Basics
- Puget Sound Indicator Name
- On-site sewage system inventory: known systems
- Progress Indicator
- Each Unit (count)
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- Topics
- On-site Sewage Systems
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No Reporter Selected
- Contributing Partners
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- Last Updated
- 07/16/2025 23:51:44
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Description
This indicator measures the number of small on-site sewage systems (OSS) known and inventoried with their local health jurisdiction. An OSS can become “known” through permit applications, inspection reports, site surveys, or spatial analyses. Inventorying and properly managing OSS is an important step to prevent and repair failures, protecting environmental and public health. This indicator helps us track the number of OSS in Puget Sound.
Progress Indicator Chart
Figure 1. Number of known on-site sewage systems across Puget Sound local health jurisdictions and marine recovery and sensitive areas in June and December of 2024.
In Puget Sound, fecal pollution impacts water quality, harvestability and food safety of shellfish, and can cause public health concerns in swimming areas. Small on-site sewage systems (OSS) treat wastewater close to where it is generated and many properties across Puget Sound are supported by this essential wastewater infrastructure [1]. However, improper management of OSS can lead to environmental and public health concerns; failing OSS are a leading pressure impacting water quality and shellfish bed health in Puget Sound[2].
Local health jurisdictions (LHJs) can construct an inventory of known systems by compiling permit records, pump-out records, inspection records, and parcel surveys. An OSS may become “known” through the following example situations:
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A homeowner applies for a permit for their OSS with their LHJ.
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A contractor reports an OSS pump-out at a parcel.
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A homeowner obtains certification through the LHJ to self-inspect their OSS and submits an inspection report of their OSS.
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An LHJ staff member reviews spatial data to identify built homes that are not connected to sewers.
Maintaining an inventory allows LHJs to track OSS in their jurisdiction. This allows them to better develop management plants to track compliance with routine inspection requirements, obtain funding for financial incentives and rebates, and provide outreach and technical assistance to OSS homeowners. Inventories also enable LHJs to identify sensitive areas that may be further impacted from failing OSS and identify OSS that may need more frequent maintenance due to age or design[1]. Predictions on future weather events, like increased flooding or sea level rise, can also be paired with OSS inventories to identify parcels that may need to seek alternatives to a residential OSS[2]. Finally, inventories highlight the magnitude of work under an LHJ’s purview, which can support arguments for increased funding and staffing of LHJs.
This indicator measures the number of OSS that are documented with their LHJ. The indicator helps us understand the current number of OSS in the region and within areas of particular concern, like marine recovery and sensitive areas.
[1] Environmental Protection Agency (2005). Handbook for Managing Onsite and Clustered (decentralized) Wastewater Treatment Systems: An Introduction to Management Tools and Information for Implementing EPA’s Management Guidelines. Office of Water, U.S. Environmental Protection Agency.
[2] Washington Department of Ecology (2023). Focus on: On-site Sewage Systems and Shoreline Management. Shorelands and Environmental Assistance Program, Department of Ecology, State of Washington. Publication #23-06-006.
Key Progress Indicator Results
At the end of the baseline period (2024), there were 550,417 known OSS in local health jurisdictions (LHJs); 86,425 of these OSS were in marine recovery and sensitive areas.
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Between June and December of 2024, there was a net change of +38,990 OSS across LHJs.
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In December of 2024, 469,397 OSS also had appropriate permits (85 percent of the total known systems). In marine recovery and sensitive areas, 76,594 OSS were permitted (89 percent of the total systems in these areas).
Methods
- Monitoring Program
Puget Sound local health jurisdictions with on-site sewage system management areas.
- Data Source
Compilation of Puget Sound local health jurisdiction data which are submitted to Washington DOH biannually.
This Progress Indicator tracks the number of documented, or “known”, on-site sewage systems (OSS) in each Puget Sound local health jurisdiction (LHJ).
LHJs in the Puget Sound region are required by Chapter 246-272A-0015 of the Washington Administrative Code (WAC) to establish local management plans describing how they will maintain inventories, identify areas where OSS can pose a risk, assess operations and maintenance requirements for areas at risk, provide homeowner outreach, and operate operations and maintenance (O&M) programs.
O&M programs collect data on OSS and inspections to protect public health and Puget Sound water quality [1]. Data management processes vary across LHJs, including practices to identify systems across Puget Sound. This indicator relies on the number of OSS permits and other documentation to identify the number of known OSS in their area.
Data are collected by LHJ staff and submitted via survey reporting form to the Washington State Department of Health (DOH) on-site sewage management program on a biannual basis. This form asks each LHJ to document their numbers of known systems. Data are then transferred from DOH to Puget Sound Partnership (Partnership), where Partnership staff manually review LHJ entries for anomalies and confirm values with DOH and LHJ staff as warranted. Data are collected at the county scale in many LHJs, though some are still working towards this goal. Documented systems within marine recovery areas and sensitive areas (sub-areas within LHJs with enhanced management requirements) are also provided in this report.
