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King County outlines $3.4 billion Mouth of Duwamish CSO program; seeks community input on SoDo impacts
Summary
King County staff told the Regional Water Quality Committee that the Mouth of the Duwamish wet‑weather facilities program — required by a modified federal consent decree — is estimated to cost $3.4 billion at completion and will likely include a wet‑weather treatment station in SoDo with a peak capacity around 240 million g.p.d.
King County Wastewater Treatment Division staff briefed the Regional Water Quality Committee Feb. 5 on the Mouth of the Duwamish wet‑weather facilities program — a suite of projects to address combined sewer overflow (CSO) discharges from five regulator stations serving SoDo, Beacon Hill and Rainier Valley. Stan Hummel, WTD’s combined sewer overflow delivery manager, told the committee the program is mandated by a modified federal consent decree and carries an estimated completion requirement of 2034.
Hummel said updated planning work and better alignment information have improved the agency’s understanding of required conveyance, outfall and geotechnical needs, but that several cost drivers are raising the program estimate. WTD’s current planning estimate for the program’s total cost at completion is $3.4 billion; Hummel said that equates to roughly $2.8 billion in today’s dollars. The program team is evaluating alternatives that include storage (tanks, tunnels, pipes), conveyance upgrades, or wet‑weather treatment stations similar to the Georgetown wet‑weather treatment station that began service in 2022. For the Mouth of the Duwamish, a wet‑weather treatment station is likely and would need a peak hydraulic capacity on the order of 240 million gallons per day, Hummel said — roughly three times Georgetown’s 70 million g.p.d. capacity.
WTD said key considerations in selecting a solution include schedule, permitting complexity (local, state and federal approvals), operations and maintenance needs for an intermittently used treatment facility, community impacts in the busy SoDo area (traffic, parking, business effects) and opportunities for community benefits. Staff noted SoDo site options could require acquisition of about 4 to 8 acres and permanent and temporary easements for pipelines and outfalls.
Hummel described program management approaches WTD is using to contain risk and cost: hiring a program manager/owner advisor independent of the engineering team, conducting value analysis and value engineering, performing geotechnical and environmental field work, and pursuing early contractor engagement and collaborative delivery approaches to improve cost‑certainty. He also cited broader market drivers — a 20–30 percent increase in construction material prices, labor escalation and competition among many concurrent large public projects — as factors raising program costs.
Committee members asked for follow‑up material on how the Mouth of the Duwamish work fits into the region’s wider CSO program (city vs. county projects), the program’s impacts on other capital priorities and rate forecasts, and technical questions about the composition of peak flows (share of infiltration/inflow vs. sanitary sewage). WTD staff and council staff committed to provide follow‑up materials and graphics at the March briefing on capital program delivery.
