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Citizens worry sewer project will decrease flow to Coldwater Spring

Posted on 25 June 2018 by calvin

Replacing 90-year-old sewer tunnel under Minnehaha Creek that is in danger of leaking sewage into groundwater

By TESHA M. CHRISTENSEN
Will a sewer tunnel project in south Minneapolis affect how much water flows at Coldwater Spring?

Some people are worried that it will, and this has prompted the Metropolitan Council Environmental Services (MCES) to revise its original plan for the regional sewer interceptor tunnel. Yet some Coldwater Spring supporters don’t think the risk has been eliminated.

Photo rightt: Nobody knows where Coldwater’s source waters come from. Tom Holtzleiter of Preserve Camp Coldwater Coalition is pushing local government agencies to pinpoint the exact location of that fracture that feeds water towards the spring. “More and more projects are going to come up, and they’re going to need to know where that line is,” stated Holtzleiter. “So far they’ve gotten lucky and not hit it. But sooner or later they’re going to run across it.” (Photo by Tesha M. Christensen)

Tom Holtzleiter has been active in working to preserve Coldwater Spring for the past 20 years. A current resident of Bloomington, Holtzleiter grew up playing at Coldwater Spring. When the Highway 55/62 interchange was redone in 2002, Holtzleiter led a group, the Preserve Camp Coldwater Coalition, which pushed for the installation of an 8-acre liner to isolate the road from the nearby spring.

Coldwater Spring, located on the southern end of Minnehaha Park in Minneapolis, is considered sacred by some Minnesota tribes and has been home to people for 10,000 years. Dubbed the birthplace of Minnesota, it was the first place European settlers lived when Fort Snelling was being built in 1820. As recently as 1976, Coldwater was an emergency drinking water supply for south Minneapolis when the city’s water supply was “putrid with algae,” according to www.friendsofcoldwater.org.

Managed since 2010 by the National Park Service as part of the Mississippi National River and Recreation Area, Coldwater is the last natural spring of size in Hennepin County.

Sewer line failing
The existing regional sanitary sewer tunnel about 40 feet under Minnehaha Creek near 3901 Minnehaha Pkwy. E. is deteriorated and in danger of leaking wastewater (sewage) into the groundwater in the future.

This sewer pipe has served the city of Minneapolis and MCES since the mid-1930s, but it now needs some attention, according to Tim O’Donnell of MCES.

“The long-term environmental risks are too great to do nothing,” stated O’Donnell.

Photo left: A sewer tunnel project near 3901 Minnehaha Parkway E. may affect the flow of water at Coldwater Spring, but the Metropolitan Council Environmental Services (MCES) is working to minimize the impact. (Photo by Tesha M. Christensen)

The regional sanitary sewer collects wastewater via local sanitary sewers from South Minneapolis (south of approximately E, 42nd St.) and about one-third of Edina. MCES conveys the wastewater through additional regional sanitary sewers through Minneapolis and St. Paul to the Metropolitan Wastewater Treatment Plant (Metro Plant) located about three miles southeast of downtown St. Paul.

Plan changed to avoid dewatering
Originally, MCES planned to replace the existing, damaged regional sanitary sewer with a new deep sewer tunnel. That would have required dewatering of the limestone bedrock—that is, temporarily lowering the groundwater level in the limestone layer by pumping it out.

Recognizing that this had the potential of impacting groundwater flow to Coldwater Spring, which is located approximately 1.5 miles southeast of the sewer tunnel, MDES decided to rehabilitate the existing tunnel instead, which will not require dewatering.

However, this will still involve drilling up to three ventilation shafts into the limestone bedrock.
According to O’Donnell, the shafts will allow fresh air to be transferred into the deep tunnel system for workers’ safety.

“These ventilation shafts are necessary to meet strict OSHA requirements for working in underground, confined spaces,” he said. “The shafts will be encased in steel and grouted in place, which will allow groundwater to flow around them without affecting the flow to Coldwater Spring.”

However, Hotzleiter isn’t so sure. He’s worried that any drilling through the limestone into the sandstone beneath will break the seal. It’s possible that if this happens, water will flow towards the Mississippi River another way, and the flow at Coldwater will trickle away to nothing.

There has also been some disagreement about how much water has flowed through Coldwater in the past, and how much is going through now.

There is a monitoring point on the southern wetland but not one on the spring reservoir north of it, so the data only shows part of the complete picture, pointed out Holtzleiter.

MDES received a mix of comments at the May 22 public hearing and in writing. “People appreciate that MCES had re-evaluated the alternatives and found an alternative that would not impact the groundwater flow to Coldwater Spring,” stated O’Donnell. “Others were concerned with temporary park and trail access impacts during construction and some tree loss. And the Friends of Coldwater and its followers, and some members of the Mendota Mdewakanton Dakota Tribal Community are opposed to MCES installing two or three ventilation shafts from the ground surface down to the regional sanitary sewer pipe that will be rehabilitated.”

The National Park Service, Minnehaha Creek Watershed District, the city of Minneapolis, and the Minneapolis Park and Recreation Board (MPRB) support the revised project.

Where is the fracture line?
Nobody knows where Coldwater’s source waters come from. The consensus is that about two-thirds of the groundwater flows to the spring through basal Platteville limestone to the north and west. Beneath this 20 feet of limestone is 70 feet of sandstone. Much of the water flowing into Coldwater moves along a fracture line, but the exact location of that fracture has never been determined. Engineers have guessed that it is near where the sewer tunnel work is planned, but it could be 20 feet away or right at the project site.

Holtzleiter is pushing local government agencies to pinpoint the exact location of that fracture.

“More and more projects are going to come up, and they’re going to need to know where that line is,” stated Holtzleiter. “So far they’ve gotten lucky and not hit it. But sooner or later they’re going to run across it.”

He pointed out that the budget of this current project is $31 million, and believes a concurrent study of the fracture line would be a “drop in the bucket” comparatively.

Construction begins next year
MDES is working to finalize the project design and will then select a contractor. Construction will take place between spring 2019 to fall 2021.

Bike/walking trails and streets may be affected during this project, but MCES is working with the city of Minneapolis and the MPRB to minimize potential disruptions.

MDES will also work with the National Park Service to increase their monitoring of water flows into Coldwater Spring during the construction project.
“In the unlikely event that there are changes to these water flows during our construction project, we will know that quickly and will make any necessary modifications to our construction,” stated O’Donnell.

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