Seattle local business news and data – Construction


March 28, 2022

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The innovative water treatment plan developed by Tetra Tech has successfully restored the water quality of Lake Ketchum.


Project: Ketchum Lake Restoration

Client: Snohomish County Surface Water Management

Lake Ketchum, a beautiful 26-acre lake located in northern Snohomish County a few miles from Puget Sound, is home to approximately 60 residents and is also heavily used by the public for swimming, fishing, boating and aesthetic pleasure. The lake provides critical habitat for various fish, amphibians, birds, and other wildlife.

As development increased in the Lake Ketchum watershed, its water quality deteriorated markedly until it eventually became the most polluted lake in Snohomish County and one of the worst in the state. Harmful algal blooms have plagued Lake Ketchum, fueled by extremely high levels of phosphorus in the water. Prior to restoration, phosphorus levels in Lake Ketchum were 13 times higher than regional standards and among the highest in Washington State. At times, phosphorus concentrations in the lake bottom waters (hypolimnion) were similar to those in the raw sewage.

Through continued collaboration between Snohomish County and the Ketchum Lake community, with technical support from Tetra Tech, the lake’s water quality has been successfully restored.

The main source of phosphorus runoff was an old dairy farm that drained into the Ketchum Lake inlet creek. The release of phosphorus from lake sediments accounted for 73% of the total annual load to the lake, with the primary inflow accounting for 24%.

In 2010, Snohomish County initiated a project to develop an actionable restoration plan for Lake Ketchum. Tetra Tech was selected to design a lake study, perform data analysis, develop a lake phosphorus model, and identify the best and most cost-effective restoration options. The county collected data during a year-long study on the sources and amounts of phosphorus entering the lake. Modeling showed that conventional management options would not achieve project objectives or were impractical due to their high cost.

The innovative final restoration plan included a comprehensive alum treatment of the lake sediments to inactivate phosphorus in the sediments, combined with a new approach of using annual alum treatments of the water column to remove the phosphorus entering the lake from external sources.

The use of small annual maintenance treatments each spring to inactivate large amounts of external phosphorus pollution has redefined the use of alum as a lake management tool. The conventional use of alum applications in lakes was primarily to inactivate phosphorus in lake sediments, thereby preventing internal loading into the lake. External water sources were controlled or otherwise treated.

Although it may seem simple, Tetra Tech’s innovative solution of performing small treatments over several years to combat external loading was a first. Careful monitoring and data collection were essential to determine the effectiveness of the project. Led by Tetra Tech, the county conducted monthly monitoring of physical, chemical, and biological changes in the lake and lake inlet to assess treatment impacts. Seven years after initial treatment, including five years of low-dose treatments, the project has proven to be highly effective in meeting and exceeding all project goals to reduce phosphorus levels, nearly eliminating potentially toxic algal blooms .

County monitoring of the project was instrumental in demonstrating this success to ensure other lake managers were aware of this innovative concept. The results have been shared many times over the past few years at local, state and national lake management conferences and on the county’s website. In addition to advancing technical thinking, the Ketchum Lake project has improved public perception of technical solutions to restore a natural body of water. Despite initial concerns, the community is thrilled with the results and continues to support the project financially for the long term. The lake is teeming with recreational activities, including fishing, boating, and swimming.

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