Wapato Lake was formed approximately 15,000 years ago during the retreat of the Fraser Ice Sheet. The lake was likely formed as a basin in glacial drift, created by a block of ice which melted after the retreat of the main ice sheet. The area was once a heavily forested rolling plain. Vashon Till underlies the area.

Keeping Wapato Lake’s water clean has been a challenge throughout the park’s history. Early efforts included adding chlorine to cut down on algae growth. Then in 1936, as part of a Works Progress Administration project to improve the park, 180,000 cubic yards of mud was dredged from the lake hoping to clean it. By 1942, the lake was closed to swimmers due to unhealthful water conditions. Since that time a variety of other methods have been tried to keep the lake water clean such as harvesting aquatic plants, adding millions of gallons of fresh water, and even draining the lake in 1981 and removing tons of polluted sediment.

Regularly draining or flushing the lake with fresh drinking water, however, is neither financially nor environmentally sustainable. In 2012, Parks Tacoma and the City of Tacoma agreed on a scientifically based approach to improve the quality of the water in Wapato Lake. Wapato Lake is actually a giant catch basin, filled by stormwater runoff from the surrounding watershed. The water contains high concentrations of phosphorus and other nutrients which promote the growth of toxic algae, harmful to people and pets.

Scientists and engineers agree that the only way to consistently reduce the concentration of phosphorus is to effectively clean the stormwater coming into the lake and move the water through the lake at a faster rate. The most efficient way to channel more stormwater through the lake is to dismantle a bypass system set up at Wapato Lake in the 1970s (based on the mistaken idea that it would help). However, this is only part of the solution. It is important to remove phosphorus and other nutrients, which feed the algae, to improve the quality of water introduced to the lake. Part of the strategy is the cautious use of alum to bind the phosphorus into the sediment, thus making it unavailable for algae growth, without harming the lake’s wildlife.

Over the past several years, frequent blooms of blue-green algae in Wapato Lake have produced potentially harmful toxins. Tacoma-Pierce County Health Department, which tracks water quality, has issued intermittent health advisories to warn people and pets to avoid areas of the lake that may have algae.

In May of 2017 Parks Tacoma hired a contractor to disburse alum, or aluminum sulfate, in the water. The alum binds with phosphorus in water and sediment, forming a cap of aluminum phosphate on the lakebed. One application of the treatment is expected to be effective for about five years.

Completion Date

May 2017

Overview

Parks Tacoma treated the water in 2017 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.

Facts

• Alum treatment is a short-term solution
• Long-term solutions are being explored
• The treatment is safe for humans

What was done at Wapato Lake?
Parks Tacoma treated the water in 2017 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, prevents 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.

Does 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.

What is a long-term solution?
A long-term solution is complex and currently under study. In 2012, Parks Tacoma 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.

Why does Wapato Lake have water quality issues?
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.