Poking the science underlying Lake Mitchell's yuck

"'Good grief!'" The farmer's remark came as he learned Lake Mitchell sediment contained nearly as much phosphorus as composted manure on his farm. That's when it hit, said Paula Mazzer, Dakota Wesleyan University professor and a fellow member of ...


"'Good grief!'"

The farmer's remark came as he learned Lake Mitchell sediment contained nearly as much phosphorus as composted manure on his farm.

That's when it hit, said Paula Mazzer, Dakota Wesleyan University professor and a fellow member of the lake Technical Advisory Team Committee: "We have almost manure-rich sediment in our lake."

Mazzer agreed for this article to explain the science of Lake Mitchell's toxic blue-green algae problem, a result of all that phosphorus.

It's not her job, she emphasized, to recommend solutions. That's the city's role.


Mazzer hails from Michigan and did postdoctoral research with the Environmental Molecular Science Institute at Ohio State University.

She taught previously at Old Dominion University in Norfolk, Va., and considers herself a bio-analytical chemist with a strong environmental focus. People with her background, she explained, develop tools like those used for liponic testing-let's all agree she understands lake science.

Lake Mitchell, Mazzer qualifies, is an engineering issue rather than a science problem, but her background allows her to grasp the issue en masse. Many, she observed, have been picking at it in pieces.

Mazzer joined a city advisory panel after Mitchell had already selected Fyra Engineering for its lake consultant. She expresses this one unscientific opinion: The Omaha firm has done a better job of working the science than it has communicating it.

For example, Fyra has yet to recommend a solution for Lake Mitchell, she said. So what has Mitchell been debating these past months?

Her answer: Spitballs.

A lake recommendation was not in the scope of Fyra's initial contract, Mazzer said. What Fyra was paid to do-and what it has done, she said-is populate a computer model to accurately reflect the interaction of numerous lake properties.

So if one thing changes in the lake, for example, what will happen to this other factor, and that one and the other thing?


Think of the model as a black box for testing lake hypotheses. Assuming the model is good, you could electronically simulate dumping a ton of alum into Lake Mitchell to bind its polluting phosphorus. You could also, of course, dump real alum - as Mitchell did at some expense and with limited success-but it's simpler and cheaper to experiment electronically, Mazzer said.

"The thing about a model," Mazzer said, "is you can test things virtually, tweak the data and see what happens."

Once Fyra completed its model, Mazzer said, Mitchell's lake committees pushed it to crunch preliminary estimates.

The $7.2 million phase-one lake dredging project offered up by Fyra was one of those spitball solutions, Mazzer said. It was more a spitball rather than a recommendation, she said.

On April 2, the Mitchell City Council considered offering Fyra a $385,000 contract to use its model and flesh out the most cost-effective lake solution.

The proposal ran into heavy suspicion. Concerns had grown over Fyra's failure to previously pursue proper South Dakota licensing, confusion over the nature of Fyra's preliminary spitballs, consternation over $18,000 in extra testing performed last February, plus overall project sticker shock.

As the evening's council meeting grew long, officials appeared to weigh their previous antes of public time and money against stepping away empty-handed with lingering doubts. After a lot of hemming and hawing, members stared at its poker table of blue-green toxic algae and plopped down $80,000 of the proposed $385,000 to get better answers.

In mid-June, Fyra will return with a much-refined spitball - still not an engineering plan. Spitball 2.0 will offer up what Fyra expects "most likely" will be the final recommendation, Mazzer said, as well as its "most likely" costs.


If the Council moves forward, engineering for Fyra's actual recommendation will require the remaining $305,000. Mazzer expects Fyra's plan will "most likely" involve a complex solution, requiring both the dredging of polluted lake sediment plus engineered caps to overlay deeper deposits.

Creating a black box

It's easy - for Mazzer, anyway - to formulate calculus equations showing how the changes in one lake factor affect another one. It's another thing, she said, to create a model reflecting how changes in one factor affect every other factor, and how they, in turn, influence each other.

What Fyra did, Mazzer said, is measure every flux - lake evaporation, spillway outflow, spring inflow, watershed size - and work its model until the changes there match closely with observed lake conditions.

Fyra used all of the historical lake data collected over its 90-year history to validate the model, she said. The data includes measurements collected over the years by students of dedicated Mitchell High School instructor Julie Olson, Mazzer said. It also includes measurements taken by Dakota Wesleyan University students enrolled in limnology, the study of freshwater bodies.

Supplementing that data with its own spot measurements, she said, Fyra created a model that closely reflects real Lake Mitchell.

"They've created a model with a lot of confidence," Mazzer said.

The additional $18,000 spent in February helped validate the model.


Phosphorus on the bottom of Lake Mitchell exists both in a loosely bound form and in a tightly bound one-locked with iron molecules.

