The US Environmental Protection Agency last week filed a voluntary motion to remand its downstream water quality decision under Section 401 of the Clean Water Act.

PolyMet said science showed its copper-nickel-precious metals project would have no downstream water quality effects.

"EPA has never said otherwise, but it did not make explicit findings when the issue arose in 2018 after the Minnesota Pollution Control Agency certified that the project will not affect the state's water quality," PolyMet said.

"This 90-day remand gives it the opportunity to do so.

"PolyMet plans to participate fully, as appropriate, in this process and believes the science will once again prevail."

It said the EPA's request for a voluntary remand did not affect a federal district court ruling from mid-February, which had ended a federal court challenge to the company's water quality permit. 

Glencore-backed PolyMet had a "big win" in late February when the Minnesota Supreme Court unanimously ruled in favour of the Clean Air Act permit issued to PolyMet by the Minnesota Pollution Control Agency. 

A 2018 updated technical report said a US$945 million Phase I development would create a 32,000 tonne per day operation, producing about 1.1 billion pounds copper, 133Mlb nickel, 5.6Mlb cobalt and 2.1 million ounces of precious metals over 20 years, with ore processed at a former steel plant.

PolyMet shares (TSX: POM) closed down 6.6% in Toronto on Friday to C$4.63, at the lower end of a one-year range, valuing it at $467 million (US$367 million).