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Thurston County On-Site Sewage System (OSS) Management Plan Update
- Greene Team
- 3 days ago
- 1 min read
Managing 55,000+ septic systems to protect public health, drinking water, and Puget Sound watersheds
Thurston County manages over 55,000 on-site sewage systems for homes and businesses not on public sewer. In April 2025, new state code (WAC 246-272A) required updating the 2016 Thurston County On-Site Sewage System (OSS) Management Plan. Greene Economics was retained by the Environmental Health Division to lead the update, which aims to protect community drinking water and the long-term health of lakes, rivers, streams, and Puget Sound.
Our team reviewed the 2016 plan and assessed its implementation. We analyzed water quality across marine inlets, shellfish growing areas, rivers, lakes, and aquifers—including Department of Health shellfish records and aquifer vulnerability maps—to identify OSS-related pollution, often indicated by elevated fecal coliform, nutrients, and E. coli. Interviews with septic professionals, shellfish groups, tribal representatives, realtors, and county staff identified high-risk areas and local priorities. We also worked closely with OSS staff to integrate new state and county code requirements.
The updated plan presents seven focus areas, including septic-to-sewer planning, sensitive area designation, climate planning, and funding strategies. Key actions recommend Eld Inlet for Marine Recovery Area designation, recognize the Deschutes River watershed as protected, and evaluate Summit Lake for sensitive area designation. The plan also calls for parcel-level OSS GIS, a countywide database with dashboards, and a dedicated climate response strategy—plus updated inspection standards, quality assurance, and homeowner outreach.
These updates strengthen Thurston County’s management of high-risk OSS, protect public health and water quality, and address climate impacts—including flooding and sea-level rise—while supporting marine water quality and shellfish areas.
