The water in Green Lake was clear when this picture was taken on January 20, 2005 (10 months after the alum treatment). The treatment is expected to provide up to 10 years resistance to toxic algae blooms.


The water in Green Lake was murky and smelly before the alum treatment. Because blue-green algae can be toxic, the city closed Green Lake beaches to wading, swimming, and wet-water boating in August 2003.

Seattle Mayor Greg Nickels with Friends of Green Lake on March 27, 2004. Friends of Green Lake led the successful campaign for the alum treatment.

Carp control at Green Lake
Learn about our monitoring program

Treatment began March 18, and was completed, on or near, April 7.

Treatment chemicals, prepared in British Columbia, were transported twice a day to Green Lake. Alum (aluminum sulfate) and buffer (sodium aluminate) were pumped into tanks on the treatment barge and sprayed into the water through PVC pipes from the rear of the barge. The barge operator guided the barge to designated treatment areas using a GPS (global positioning system).

During treatment, to prevent acidic conditions that could result in fish kills, environmental consultants monitored water pH and alkalinity and additional water quality parameters.

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©2009 Friends of Green Lake
PO Box 30544, Seattle, WA 98113

Webmaster - Ellen Hewitt
Updated October 12, 2009