The case involving the clean up of a mercury dump site upstream from Grassy Narrows will be heard by judges at the Supreme Court.
The legal dispute centres on whether the Ontario government can force Weyerhaeuser and Resolute Forest Products to clean up a contaminated site.
In 2011, the province ordered the companies to remediate the former mill site in Dryden, where some 9,000 kg of toxic effluent was dumped in the English-Wabigoon River system in the 1960s.
Researchers have since found mercury is still leaking from the site into local waterways.
Domtar currently operates the mill in the community. However, the company has said responsibility for cleaning up mercury from previous owners was transferred to the province in 1985.
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