Tempe Town Lake

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Tempe plans to raise quality of Town Lake
By Garin Groff, Tribune
December 26, 2006

Tempe will begin diverting enormous amounts of water around its Town Lake this spring to keep bad water from fouling swimming events that draw people from across the nation.

The city plans to pump water after a year when bad water quality forced Tempe to cancel one event and change two others.

Tempe has already tried pumping water and treating it with chemicals. Neither did the trick.

The latest pumping effort will carry much more — 15 million to 20 million gallons a day — and should ensure swimmers aren’t left high and dry again, said Nancy Ryan, Rio Salado project manager.

“We do have a high level of confidence that it’s not going to be a problem,” she said.

The issue is a smaller lake that’s formed east of Town Lake. It spills dirtier water into the larger lake, fostering algae growth and making the water unsafe for swimming.

The pumping should lower that smaller lake by perhaps two feet and make it easier to control pH levels, said Basil Boyd, a water resources hydrologist for the city.

The city monitors pollution levels by testing the water’s pH, a measure of acidity in the water. Tempe considers the lake safe for swimmers if pH levels are 9.0 or lower. If levels exceed that, there are health risks, from eye irritation to bacterial infection.

Tempe tried pumping water earlier this year through a storm drain on the lake’s south side. Most of the pipe is large, but a 12-inch section on the east end restricted the flow to 3.5 million gallons a day.

That was a relative drop in the bucket compared with how much water pools to the east. The water comes from storm drains that carry runoff from streets in several cities, from mining operations in the river bed and from a reclamation plant in Mesa. Those sources have higher pH levels than the lake’s source, delivered through Salt River Project canals.

The smaller lake formed after the normally dry Salt River flowed in 2005. When the river stopped flowing, the small lake didn’t go away — much to the surprise of water experts in Tempe and Mesa.

“We don’t know why it’s still there,” said Bill Haney, director of Mesa’s water division.

Mesa will eliminate the water it puts in the river by February or March. That amounts to 4 million gallons a day, which will instead be pumped into an underground storage project. That won’t put a dent in the small lake’s size, Haney said, because the storm drains are vastly larger sources of water.

But Tempe’s pumps will divert enough water, Boyd said. The effort will cost about $600,000 a year for pump rental and fuel. The pumps will run from March through May and again from September to November, when triathlons and other swimming events are held. The city won’t use the pumps in winter or summer, allowing the water to flow into the lake at a time when swimmers don’t use it.

Tempe relied on chemicals this year to lower pH but had mixed results. The city moved one event to a pool and canceled another because the pH rose above 9.0, the federal safety standard. The treatments lasted about a week and dropped the levels into the safe range.

Even with pumping, Basil expects the pH will get too high at times. But by eliminating the flow of dirtier water when swimmers are in the lake, the pH will stay lower and treatments will last longer. “You’re going to require treatments perhaps a couple times each swimming season,” Boyd said.

Contact Garin Groff by email, or phone (480) 898-5938