Concentration of high phosphorus levels in Lake Mitchell sediment catches engineer team by surprise

The investigation into the sediment concluded that the highest concentration of phosphorus was found in the northwest portions of the lake, especially near the west end bridge.

Algae in Lake Mitchell. (Matt Gade / Republic)
Algae in Lake Mitchell during the summer of 2020.
Republic file photo

MITCHELL — An engineering team that’s been putting together a dredging design to curb Lake Mitchell’s algae woes was in for a bit of surprise when they discovered just how high some of the sediment's phosphorus levels are.

Eric Lund, an engineer with Minnesota-based Barr Engineering, has been leading the lake dredging design work over the past two years and provided results of the second design phase during Monday's Mitchell City Council meeting. After analyzing the sediment along the lake bottom, it helped Lund and the team of engineers develop project recommendations to improve the lake's water quality that's been hampered by algal blooms.

"The water quality has continued to degrade over the past several decades. Removal or treatment of the sediment in the lake would be expected to reduce that internal loading, similarly (Firesteel) watershed improvements would be expected to reduce that external loading," Lund said of his recommendation to do both in-lake and near-lake projects in the Firesteel watershed.

To determine the levels of phosphorus – known for causing algal blooms – coming from the sediment, Lund said samples at multiple areas of the lake were collected. The results revealed the soft sediment situated toward the surface of the lake bottom was releasing alarming amounts of phosphorus compared to the deeper native sediment.

“We did a column release study where we bring the sediment into our lab and try to create different types of conditions, and then measure over time how much phosphorus is being released into that sediment,” Lund said. “The release rate for that soft, accumulated problematic sediment that’s come and settled at the bottom of the lake – the average release over two weeks of these 12 different samples was about 4,400 parts per billion.”


Lund said the native sediment only released 145 parts per billion (ppb) of phosphorus in the same time period, which he noted was 10 to 20 times less than the surficial accumulated sediment.

The engineers’ investigation into the sediment concluded that the highest concentration of phosphorus was found in the northwest portions of the lake, especially near the west end bridge where Firesteel Creek flows.

Shown here is the portion of Lake Mitchell's shoreline along the west end bridge where phosphorus levels are extremely high.
Sam Fosness / Republic

Considering the city has identified Firesteel Park as a spoil site where the dredged sediment would be disposed of, the high concentrations of phosphorus-filled sediment being located on the northwest side of the lake would reduce the length of sediment hauling.

How much phosphorus needs to be reduced for clean lake?

After compiling the data from the sediment evaluations, Lund provided several lake improvement scenarios for city leaders to consider.

The scenario that calls for reducing 50% of the phosphorus in the lake, paired with a 50% reduction in phosphorus in the Firesteel watershed, estimated Lake Mitchell’s phosphorus levels would dip to 430 ppb, well below the 870 ppb of phosphorus that the lake has been experiencing.

The graphic claimed reducing 50% of the phosphorus combined with an 80% phosphorus reduction in the Firesteel watershed would dip the lake’s phosphorus levels to 280 ppb.

The do nothing on in-lake solutions scenario, paired with a 50% reduction in phosphorus in the Firesteel watershed would decrease phosphorus levels to an estimated 680 ppb, marking a decrease of roughly 200 ppb from the existing levels.

“Doing either will reduce total phosphorus concentration. Without both, it seems very unlikely that water quality targets would be consistently achieved,” Lund said. “Continue to invest in watershed improvements.”


Green algae flows along the western bank of Lake Mitchell just north of the amphitheater on Wednesday afternoon. (Matt Gade/Republic)
Green algae flows along the western bank of Lake Mitchell just north of the amphitheater.
Republic file photo

Another area the team of engineers dove into is whether the amount of carp in the lake are contributing to water quality issues. According to Lund, the electrofish survey that was conducted on the lake showed the volume of carp didn’t indicate the fish were contributing to Lake Mitchell’s water quality woes. He noted a drawdown of the lake water would result in a loss of fish and changes in the fishery.

“There’s going to be a change in the fishery. What that looks like is a conversation for discussion. I think there will likely be some loss and some that will leave,” Lund said, noting he hopes the change would ultimately be for the better of the body of water.

A key question the crew of engineers set out to answer is what should the lake’s water quality strive to be?

Lund said a provisional goal in previous studies was reducing the concentration of phosphorus in the lake to 90 ppb, which was estimated to result in a 50% frequency in algal blooms. In recent years, the lake has seen phosphorus levels fluctuate around 900 ppb, according to the data presented.

“Folks have acknowledged 90 is a hard goal to attain. Over the time period we have been trying to address these issues, the lake quality has continued to get worse,” he said, noting the 90 ppb goal was made over a few decades ago when water quality wasn't nearly as poor as it is now.

The data Lund provided during Monday’s meeting showed phosphorus levels have consistently spiked over the past few decades.

“In 2002, the annual average total phosphorus concentration during that summer was about 150 parts per billion with an average mean of 300. Fast forward to the last six years of data, we’re looking at about 600 to 1,200 parts per billion with a mean of 900 parts per billion,” Lund said of the increase in phosphorus levels over the past two decades.

Firesteel Creek – which flows into the lake – has also seen increases in phosphorus levels over the years, the report showed. Numerous studies have claimed the creek is one of the biggest contributors of the lake's algae problems, as it unloads agricultural runoff and nutrients into the lake. It's also why the city has been focusing on Firesteel watershed improvements.


Council member Steve Rice asked how much impact the city’s 30-acre wetland along Firesteel Creek will have on phosphorus levels entering the lake. Although Lund couldn’t provide a clear answer on the impact the project will have on phosphorus reduction, he speculated the wetland will “get you something.”

Mayor Bob Everson joined the presentation at that point and explained quantifying such a number is very difficult, but he indicated crews are attempting to analyze the wetland’s estimated impact.

Everson concluded the presentation by highlighting the city’s ongoing work to construct additional wetlands along the Firesteel watershed to reduce the phosphorus flowing into the lake. The city has brought another engineering firm, Houston Engineering, formerly known as Fyra, in the mix to help the Firesteel watershed improvement efforts.

“Things are looking good,” he said.

The design of the wetland that Ducks Unlimited has been working on for the past year will cover about 30 acres of land on the former Kelley property, where the wetland will be constructed to reduce the phosphorus and sediment flowing into the lake.

While the team of engineers recommended both an in-lake and watershed solution, the council has yet to approve a Lake Mitchell dredging project as an in-lake solution. Previous cost estimates of a dredging project hovered around $20 million.

Sam Fosness joined the Mitchell Republic in May 2018. He was raised in Mitchell, S.D., and graduated from Mitchell High School. He continued his education at the University of South Dakota in Vermillion, where he graduated in 2020 with a bachelor’s degree in journalism and a minor in English. During his time in college, Fosness worked as a news and sports reporter for The Volante newspaper.
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