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Green Lake alum treatment begins April 4

Seattle Parks and Recreation will treat Green Lake with aluminum sulphate (alum) from April 4-18 to reduce phosphorous levels.

Seattle Parks and Recreation awarded the contract for the alum treatment to HAB Aquatic Solutions. The contractor will close the parking lot north of the Green Lake Small Craft Center, 5900 West Green Lake Way N. to stage alum treatment equipment. Staging and treatment is anticipated to take two weeks beginning Monday, April 4 and be completed by Monday, April 18, 2016.

The 2016 alum treatment is intended to reduce phosphorus levels in the lake, limiting cyanobacteria (blue-green algae) blooms and improving recreational use of the lake. The alum treatment is applied from a barge on the lake. The treatment is pumped from a truck staged in the parking lot through hoses that will run over the Green Lake path to the barge. These hoses will require a small ramp to allow Green Lake path users to travel safely over them.

The alum will be applied near the water surface and it removes phosphorus from the water column as it forms small clumps and settles. It then covers the bottom sediments to further prevent the internal release of phosphorus from the sediments. The alum treatment is effective immediately upon application.

Previous lake-wide alum applications in 1991 and 2004 effectively limited algae blooms and improved water quality. The alum treatment is expected to be effective for up to 10 years.