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Serving Northern St. Louis County, Minnesota

LAKE VERMILION STATE PARK

Sulfide complicates development

Acid-generating rock found during road work in park

Marshall Helmberger
Posted 10/23/14

REGIONAL— The presence of concentrated sulfide rock in pockets between Tower and Ely has complicated plans for reconstruction of Hwy. 169 in parts of Breitung and Eagles Nest townships. It has also …

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LAKE VERMILION STATE PARK

Sulfide complicates development

Acid-generating rock found during road work in park

Posted

REGIONAL— The presence of concentrated sulfide rock in pockets between Tower and Ely has complicated plans for reconstruction of Hwy. 169 in parts of Breitung and Eagles Nest townships. It has also complicated development planning and construction in the new Lake Vermilion State Park, which includes some of the same rock formations that are causing concern along Hwy. 169.

That connection was raised recently by former state Rep. Tom Rukavina, who said rock blasting for the new state park access road, which is very close to Hwy. 169, likely cut through sulfide-bearing rock. He used the example to cast doubt on claims by officials from the Minnesota Department of Transportation and others that the sulfide issue was a legitimate concern. “Either the DNR is breaking the law, or you’re giving us a line of B.S.,” Rukavina said at an Oct. 10 highway update at Eagles Nest Town Hall.

The real story, not surprisingly, is more complicated.

“We did run into some acid-generating rock during road construction,” said Christa Miller, who is overseeing Lake Vermilion park development for the Department of Natural Resources.

In fact, all of the 2,861 cubic yards of rock blasted to make way for the new access road contained at least some sulfides, according to Zach Wenz, with the DNR’s Division of Lands and Minerals, who has been overseeing the issue for the agency. “Sulfide concentration was highly variable,” stated Wenz, with concentrations ranging from barely detectable to 8.2 percent by weight. The overall average percent was 0.74 percent by weight, based on DNR analysis of the rock.

According to Wenz, chemical testing of the blasted rock showed sufficient “neutralization” to inhibit acid rock drainage in the short term. That’s prevented the large stockpile of blasted rock, currently stored along the park road, from presenting an immediate pollution risk. Even so, the DNR had initially covered the pile with a large tarp to protect the pile from rainwater that could percolate through and eventually generate acidic runoff. That tarp deteriorated, however, and the pile is currently uncovered while the DNR arranges for a new covering.

So far, said Wenz, DNR officials have yet to detect any acid runoff from the pile, but it’s only been in place for a matter of months.

DNR officials are currently developing a mitigation strategy to permanently address the danger posed by the waste rock. According to Wenz, the agency is using federal guidelines and best practices developed by the Federal Highway Administration and some eastern states, such as Pennsylvania and Tennessee, in the wake of highway projects there that required expensive clean-up efforts after exposing large quantities of sulfide rock during construction.

While a final plan for dealing with the waste rock has not yet been announced, it will likely involve mixing the rock with a neutralizing agent, such as lime, and then encapsulating the material under a waterproof covering. “Once exposed to air and water, that’s then the trouble happens,” said Miller.

Some method of monitoring discharges from the material will likely also be required, according to Wenz. Currently, DNR officials expect to complete that work as part of campground construction, set to begin next spring.

While the sulfide-bearing rock was not identified until road construction in the park was underway, Wenz said that since the discovery of the issue, the DNR has been conducting test drilling ahead of any significant rock removal. The detection of high sulfide ore did prompt road contractors to relocate a segment of the access road in order to avoid one sulfide hot spot. That required a sharper turn than originally planned, which forced contractors to install a guardrail to meet safety standards.

The DNR’s policy of avoiding sulfide hot spots has led to other changes in park plans as well. “We have changed locations of some facilities for that very issue,” said Miller. “And we’re preemptively rock testing before development. We want to avoid this rock wherever possible.”

At this point, it isn’t clear how much it will cost to mitigate the risks associated with the sulfide rock that already was exposed in the road-building process. But the park’s experience is worth noting given the implications for the much larger amount of rock blasting that would be required for any of the planned upgrades of Hwy. 169. While the park road project involved the removal of 2,861 cubic yards of rock— or about 286 large dump truck loads— the south realignment favored by many Ely officials would involve blasting approximately 266,000 cubic yards of rock, or nearly one hundred times more than was removed in the park. The so-called middle route, which would reconstruct the highway mostly within the existing corridor, would entail the removal of 69,000 cubic yards. A third alternative, which would require an extensive detour, would involve blasting 138,000 cubic yards of rock.

While some of the waste rock generated by any of the construction scenarios could potentially be encapsulated under the new highway, federal guidelines generally limit that to rock that’s relatively low in sulfides, usually less than 0.5 percent by weight.

While the DNR’s experience in the new park may be instructive to the larger highway project, MnDOT officials have not discussed the matter with their DNR colleagues, according to Duane Hill, senior engineer at MnDOT’s Duluth office.

But Hill said DNR officials did ask MnDOT about the issue prior to beginning the roadwork.

The issue of sulfides is a new one in Minnesota, said Hill, but MnDOT has been working to develop strategies to address the risks associated with the acid-generating rock. “We’re planning on a problem,” said Hill. “We’re assuming all the rock will need to be mitigated.”

The easiest strategy, said Hill, is to avoid sulfide hot spots whenever possible. He said once a route is selected, MnDOT will conduct test drilling in areas where they expect to blast significant amounts of rock. Hill said if hot spots are detected, the road can be redesigned to avoid them. That could involve adjustments in the route, or raising the road surface with large quantities of fill to minimize the depth of rock cuts.

But Hill noted that pre-drilling along the corridor will not find all of the sulfide hotspots. Because the hot spots are often small and isolated, Hill said “it can be sort of like looking for a needle in a haystack.”

That means MnDOT will almost certainly run into pockets of high sulfide in the area, but Hill said MnDOT is looking at a variety of strategies to mitigate that. In some cases, where the percentage of sulfide is relatively low, the blasted material can be used as fill under the road surface, said Hill. Where sulfide hot spots are exposed along rock cuts, MnDOT can apply lime in the ditches to neutralize any acidic runoff.