Federal regulators say the West Virginia Department of Environmental Protection improperly left 255 streams totaling 1,000 miles off a list of impaired waterways it submitted for approval last fall.
The Environmental Protection Agency said Monday they must be added to comply with the federal Clean Water Act.
In January, environmental groups threatened to sue EPA if it didn't force West Virginia to enforce the law.
They accused the DEP of colluding with polluters to help them avoid hundreds of millions of dollars in cleanup costs.
They also criticized lawmakers for passing weaker water-quality standards last year. The DEP subsequently argued that its list didn't constitute a revision of standards.
The EPA will publish its decision April 15 then take public comment for 30 days.
A DEP spokeswoman didn't immediately comment.
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