First-of-a-kind digital map pegs acid risks for MnDOT

HERMANTOWN, Minn. -- In 28 years as a geologist working in and out of industry, Dean Peterson has done his share of field mapping. He loves the work -- camping under the stars, sweaty hikes to document rock outcroppings and other phenomena throug...

Dean Peterson, program manager for NRRI’s economic geology group, developed a mapping application MnDOT will use to identify areas where shallow bedrock has the potential to produce acids if disturbed by road projects. Steve Kuchera / Forum News Service
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HERMANTOWN, Minn. - In 28 years as a geologist working in and out of industry, Dean Peterson has done his share of field mapping. He loves the work - camping under the stars, sweaty hikes to document rock outcroppings and other phenomena throughout Northern Minnesota.

His work has contributed to our understanding of the state’s terrain and slots in alongside the likes of the late University of Minnesota professor Frank Grout - “one of the best geologists in the history of mankind,” Peterson said.

Now in his second stint at the University of Minnesota Duluth’s Natural Resources Research Institute in Hermantown, Peterson has consolidated the hard-copy work of well-diggers and scores of other geologists into digitized format. Map segments that once stressed file cabinets and filled archaic floppy discs are now built into a digital map of the state.

“This data set is like none other,” the 56-year-old Peterson said. “It’s really valuable. To have somebody else do this (from scratch) would take them decades to remake it again.”

The material is scientifically valid and each click informs the user with embedded details such as rock type, mineral content, precise location and mapping origins which can date back decades.


“They did a great job on it,” said Jason Richter, chief geologist for the Minnesota Department of Transportation in St. Paul.

Earlier this year, MnDOT became the first agency to tap into a portion of the data - having commissioned Peterson and NRRI to create a map outlining potentially acid generating rocks. The MnDOT map filters out areas of bedrock with the potential to contain sulfur minerals, or sulfides. Latent when underground, sulfides risk becoming sulfuric acid, which can contaminate waterways.

“If you have pyrite, which is just iron sulfide or any kind of sulfide, plus air plus water - you need all three of those - what you get is an iron sulfate which will dissolve in water and you get sulfuric acid,” Peterson said. “That’s just what happens. It’s very much a natural process.”

MnDOT’s 17-year odyssey with a road project completed last year between Ely and Tower - the $16.4-million Eagles Nest Lake Area Project along Minnesota Highway 169 - was the impetus for the agency’s pursuit of a sulfide map.

The project corrected 6 miles of curving, shaded highway notorious for breeding the black ice responsible for nine fatalities, 60 injury crashes and innumerable slide-offs in the 30 years prior to reconstruction. Some residents’ unrest with the project coupled with the discovery of sulfide crystals in the rock along the route served to create one delay after another. It took 18 iterations before the agency settled on a plan to straighten the roadway and make it safer.

“The Eagles Nest project, that was our first experience with this sulfur issue,” Richter said, before gently pressing sulfides’ hottest button: proposed copper-nickel mines. “Together with PolyMet and Twin Metals, (Eagles Nest) created an awareness that prior to that we weren’t doing any testing, weren’t addressing the sulfur issue. The last few decades sulfur and sulfides have come to the forefront and we’ve been thrust into that same awareness.”

Both Peterson and Richter explained that the presence of sulfides won’t necessarily derail a state highway project, but the knowledge will allow planners to look at options and find ways to mitigate issues before contractors start blasting new formations out of bedrock. A Duluth engineering company is using the NRRI sulfides map to write a guidance manual on what to do when encountering sulfides, Richter said.

“Ultimately it’s a risk assessment map - the screening tool that our project managers in northern third of the state can use if they anticipate doing any rock excavation,” Richter said.


With the new map, MnDOT can not only identify bedrock with high potential for sulfides, but also how deep the bedrock is.

The longtime and ongoing Minnesota Geological Survey has coupled with well-diggers, who are required by the state to report terrain contents, to build hundreds of thousands of points of data on how deep one has to go in any given place before striking bedrock.

“If it’s more than 50 feet MnDOT doesn’t really care,” Peterson said. “They’re not going to be exposing that rock on the surface.”

Peterson’s digitization in full of the state’s geologic archives appears as a psychedelic swirl of colors superimposed over the state. Bedrock formations are color-coded and the type and mineral makeup of the rocks varies wildly.

The map is able to be queried and distilled into layers. Want to know where best to look for gold? Go to the long east-west creases in north-central Minnesota highlighted in red on its own map. Building a bike trail and want to mix up the forest scenery? There is a map entirely void of everything save Minnesota’s rock outcroppings - heavily concentrated in the north and east.

“That’s where we know the truth,” Peterson said of the outcroppings. “We can see it. I can take my hammer to it.”

Conversely, the map reveals fault lines hidden underground.

“The story of a map is a lot in the lines,” Peterson said, “and if it’s done right it will tell you what kind of rock is on one side and what kind of rock is on the other.”


Because the map has potential for a wide variety of uses, NRRI is having ongoing discussions on how to proceed with protecting its new property while also using and possibly even monetizing it.

Proud to have curated the life’s work of an entire profession, Peterson recalled how field-mapping geologists like him commonly climbed to the highest spot around for their lunches.

“You’ve worked your butt off and you’re sweating, there are twigs in your shirt collar and everywhere,” he said. “You go where you’ve got a view. You want to eat and then you take about 10-minute snooze. You sit back, feel the wind, close your eyes and it rejuvenates you.”

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