The cooperation stemmed from a requirement that Boise monitor mercury levels in fish downstream of its wastewater treatment plants, which discharge into the Boise River, which flows into the Snake. Methylmercury is a chemical component of water coming out of sewer plants.
Though the plants aren’t designed to treat mercury, most of it separates from the rest of the sewage because it is 20 times heavier than water, Boise Public Works spokesman Colin Hickman said in an email. In fact, tests indicate they remove 95 percent of the total mercury that flows into them from the city’s sewer lines, Hickman said.
The state of Idaho observes methylmercury standards with the goal of protecting human health. Besides brain and nervous-system damage, long-term exposure to methylmercury is suspected to cause human health problems such as cancer and poisoning of the blood vessel system, reproductive system and immune system, according to a study published in 2012 in the Journal of Preventive Medicine and Public Health.
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The Geological Survey set up sampling sites in the Boise River near Parma, Middleton, Murphy and at Eckert Road — upstream of the sewer plants, Hickman said. The agency also placed a monitoring site in the Snake River near Nyssa, just below the confluence of the Boise River near, and another one in the Brownlee Reservoir.
Samples collected from catfish near the confluence and smallmouth bass in Brownlee showed methylmercury concentrations that exceeded the state of Idaho water quality criteria, Hickman said. Samples taken in 2014 and 2015 showed no excessive methylmercury levels, he said.
Now, the city of Nampa, which lies downriver of Boise, is joining the mercury monitoring program. Nampa will pay around $4,300 per year for the program. The cost is based on the number of residents in the city.
Boise so far has paid more than $86,000, Hickman said. Part of that money went to the formulation of the monitoring plan, which other cities can copy. The Geological Survey is contributing two-thirds of the money Boise pays, Hickman said.