Started March 18, 2004
Completed April 7, 2004

In March of 2004 Approximately 10,000 gallons of aluminum sulfate (alum) were pumped into Greenlake. Alum is a chemical compound often used in wastewater treatment and sometimes as a food additive. Because alum on its own would make the lake too acidic to support aquatic life in the lake, about 1,000 gallons of a second chemical, sodium aluminate, were applied simultaneously to 'buffer' the alum, maintaining near-neutral pH in the lake. The addition of alum and sodium aluminate to the lake posed little risk to humans, aquatic plants, and wildlife.

When the chemicals are added to the water, a flaky white solid 'floc' forms. Aluminum in the floc will chemically bind phosphorus dissolved in the water, and then sink to form a layer on the bottom that will limit internal phosphorus recycling from lake sediments. The treatment resulted in dramatic improvements to the water quality and appearance of the lake.

Greenlake is now very swimmer friendly. Algae growth and closures were sometimes a problem before the City of Seattle performed a comprehensive cleanup. It is now one of the cleanest public swimming areas in the Seattle area.

King County's Swimming Beach Monitoring Program shows current conditions at several swimming beaches, and Greenlake's water quality in particular.