Are Lewiston sinkholes an issue for manure digester site?

Jul 23, 2023

by CHRIS ROGERS

The jury is still out on whether the prospective site of a manure digester — aka a biogas plant — in Lewiston is suitable. There are two sinkholes nearby, and an official from the biogas company, Nature Energy, said last Friday the firm is still evaluating the site's geology.

After scrapping earlier plans for a plant to convert manure and food waste into renewable natural gas and fertilizer in Wilson Township outside Winona, largely due to the risk of sinkholes, Nature Energy has been pursuing a plant somewhere in Lewiston. In April, the Lewiston City Council voted to tentatively support and explore the idea.

At a May 17 public information meeting, Lewiston resident Michelle Benke asked the question on everyone's mind: "Where are you putting this?"

Nature Energy Vice President of Project Development Bill Parmentier said city officials recommended the site of a former wastewater treatment plant south of town, which is currently a city park known as Prairie Trails Park. According to Minnesota Department of Natural Resources and Winona County maps, the roughly 60-acre site — about twice the area Nature Energy needs — has a sinkhole in the middle of the property and another immediately adjacent to the north.

"We are investigating that as a possibility. I can't say with any certainty that that is going to be the ultimate place because it may work out or it may not work out," Parmentier said. "… As you know, the topography of Southeast Minnesota is riddled with karst, and we know that and we take that seriously. And so, that's a very important part of our investigation."

Karst refers to Southeast Minnesota's uniquely porous geology, where holes in the bedrock allow sinkholes to form and make it relatively easy for pollutants on the surface to reach drinking water aquifers. That includes pollution from fertilizer and manure, which contribute to nitrate contamination in some wells in the Lewiston area.

The proposed biogas plant would handle millions of gallons of manure and fertilizer byproduct. The company said the resulting fertilizer could help reduce nitrate runoff and leaching because it is in a form that plants can immediately absorb, making it more conducive to precise nutrient management. However, some citizens have asked about the potential for a spill or accident. In 1991, the sinkhole at the former wastewater treatment plant site — which was then operational — opened up, draining an estimated 7.7 million gallons of semi-treated sewage into the groundwater.

Previously, Nature Energy had been planning to build its biogas plant on Winona Area Industrial Development Association property in Wilson Township, where there was also a sinkhole. In April, the company said it had scrapped those plans in favor of Lewiston. "… We have determined, after extensive geological testing and consulting with our local soil and geological experts, that the topography of the existing [Wilson] site is not suitable to safely build and operate a Nature Energy facility," Nature Energy's Bob Lefebvre explained in an April 14 email.

Parmentier echoed that, saying that after reviewing an initial handful of soil borings for the Wilson site, Nature Energy had conducted an additional 80 borings to make sure the site was suitable. "The karst was simply too high," Nature Energy Vice President of Business Development Jesper Nielsen said at the May 17 meeting. "… We did the first six borings: Everything looked good. And after we did 80 borings, we found out it was not safe to be there."

At the site in Lewiston, Parmentier said city officials shared borings conducted by a company that had previously explored developing the former wastewater treatment plant site but decided against it. He said that Nature Energy was looking at those results to begin evaluating whether the Lewiston site was suitable. Parmentier said the company might do further borings after that, as it did in Wilson, and, if the site proves unworkable, it might look for alternative locations in Lewiston. "This is our project. We want to live here. We need to find the right place," he said.

At the May 17 meeting, Nielsen described an initial determination as forthcoming. "We can say now that our construction people will have meetings this coming week," he said, "and we will determine if the information we have now will say ‘go’ or ‘no go’ for that site within the month of May. So it's not something that's going to happen over the summer or anything. We as developers want to have a clear answer to go ahead or not as soon as we can. And clearly because of our findings in Wilson, this is the focus right now — is that site suitable …?"

However, Nature Energy has not yet reached a "go or no go" determination. In a May 26 email, Nielsen said, "At this stage, we are still in the evaluation phase for the Lewiston site." He added that the company always thoroughly vets its sites and would share updates in the future.

As for the possibility of a different site in Lewiston, City Administrator Bobby Falcon mentioned in an email earlier this spring that there are other sites within city limits and adjacent to city limits that would be large enough to accommodate Nature Energy's facility. According to state officials, any site selected would need to go through an environmental study called an environmental assessment worksheet, which includes an opportunity for public comment.

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