Permit Sonoma › Divisions › Engineering & Construction › Well and Septic Systems › Septic Systems › OWTS Manual › OWTS Manual Update Process › Proposed OWTS Manual August 13 2024

Revised OWTS Manual | August 13, 2024

The County of Sonoma is pleased to release the latest draft of our Onsite Wastewater Treatment Systems (OWTS) Manual. We held Zoom meetings the week of June 24, 2024, to receive public comments. Staff will be presenting a Local Agency Management Program and the revised OWTS Manual to the Board of Supervisors on August 13, 2024 (revised date).

Below is a summary of why the OWTS Manual is being revised, the implications to existing and new development, how this manual interacts with the Total Maximum Daily Load (TMDLs) in the County, and an overview of the changes in this version. We also provide links to comments and responses received from the OWTS design community as part of developing the proposed revisions to the OWTS Manual.

Draft OWTS Manual:

OWTS Manual version 8.0 (PDF: 10.3 MB)
OWTS Manual version 8.0 edits shown (PDF: 10.9 MB)

Frequently Asked Questions (FAQs)

Why a revised OWTS Manual?

State law mandated the State Water Resources Control Board to adopt standards for regulation of OWTS commonly known as septic systems. The State Water Board has complied with state law through the adoption of the State’s OWTS Policy. The State’s OWTS Policy requires local jurisdictions to apply to the appropriate Regional Water Board (RWB) by submitting a Local Agency Management Program (LAMP), for their review and approval.

The County of Sonoma does not yet have an approved OWTS program. In August 2019, the County submitted a LAMP to the RWB, however, the OWTS program was not approved. The RWB required several revisions.

County staff have made the necessary revisions to the LAMP and OWTS Manual. Additional edits are being proposed which include clarifications, filling gaps in the regulations, and adding regulations to address stream or river impairments and TMDLs.

What are the implications to existing or new development?

Existing Development:

If you are not proposing any development, there is no impact from the revised OWTS Manual. Absent a failing system or public health hazard, Permit Sonoma has no legal authority to require a landowner to upgrade their OWTS.

If your OWTS is failing or needs corrective action, the revised OWTS Manual adds a new section to address these systems. In Section 20 Tier 4 Corrective Action the manual presents a range of options from repairs to substantial conformance.

The idea behind a “substantially conforming” system is not new but is being revisited. Many sites or parcels are physically constrained due to steeper slopes, stream setbacks, existing development, and other land encumbrances. The concept is to evaluate the site, determine the available site area for an OWTS and to design the best OWTS that can fit on that site given the physical limitations.

There are limitations for the use of substantial conformance. For example, substantial conformance is not to authorize new or additional development or re-construction of existing structures. The goal is to upgrade the OWTS, beyond a repair, with the understanding a fully code-compliant system would never be achieved due to the site limitations.

New Development:

For new development or added development, the revised OWTS Manual has revised Section 6 Requirements for Approval of Building Permit.

This section goes through various scenarios of development including new structures (dwellings, accessory dwelling units (ADUs), guest house, garages, swimming pools) and improvements to existing structures (remodels, additions). This section discerns if proposed development creates new or additional waste flow and/or if the proposed development may physically impact an existing OWTS. Depending on the responses, nothing needs to be done (interior remodel), a new OWTS may be required (new dwelling) or the existing OWTS may need to be evaluated (bedroom addition).

Section 6 is very detailed and nuanced and this is a simplified summary. If you are looking to develop or add structures to your parcel, please review Section 6.

How does the revised OWTS Manual affect reserve replacement areas?

Reserve areas are critical to sustainable development. The idea is a site is developed with the primary OWTS and either one or two replacement areas. One or two remain fallow while one is in use. When needed, the reserve area becomes the primary OWTS while the old primary OWTS is allowed to rest and recover. One clarification in this version is if a reserve area is used, that does not then require a new reserve area. The concept is to rotate through the reserve areas, not to replace them when used (Section 4.3.C.3)

Development has the potential to be placed over the replacement area(s) thereby affecting the long-term sustainability of the site. As development is proposed, the location needs to be reviewed relative to the reserve area(s). If the proposed development impacts a reserve area, the options include creating a new reserve area or avoiding the existing reserve area.

Unfortunately, many sites were developed without fully vetted reserve replacement areas. When new development is proposed and it is determined a reserve replacement area was not vetted, a reserve replacement area will be required. This only applies if the proposed development results in the parcel being more than 50% encumbered. See Section 6.6 Reserve Replacement Area and Building Improvements for additional details.

How does the revised OWTS Manual address TMDLs and Impaired Water Bodies?

There are three water bodies within Sonoma County designated as impaired due to pathogens and/or nutrients. These are the Russian River, the Petaluma River and Sonoma Creek. The County is proposing Section 19 OWTS Policy Tier 3, Total Maximum Daily Load and Advanced Protection Management Program to address these impaired water bodies.

The State’s OWTS Policy Tier 3 provides three approaches for water bodies that are designated as impaired. These options include implementing an approved Total Maximum Daily Load (TMDL) Action Plan, implementing local requirements known as Special Provisions, or implementing the requirements within the State OWTS Policy Tier 3 section 10. The selected option is presented through a document known as an Advance Protection Management Program.

The San Francisco Regional Water Board (RWB) has adopted TMDLs for the Petaluma River and Sonoma Creek and OWTS Manual Section 19 implements the TMDL Action Plans for those watersheds.

There is no TMDL for the Russian River currently, so the County is proposing Special Provisions. The main requirement is for new or replacement OWTS to include supplement treatment and/or enhanced dispersal systems which are defined in the OWTS Manual Section 3 Acronyms and Definitions.

