Serving Northern St. Louis County, Minnesota

Unusual drainage eyed along Hwy. 169

Test results show elevated levels of sulfate, other chemicals, near Six Mile Lake realignment

Marshall Helmberger
Posted 11/14/18

EAGLES NEST— Unusual-looking drainage coming from under the surface of the newly-reconstructed portion of Hwy. 169 in Eagles Nest Township has prompted MnDOT officials to collect water samples in …

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Unusual drainage eyed along Hwy. 169

Test results show elevated levels of sulfate, other chemicals, near Six Mile Lake realignment

Posted

EAGLES NEST— Unusual-looking drainage coming from under the surface of the newly-reconstructed portion of Hwy. 169 in Eagles Nest Township has prompted MnDOT officials to collect water samples in hopes of learning more about the chemical reactions that appear to be taking place in the road bed.

One particular seep, located just east of the highest spot on the new highway, has generated both white and orange precipitate that is accumulating on the north side of the road. Initial testing, by both MnDOT and members of the public, suggests that water flowing under the road is experiencing a rise in levels of sulfate, calcium, chlorides, and alkalinity, along with a sharp rise in conductivity, which generally indicates the presence of dissolved chemicals, such as sulfate and an assortment of chlorides.

Project Manager Michael Kalnbach said it’s too early to say whether the test results are significant. He said he remains confident that the mitigation plan that MnDOT is implementing will address concerns about the sulfide content of some of the 132,000 cubic yards of rock that contractors removed during the construction of the new portion of highway.

Kalnbach said members of the project’s technical working group have been informed of the testing. “They are aware of the water sampling results,” he said. “Based on the results, they will help decide what, if anything, needs to be done. At this point, we’re following what was in the mitigation plan.”

The project’s mitigation plan includes the drilling of test wells and regular water sampling to provide early detection if the sulfide rock is generating acid drainage, as is common when sulfur-bearing rock is exposed to water and oxygen. At this point, the drainage does not appear to be highly acidic, which could mean that the large volume of limestone, which was added to portions of the road bed, is doing its job. Road builders added 3,645 tons of limestone to neutralize pockets of sulfide within the rock as it was blasted to make way for the improvements to the highway. That’s 22 percent more than the original engineers’ estimate, reflecting a larger-than-projected amount of sulfide rock. Of the 132,000 cubic yards of rock removed for the project, 32 percent tested at or above 0.1 percent sulfur, which is the level where mitigation is required. At least one sample tested as high as 1.13 percent sulfur.

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