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Wapato Lake FAQ

  • What was done at Wapato Lake?
    Metro Parks treated the water to ensure that it is safe for recreational opportunities, such as youth fishing, human-powered boating and paddle boarding, as well as for children to dip fingers or toes in.
     
  • What is the problem?
    The lake is polluted by phosphorus in sediment at the bottom of the lake, and stormwater runoff brings more phosphorus into the water. Phosphorus is a nutrient that promotes the growth of toxin-producing, blue-green algae when temperatures are warmer.

    The threat of harm from exposure to the toxins makes the lake periodically unsafe for recreation; the frequency of warnings from the Tacoma-Pierce County Health Department has increased over the past several years.
     
  • What’s the solution?
    Treatment with alum, or aluminum sulfate, will prevent algae growth by locking up the phosphorus. The alum binds with phosphorus in the water and in sediment and forms a cap of aluminum phosphate on the bottom of the lake.
     
  • When will this take place? How long will it take?
    Treatment took place on May 9-10, 2017
     
  • Will the treatment affect people?
    No.
     
  • Is this a permanent solution to the water pollution problem?
    No, but it is expected to benefit the lake for about five years.

    A long-term solution is complex and currently under study. In 2012, Metro Parks and the City of Tacoma, with the help of the University of Washington Tacoma, began to research ways to improve the quality of the lake water by flushing treated stormwater quickly through it. Tests of a possible treatment system are now underway on a small scale.
     
  • Wapato Lake water quality issue
    The 34-acre lake is an element of Tacoma’s stormwater detention and conveyance system. The North pond of Wapato Lake has two main storm water outfalls that collectively drain over 900 acres of residential and commercial properties; the majority of this water bypasses the main lake as it makes its way to Puget Sound via the Chambers Creek drainage network.  

    Some stormwater runoff from surrounding neighborhoods does filter into the lake. That means polluted runoff seeps into the lake and contributes to the phosphorus load that already exists in the sediment at the bottom of the lake. This has been a problem throughout the lake’s history. Most recently, this has resulted in warm-weather blooms of blue-green algae, which produces toxins potentially harmful to people and pets.

    Frequent warnings against exposure have been posted by the Tacoma-Pierce County Health Department.
     
  • Treatment plan
    Metro Parks plans to combat the immediate problem by limiting algae growth. To do this, Metro Parks will use chemistry as a barrier against phosphorus, in both the water and the sediment at the bottom. This will remove the nutrients that algae needs to thrive.
     
  • Chemical process
    The active ingredient is alum, or aluminum sulfate. In water, aluminum sulfate becomes aluminum hydroxide – often found in over-the-counter antacids.

    Aluminum hydroxide is heavier than water and will drop to the lakebed in the form of a fluffy flocculent, often called floc. The floc will bind with phosphorus in the water and sediment and form a blanket of aluminum phosphate on the bottom.  

    Aluminum phosphate is insoluble – it doesn’t dissolve – and prevents phosphorus in the sediment from leaching into the lake.
     
  • What steps has Metro Parks taken to ensure safety for wildlife at the lake?

    Metro Parks has hired Herrera Environmental Consultants to oversee the job. Herrerra has a track record of successful alum treatments of a number of Western Washington lowland lakes similar to Wapato Lake, including Seattle’s Green Lake. The company had no involvement in the 2008 treatment.

    Water quality, specifically pH and other variables, will be rigorously monitored by the University of Washington Tacoma during treatment. Herrera will oversee UW’s and the contractor’s work.