• Magnuson Beach closed after high concentrations of bacteria found

    By: KIRO 7 News Staff


    SEATTLE - Officials with the King County Department of Natural Resources and Parks have closed Magnuson Beach after finding high concentrations of bacteria in the water.

    Bacteria are measured as colony forming units (CFU), which is a count of the number of bacteria, per 100 mL of water (about a half-cup of water),” officials said.

    The beach was closed after water samples taken on May 20 showed high concentrations of E. coli. 

    Bacteria samples taken from the water were recorded at a value of 4733 CFU/100 mL, which is far above safe for swimming.

    Water with high concern will reach an individual day value of 1000 CFU/100 ml, officials said.

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    Officials said low concern or safe swimming water is usually around 200 CFU/100 ml.

    Three water samples are collected from different parts of a beach and officials use an average for determining the bacteria value for the day.

    Public health officials said a beach can also be closed if the average of the E. coli values for the three most recent sampling days is over 200 CFU/100 ml and is calculated using a geometric mean.

    To reopen a beach, two requirements must be met, according to the King County health officials post: 

    •    The three-day average (geomean) E. coli value is below 200 CFU/100 mL, and
    •    The daily E. coli values from the two most recent sampling days are both below 200 CFU/100 mL.

    The season for monitoring for bacteria has begun and routine monitoring for harmful algal toxins is expected to begin in June.

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