Known systems may not be permitted nor regularly inspected. Regular inspections ensure proper maintenance of OSS and correction of equipment issues or failures. Despite this, OSS inventories help LHJs track compliance with inspection requirements to ensure OSS are properly functioning.
LHJs submit biannual reports which illustrate a snapshot of OSS data at the time of reporting. This means that OSS may be double counted if multiple reports are summed together. We avoid this by reporting numbers attached to specific report submittal dates and by reporting net change over time.
Current reporting period: 2024
Baseline period: 2024
Each reporting period for this indicator is one calendar year; each calendar year includes two biannual reports submitted by LHJs covering January through June and July through December, respectively. We will evaluate trends over time by comparing the regional indicator value of the most recent reporting period (one year) to the regional value of the baseline period (2024). 2024 is selected as the baseline period because it is the first year for which LHJs reviewed local methods and implemented widespread updates to their data systems.
Limitations :
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Data comprehensiveness and quality varies by LHJ, depending on available resources. LHJs are often understaffed and under-resourced, which can impact their ability to comprehensively assess the inspection status of OSS, both within marine recovery and sensitive areas and across their full county. Beginning in 2022, LHJs completed a significant effort to update databases and data systems to provide the data reported here. Most database updates are still in progress as of July 2025, and staff continue to work with technology staff to develop workflows that allow them to query this data accurately. Dedicated and sustained funding to LHJs can help improve the quality and comprehensiveness of local data.
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Data are currently non-spatial. This impacts our ability to understand the relationship between water quality and OSS management. We can note which OSS are in marine recovery or sensitive areas, but not specific locations therein. Improving the spatial data capabilities of LHJs is a regional priority.
[1] Washington State Department of Health, (n.d.). On-site Sewage Systems (OSS). Wastewater Management, Department of Health, State of Washington. Accessed at https://doh.wa.gov/community-and-environment/wastewater-management/site-sewage-systems-oss#:~:text=Small%20on-site%20sewage%20systems%2C%20also%20known%20as%20septic,flows%20of%20less%20than%203%2C500%20gallons%20per%20day.
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- Critical Definitions
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On-site sewage system (OSS): Integrated system of components, located on or near the property it serves, that stores, treats, and manages the dispersal of sewage [1].
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Small OSS: An OSS with flows less than 3,500 gallons per day [2]
[1] Chapter 246-272A WAC: (n.d.). https://app.leg.wa.gov/wac/default.aspx?cite=246-272A&full=true.
[2] Shellfish Beds Interdisciplinary Team, Puget Sound Partnership, Washington State Department of Health. (2023). Shellfish Beds Implementation Strategy: Protect and Restore Shellfish Beds in Puget Sound. https://pspwa.box.com/s/d3u2rf092imo9pt4foqgrw3m6p6rj9yv.
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Interpretation of Results
This Progress Indicator report represents the first year for which we have improved data on OSS management. We thus apply the “Limited Data” designation to this indicator and will address indicator trends over time as we compile additional years of data.
At the end of the baseline period (2024), there were 86,425 known OSS in marine recovery and sensitive areas.
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Between June and December of 2024, there was a net increase of +22,744 known OSS in marine recovery and sensitive areas.
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In 2024, a larger proportion of systems were permitted in marine recovery and sensitive areas compared to the full local health jurisdictions (LHJs).
Figure 2. Number of permitted (blue column sections) and unpermitted systems (yellow column sections) across Puget Sound local health jurisdictions and marine recovery and sensitive areas in June and December of 2024.
The number of known OSS varies by county across Puget Sound (Figure 3).
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At the end of 2024, Pierce county had the highest number of known OSS, followed by King and Snohomish counties. Jefferson and San Juan counties had the fewest number of known OSS.
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At the end of 2024, Whatcom county had the largest number of known OSS in marine recovery and sensitive areas. King county had the smallest number of known OSS in marine recovery and sensitive areas.
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These observations do not reflect the populations of these counties nor the sizes of marine recovery and sensitive areas, which likely vary across LHJs.
Figure 3. Number of known on-site sewage systems across local health jurisdictions (blue columns) and within marine recovery and sensitive areas (orange columns) at the end of 2024, by county.
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Puget Sound local health jurisdictions dedicate significant time and resources to developing and maintaining inventories of OSS.
Trends in this indicator are affected by:
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Dedicated and sustained funding for LHJs to identify and inventory OSS,
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Technical capacity and suitable software for LHJs to manage and update OSS inventories,
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Housing development, especially housing growth outside of urban areas where reliance on OSS is more likely, and
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Feasibility to extend sewer lines or design large community OSS within or near urban areas.
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Overview of OSS and homeowner resources, Department of Health (DOH)
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Septic system basics, DOH
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On-site sewage systems, Washington Administrative Code Chapter 246-272A
Datasets
No datasets uploaded.
Reporting Guidance
- Reporting Instructions
Subcategories
No Subcategories for this Puget Sound Indicator.