The Minnesota Supreme Court is weighing whether the groundwater around the U.S. Steel's leaking Minntac tailings basin should be regulated as drinking water, and Minnesota regulators say the outcome could have vast impacts on other water permits throughout the state.

At issue is whether the Minnesota Court of Appeals erred last year when it concluded drinking-water standards do not apply to groundwater and, therefore, Minnesota Pollution Control Agency can't issue permits with conditions setting groundwater as that class of drinking water.

But the Fond du Lac Band of Lake Superior Chippewa, MPCA and environmental group WaterLegacy disagree.

In oral arguments held Monday morning, Stacey Person, an attorney for the MPCA, said that if the Supreme Court didn’t reverse the lower court’s decision, it would negate water permits that rely on the same requirements, creating “a huge void in the protection of groundwater in this state.”

“The current groundwater limits in existing permits across the state would become unenforceable. No one, neither (MPCA) nor the regulated community, would know what limits apply, or even how to determine what the limits should be," Person said. "Basically, it would be a free-for-all on groundwater pollution until the regulatory void created by the Court of Appeals’ decision gets filled.”

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But U.S. Steel attorney Jeremy Greenhouse argued the Court of Appeals “got it right.”

“The permit conditions based upon MPCA’s erroneous application of the Class 1 standards to groundwater would require U.S. Steel to construct and operate an outsize water treatment plant,” Greenhouse said. “Also, that the standards, which have no legal basis, and which are undisputedly not necessary to protect human health, can be met in groundwater that will almost certainly never be used as a source of drinking water.”

The sides disagree over whether two different rules, when read together, actually classify groundwater as Class 1 water, or drinking water, which has more stringent requirements, and whether the MPCA has authority over the waters.

Several justices also expressed confusion.

“In doing this statuary or rule interpretation, I’m struggling a little bit to find where in the two administrative rules there’s a classification of groundwater as a Class 1 water. It seems to me that the potentially better analysis is that those rules are ambiguous, and then (the MPCA’s) history that (Person) just laid out maybe becomes very relevant and interpretation of what those rules taken together mean,” Associate Justice Gordon Moore said.

Person said the rules were clear, but if the court found them ambiguous, then it should look to defer to MPCA to “tell us what that regulation means.”

RELATED: Read more about Minntac

Paula Maccabee, attorney for WaterLegacy, said the rules work together because they give the MPCA the authority to protect potable water — water used for drinking or food preparation — by “stopping the pollution in the beginning” and recognizes that aquifers are interconnected.

“That’s why all groundwater has to be designated for the drinking water use, because once something is spilled into the groundwater, it gets everywhere,” Maccabee said.

The groundwater as drinking water rules came in a water pollution discharge permit issued by the MPCA in late 2018, the first for Minntac since 1987. While the 1987 permit had been up for reissuance since 1992, the MPCA had administratively extended it for 26 years.

After the Court of Appeals reversed the permit last year, the Fond du Lac Band, MPCA and WaterLegacy appealed the decision to the Supreme Court.

Fond du Lac Band attorney Sara Van Norman acknowledged the unlikely alliance fighting the lower court’s decision.

“All three of us are on the same side here, and that hasn’t been true for other issues in this case and hopefully that speaks for itself,” Van Norman said. “We all are making the same argument under the plain language of these rules.”

The five-year permit issued in 2018 had set long-range goals for U.S. Steel to meet for reducing pollutants, like sulfates, that are leaking out of the site's 8,000-acre tailings basin — where a wet slurry of mine waste left over after taconite pellets are processed resides — into nearby surface and groundwater.

Minntac is the largest taconite iron ore mining and processing operation in the U.S. In 2019, it shipped 12.9 million tons of pellets and employed 1,460 people. It went online in 1967.