Sections

Weather Forecast

Close

Lake Mitchell restoration report arrives

Lake Mitchell in the summer of 2017. (Evan Hendershot / Republic)1 / 4
A photo of the proposed by-pass system to improve water quality at Lake Mitchell. (Fyra Engineering graphic)2 / 4
Scenario 2 cost estimates from Fyra Engineering for Lake Mitchell restoration. (Fyra Engineering graph)3 / 4
Scenario 1 cost estimates from Fyra Engineering for Lake Mitchell restoration. (Fyra Engineering graph)4 / 4

The wait is over, but the cost estimates are unprecedented.

Mitchell city officials forewarned that the costs appearing in Fyra Engineering's Lake Mitchell restoration report are preliminary estimates and are by no means what will be presented to the City Council in January, but stacked side-by-side with other recent projects in Mitchell, the estimates to solve a decades old problem are stratospheric.

Two separate estimates, one for $87.01 million and another at $31.218 million, were the guiding proposals offered Tuesday at City Hall to the restore algae-stricken Lake Mitchell in the coming years. The early estimates will still need to go through a review process before a public meeting in January, and were part of a $73,725 report commissioned by the board in October 2016.

Aside from the larger cost solutions, the report also includes a proposal of an aluminum sulfate treatment that could cost as much as $6 million, "with the hope of decreasing this cost by using more viable alternatives."

It was also briefly mentioned that draining the lake could be beneficial, as it would make dredging Lake Mitchell easier and more effective. But a proposal to drain the lake was thrown out as more of a method to gauge public interest, Fyra's Mike Sotak said after the meeting. Sotak said draining the lake could make some portions of the restoration easier and more affordable.

As curious, but skeptical looks darted around Council Chambers, Fyra's Charles Ikenberry and Sara Mechtenberg said the cost proposals are there to offer the city some "perspective."

"It's realistic, but probably not feasible," Ikenberry said.

While the estimates may come with some sticker shock to Mitchell residents, city officials have repeatedly noted the city would likely be on the hook for 40 percent of the total cost. The additional 60 percent is expected to be funded through grants and other available monies.

The first restoration scenario, which comes with the early estimated price tag of $87.01 million, would focus on watershed-based practices to reduce the nutrient load within the lake by 50 percent. Nutrients and phosphorus are major contributors to algae growth within the lake — algae which turned Lake Mitchell's bays into a thick, green, odorous soup last summer.

The second scenario, at an estimated $31.218 million, would utilize a "by-pass system," retention system and an alum injection. The by-pass system, as Fyra calls it, would divert nutrient-rich water away from Lake Mitchell.

The water could be caught in something similar to a retention pond, and could then be sent to the James River through a pipe beneath the lake. Mechtenberg acknowledged the lack of conservation in this approach, but Sotak said the water ends up in the James River regardless of whether it's pumped directly to the river or goes through Lake Mitchell.

The second scenario is designed to "reduce the volume of water and the associated pollutant load delivered to the lake, and to primarily only allow alum treated water to reach the lake on a regular basis."

When it's all said and done, Sotak said paring scenario two down to around $20 million is within the "realm of possibility."

While the preliminary proposals come with hefty cost estimates, they also come funding opportunities. Among others, Fyra lists the following agencies as possible sources of funding: Natural Resources Conservation Service (NRCS); S.D. Game, Fish & Parks; Ducks Unlimited; S.D. Department of Environment & Natural Resources; James River Water Development District; Environmental Protection Agency; and Pheasants Forever.

Mitchell committees will comb through the report in the coming weeks, and if the City Council ultimately decides to move forward with Fyra, phase one implementation could begin as early as next summer.

The next steps, Sotak said, would be to continue acquiring more data, optimize performance compared to dollars spent and fine tune the alum injection needs.

After reviewing the long-awaited preliminary report, Lake Mitchell Advisory Committee member Mike Vehle was encouraged.

"I think it's good to have all kinds of information," Vehle said after the meeting. "They've done a lot of research, and in order to have a comprehensive solution you've got to have a lot of data to get there."

Causes and solutions

According to Fyra's report, 66 percent total phosphorus flowing into Lake Mitchell is in the dissolved form, meaning it's dissolved in the water. Common conservation practices are designed to reduce semi-attached phosphorus.

Phosphorus enters the lake at the highest concentrations during high-flow events, which Fyra believes is important in considering management practices and scenarios. Fyra officials added that approximately half of the nutrient loading is from the watershed, and the other half is from within the lake.

Fyra also noted that the water quality issues of pollutant loading and algae blooms date back to studies published in 1985.

With these issues in mind, here is a brief rundown of potential solutions in Fyra Engineering report.

Solution: Land practice improvements.

Pros: This includes conservation practices, which would be aimed at reducing runoff into the Firesteel Creek.

Cons: Due to the size of the watershed, the net effect may not be immediately noticed.

Solution: Livestock management practices.

Pros: Could reduce the amount of manure potentially entering the waterways.

Cons: Costs vary widely, and implementation is voluntary by individual landowners.

Solution: Farm ponds/wetlands.

Pros: NRCS and Ducks Unlimited funds could be available for cost assistance.

Cons: Since a large portion of the phosphorus in the runoff is in the dissolved form, farm ponds "will not provide the effectiveness in this watershed compared to a similar application" compared to a watershed under other conditions.

Solution: Regional detention.

Pros: This alternative would not completely solve the issue. It could, however, be a key component in achieving goals.

Cons: Significant land rights or easements would need to be acquired.

Solution: Large wetland complex.

Pros: A large wetland complex at the mainstem of Firesteel Creek near Lake Mitchell could "achieve substantial nutrient uptake (at) the beginning of its lifetime."

Cons: Significant land rights or easements would need to be acquired.

Solution: Lake by-pass system.

Pros: Nutrient-rich flows would be diverted away from the lake.

Cons: More information is needed on the source and timing of the release of nutrients.

Solution: Phosphorus inactivation.

Pros: "Alum is one of the few management practices that is able to treat the dissolved phosphorus portion of the load."

Cons: Alum has been used at Lake Mitchell before.

Solution: Localized dredging.

Pros: Could be used to target the removal of soft sediments that are contributing to high internal load rates.

Cons: The lake has been dredged before in the 1980s.

Solution: Shallow vegetation management.

Pros: Increasing aquatic vegetation adds direct competition with algae for nutrients.

Cons: This can require a lot of shallow depths, which Lake Mitchell does not have.

Solution: Lake segmentation.

Pros: Enhances aquatic vegetation and raises water levels.

Cons: It's challenging to identify a portion of the lake that could be segmented off without public disapproval.

Solution: Hypolimnetic aeration or withdrawal.

Pros: Installs bubblers to inject oxygenated air into the water column.

Cons: The impact on Lake Mitchell is unknown.

Solution: Phosphorus inactivation.

Pros: Whole lake treatments can be "very effective" in reducing phosphorus if "dosed properly."

Cons: Could use aluminum sulfate, which has been used before.

Solution: Algae harvester.

Pros: Instead of preventing algae growth, this could be used to remove algae during "the worst of the blooms."

Cons: Could cost "a few to several hundred thousand dollars."

Solution: Pumping from the James River.

Pros: It is feasible to consider.

Cons: There are complications with water rights, and this was not pursued by Fyra.

Advertisement
randomness