Insights

Greene Economics led the 2025 update of Thurston County’s On-Site Sewage System Management Plan to meet new state code and strengthen public and environmental health protections for drinking water, shellfish beds, and Puget Sound. The plan targets vulnerable on-site sewage systems—especially in sensitive and flood-prone areas—enhances data and climate change planning, protects sensitive watershed, and supports more than 55,000 systems serving homes and businesses countywide.

King County has more than 85,000 on-site sewage systems, many nearing the end of their working life. Our cost and equity analysis estimates what it will take to address failing septic systems and shows the county where to focus sewer investment and financial assistance.