as wake action and corrosion.
Project History
Check out the full timeline below by expanding each year.
| As a condition for approval of the final maps for Tracts 378, 379, 380 and 381 of the Oak Shores central neighborhood, General Resource Development Corp (“GRD”) had agreed to construct wastewater collection, treatment, and disposal facilities to accommodate effluent from the project. The agreements stipulated that GRD would post a bond for the construction and dedicate the wastewater facilities to the County within two years of the date of completion of construction, after proving successful operation. |
| In 1973, GRD completed construction of most of the Oak Shores Sewer System, though they failed to address deficiencies noted by the County and the Regional Water Quality Control Board (RWQCB) in the two years following. |
| In 1976, the County of San Luis Obispo (County) established CSA 7A and assumed responsibility for operating the wastewater collection and treatment system after GRD was unable to fulfill their responsibilities to complete construction and prove successful operation as had been stipulated in the previous development agreements. |
| In 1988, the County established Ordinance No. 2338 establishing the Rules and Regulations for the CSA 7A wastewater collection and treatment system. |
| In 2003, the CSA 7A wastewater collection and treatment system reaches 30 years of age. |
| In 2004, the County conducted the CSA 7A Interceptor Bypass Study. The study reviewed the existing wastewater collection system and identified improvements which would allow abandonment of the interceptor sewer pipelines and the 60-foot-deep Lift Station No. 3. The study concluded that abandonment of the interceptor sewer pipelines would require construction of eight (8) new lift stations and approximately 22 sewer ejector pump systems at individual properties. The proposed improvements would reduce the potential for leakage of raw sewage directly to the lake from the interceptor sewer pipelines, eliminate the risk of shutdown of the pumping and treatment facilities due to inflow to a submerged collection system, allow maintenance staff to have year-round access to the entire collection system, and allow Lift Station No. 3 to be replaced with a new lift station that is both more accessible and easier to safely maintain. |
| In 2011, a cleanout pipe broke below lake level in the eastside interceptor sewer pipelines, which could have resulted in a sewer spill into Lake Nacimiento. The pipe was repaired by a diver, and the County issued a boil water notice advising all users of Lake Nacimiento to avoid domestic and recreational use of the water. |
| In 2013, the CSA 7A wastewater collection and treatment system reaches 40 years of age. |
| In 2015, the County contracted with MNS Engineers to prepare the Risk Assessment Study on the Interceptor Sewerline System in CSA 7A. The study examined the potential risk of failure of the existing eastside & westside interceptor sewer pipelines and provided a risk reduction analysis for multiple options. The study presented Improvement Option 3b. as the most feasible option, which included removal of 5,776 feet of the exposed and susceptible segments of the interceptor sewer pipeline from service and lining the remained portions with an in-situ structural liner. |
| In 2021, the County received grant funding in the amount of $1.25 million from the American Rescue Plan Act (ARPA) to fund preparation of a technical memorandum and commence design and preparation of construction documents for the Interceptor Bypass Project. |
| In 2023, the CSA 7A wastewater collection and treatment system reaches 50 years of age. In 2023, Advantage Technical Services completed an inspection report for CSA 7A Lift Station No. 3. The report’s recommendations #5 and #7 focused on the need for increased corrosion resistance and development of an inspection program. These recommendation have been included into the overall scope for the project. |
| In 2025, two manhole structures on the westside interceptor pipeline system were damaged due to soil erosion. The County and Oak Shore HOA staff responded quickly to repair the damage. To mitigate any future damage the Oak Shores HOA requested that the County support the enforcement of a “No Wake Zone” within the Monterey County Parks jurisdiction of Lake Nacimiento.
 Oak Shores, California – Before and after photos showing the repair efforts that were made to a manhole structure on the westside interceptor pipeline system in January 2025.
 Oak Shores, California – Before and after photos showing the repair efforts that were made to another manhole structure on the westside interceptor pipeline system in June 2025.
 Oak Shores, California – The proposed “No Wake Zone” from the Oak Shores HOA that would support mitigation of future damage to the CSA 7A interceptor sewer pipeline and structures along the lakeside.MNS Engineers completed the Oak Shores Interceptor Bypass Project Feasibility Study , a component of final design and construction document preparation for the Interceptor Bypass Project. The feasibility study reviewed past studies and evaluated the proposed collection system improvements in preparation for developing plans, specifications and costs estimate documentation. |
Project Elements
The Interceptor Bypass Project includes several key elements designed to improve system performance and reliability. These include installation of a new bypass pipeline, construction of new maintenance access structures, and upgrades to existing infrastructure to handle future capacity needs. Together, these improvements will enhance operational flexibility and reduce the risk of service interruptions.
Special Considerations for the Project
Lake Nacimiento serves as both a regional drinking water source for the cities of Paso Robles, Templeton, Atascadero and San Luis Obispo and as a recreational body of water for uses such as boating, fishing, and swimming. There are two existing boat launch ramps at the Oak Shores Resort and other ramps located around the lake shores. Completing this project would continue to ensure public health and safety for all the users and uses of Lake Nacimiento.
The interceptor sewer pipelines are located below the high-level of the lake, and so construction activities will need to consider factors related to working within the lake including coordinate with lake users and uses, timing on the water levels, and activities to be done within jurisdictional waters, all of which increase the complexity and cost of the project. For example, the project will require special permits from federal agencies like the Army Corps of Engineers.
Project Timeline
| Year | Actions |
|---|---|
| 2025 |
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| 2026 |
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| 2027 |
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| 2031 |
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Grant Opportunities and Efforts
TThe County has secured $4 million in American Rescue Plan Act funding for CSA 7A, including $1.25 million dedicated to the design of Element 1. To help offset project costs, the County will continue exploring additional grant opportunities. Once the design phase is complete, additional grant applications can be submitted. In the meantime, we have developed an informational funding sheet to raise awareness with potential grantors.
Current Estimated Interceptor Project Cost
| Estimated Cost | Project Element |
| $20M - $24M | Element 1 - Interceptor Sewer Pipeline Bypass and Abandonment |
| $6M - $8M | Element 2 - Interceptor Sewer Pipeline Rehabilitation |
| $1M - $2M | Element 3 - Lift Station No. 3 Coating |
| $5M - $7M | Estimated Debt Service |
| $32M - $41M | Total Estimated Project Cost |