Phosphorus binds with both the iron chemical form Fe3-household rust - and its alternative Fe2. In most lakes, phosphorus binds with one form or the other. In Lake Mitchell it binds with both. That became part of February's testing.

Under low oxygen conditions, iron releases phosphorus back to water easily, Mazzer said, but the two forms release it at different rates, so the tests will be used calculate an overall rate.

February's main testing objective, however, was the use of sound waves to accurately calculate sediment depths. Sonography works best under a lake capped with ice.

Deep core samples were drilled to verify the sonography findings, Mazzer said and they revealed loosely and tightly bound sediment layers where Firesteel Creek flows into the lake. Those alternating layers had thrown off the sonogram profile, so the discovery made the black box that much better.

Phosphorus as algae handle

The ultimate goal is to use the black box to reduce lake phosphorus - the handle for tackling toxic blue-green algae.

Phosphorus availability limits the potential of plant growth regardless of whether other nutrients are present.


Evolution endowed toxic blue-green algae with an advantage to exploit the tons of phosphorus available in Lake Mitchell. Each summer, algae blooms spread like sickly green paint spills across the lake surface and then die, releasing phosphorus back to the water for a subsequent bloom.

To ease its own return, this algae creates toxins to kill off competing plants and fish, enabling it to feast alone and create increasingly bigger blooms.

"The toxin kills everything in the lake except the algae," Mazzer said. It's toxic to human and animals as well.

"To break that cycle," she said, "you have to drastically reduce the nutrients."

Half and half

Roughly half of Lake Mitchell's phosphorus is new, pouring into the lake every spring with Firesteel flooding. The other half has been cooked into lake sediment over the past 90 years and cycles repeatedly, taken up by plants in summer and released back to water and sediment in fall.

Phosphorus that will add to the coming bloom is even now pouring into Lake Mitchell on snowmelt scouring the extensive watershed of a winter's worth of livestock and wildlife waste. Nutrients from a winter's worth of dead vegetation is also entering with the water.

Snowmelt "may be flushing the lake," Mazzer said, "but it's flushing the lake with high-phosphorus water."


The phosphorus will concentrate once the weather warms. In June, evaporation will commence, the spillway will dry up, and rot will quicken.

Last summer, Mazzer scooped around green soup at one point on the lake to gather water with a phosphorus reading of 1,600. Healthy lake water - one with fish, plants and good water quality - would measure 80.

"Eighty," Mazzer said, "is a good target."

Science of money

A twenty-fold phosphorus reduction can't be achieved only by scraping polluted sediment from the lake's bottom. It can't be achieved just by minimizing the phosphorus flowing in on spring flows.

The model, Mazzer said, shows it will take a combination of both.

"We can't just tackle the sediment," Mazzer said. "We have to tackle both halves."

Fyra's push to tackle the lake sediment first, she said, relies as much on political science as it does on physical science. It involved political maneuvering to set the city up for federal funding to clean flows from the watershed.

When it comes to the watershed currently, Mazzer said, there exists little low-hanging fruit. Fyra's analysis shows most of the phosphorus entering on Firesteel arrives in the dissolved form rather than in soil erosion.

On average, she said, "Area farmers and ranchers are doing a good job of limiting soil erosion."

It makes good sense, she said.

"We've got buy-in from farmers and ranchers," she said. "It's in their own best interest."

There are a few individual operations that could be encouraged to do better, but even huge improvements there would have marginal effects overall, she said. The massive reach of the watershed and the small amount of water squeezed from every acre makes cleanup there a complex matter.

Efforts to improve the watershed must continue, she emphasized, to hit the target of 80. One spit-balled plan, Mazzer said, is to build a water cleansing impound to hold and treat spring Firesteel flooding upstream before it enters the lake. The impound would rely on a mix of marsh-like vegetation and water treatment to capture the phosphorus. The impound would likely be dry in summer.

By doing that and addressing lake sediment, "We bring concentrations down to around 80."

Should Mitchell choose an impound, Mazzer said, Fyra would help the city find external funding.

Federal money exists to help create the upstream impound, she said. There's little federal money available to tackle phosphorus already in the lake.

The lake, meanwhile, is under city control. It requires no purchase of land and cleanup runs athwart of fewer environmental rules.

The city's commitment, demonstrated by addressing the lake sediment, Mazzer said, would ease its path to obtaining federal funds for the long-term project.

"So if we want to get the other funding," she said, "we tackle the part we're going to end up paying for anyway."

Algae turns Lake Mitchell green near the Mitchell Prehistoric Indian Village on Tuesday. (Evan Hendershot / Republic)
A Lake Mitchell bay is filled with algae near the Mitchell Prehistoric Indian Village on Aug. 8, 2017. (Evan Hendershot / Republic)

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