Well, apparently they needed the existing environmental regulations to be waived. It was never revealed why, e.g., what chemicals they wanted to dump. They could simply be required to re-use their wastewater in a closed system, like nuclear power plants (for a different reason, heat generation). Then it can be as dirty or clean as it needs to be.

Also, the water doesn't belong to us. Use of Great Lakes water is governed by a treaty with Canada and administered by an international body that includes the states and provinces in the Great Lakes watershed.