The area subject to the Special Provisions is a 600-feet width on each side of the water bodies listed in Section 19 and came from the Federal Clean Water list of impairment presented in the State’s OWTS Policy Attachment 2 and Attachment 3. The area is not the entire Russian River watershed.

There are exceptions to the supplemental treatment requirement if the site has three feet of good soil within 200 feet of the stream/river or two feet of good soil between 200 and 600 feet of the stream/river.

What are the significant revisions in this version of the OWTS Manual?

Reserve replacement areas. Language has been proposed clarifying the use of a vetted reserve replacement area does not require another replacement area. (Section 4.3)
Easements. Clarifying language regarding abutting lots and easements. The intent is to allow easements on abutting lots and not allow easements to a parcel that is one or more parcels removed from the subject parcel. (Section 4.5)
OWTS replacement under a repair permit. OWTS Manual v7.0 authorizes an OWTS repair permit to include up to 50% of a replacement system. The RWB denied the prior LAMP due to this provision. Staff have revised this provision to allow an OWTS repair permit to include up to 25% of a replacement system. (Section 4.10)
Many variances are “tried and true” and were standardized in the current OWTS Manual Section 17 Table 1. These are proposed to be moved to the relative sections as exceptions. Others are presented in Section 4.16. This approach allows the “variance” to be accepted as part of the OWTS design versus requiring a separate “variance” application. (Section 4.16; Section 17)
Percent Encumbrance and Reserve Areas. Proposed development can affect reserve replacement areas by building over the top of them. The proposal is to require a reserve replacement area(s) when the proposed development creates more than 50% land encumbrance. This can be satisfied by documenting an existing reserve area(s) or creating one. If the proposed development creates 50% or less percent encumbrance, the reserve replacement area(s) is/are required to be shown only if they exist. Development within an encumbered area (stream or well setback for example) does not add to the percent encumbrance. (Section 6.6)
Shifting site evaluation work to designers. Site evaluations take three forms:
- Soil depth and type
- Percolation rate
- Groundwater elevations
The concept is to shift this work to solely design professionals, freeing up staff to focus on plan reviews. Revisions were needed to eliminate applications for site evaluations, to require the site evaluation information be submitted in the OWTS Design Report when the OWTS application is submitted, and clarification on how to document the site evaluation information. (Section 7)
Site Evaluation Area and Site Evaluation Radius. Each site evaluation location has an effective radius. Combining these areas creates a Site Evaluation Area where the OWTS can be located. The effective radii for each type of site evaluation are proposed to be (Section 7-13):
- Soil evaluations: 50 feet
- Groundwater elevations: 50 feet
- Percolation tests: 25 feet
Separation distances for Mound and At-Grade OWTS. OWTS need to be separated to ensure proper functionality and to protect the downslope soil for future use. The separation distances are frequently debated on a project-by-project basis due to not being in the standards. The concept is to create minimum setbacks that ensure proper functionality and reduces delays in the plan review process.
- For Mound OWTS the proposed setback allows abutting the toe of primary OWTS sand layer to the toe of the second or reserve OWTS sand layer. (Section 13.3)
- For At-Grade OWTS the proposed setback allows the abutting the toe of the primary OWTS fill layer to toe of the second or reserve OWTS fill layer. (Section 13.4)
These systems have internal distances from the point of dispersal to edge of the sand toe (Mounds) and to the edge of fill (At-Grades) that allows for abutting of the external edges.
Drip Systems. The current OWTS Manual provides for sub-surface drip OWTS. The revised manual proposes an “at-grade” drip system provide there is a minimum of 24 inches of good soil below and a minimum of six inches of cover soil. (Section 13.5)
Operational Monitoring Program. The current OWTS Manual requires non-standard OWTS to be in the Operational Monitoring (OPR) Program. In 2023, the County revised this program to only require OWTS with a supplement treatment unit be placed in the OPR program. The proposed OWTS Manual catches up to this change. (Section 14)
State OWTS Policy Tier 3. Section 19 has been added to address impaired water bodies, TMDLs and Advanced Protection Management Plans. (Section 19)
State OWTS Policy Tier 4. Section 20 has been added to address OWTS that need corrective action. (Section 20)
Cesspool Conversion. A subsection of Section 20 allows for the conversion of cesspools. Cesspools are prohibited by the RWB’s and the State OWTS Policy. The County provides for a cesspool conversion that waives the typical site evaluation requirements to encourage removing cesspools from the landscape. As part of Section 20, a cesspool conversion does not authorize new or additional development or re-construction of existing structures. (Section 20).
Waterless Toilets. In 2023, the County revised section Sonoma County Code section 26 to allow for non-flush toilets (waterless toilets). The proposed OWTS Manual catches up to this change. (Section 21)
OWTS not authorized by the State’s OWTS Policy. The proposed OWTS adds a section to provide direction if a OWTS does not meet the minimum standards of the OWTS Policy. (Section 22)

What (and when) is coming before the Board of Supervisors?

Staff will be presenting a Local Area Management Program and the revised OWTS Manual to the Board of Supervisors on July16, 2024. Staff are seeking authorization to submit these two documents to the North Coast RWB as the County’s application for a local OWTS program. This is being done pursuant to the State’s OWTS Policy.

Have other people commented on this OWTS Manual version?

Yes. County staff have been working with design professionals through the Land Use and Advisory Panel (LUAP) and with members of the Lower Russian River Citizen’s Advisory Group. Their comments and the County responses can be viewed below. Their comments are sorted by OWTS Manual section.