Here’s more from KTUU on the announcement:
The Draft EIS described a total embankment breach, although a catastrophic failure, as extremely unlikely and a “worst-case” scenario. In its July comments, the EPA raised concern that “given the occurrence of multiple large-scale tailings dam releases in recent years at modern operating mine facilities, the possibility of a dam breach may not be too remote and speculative.”
Although the Army Corps and cooperators conducted a Dam Failure workshop, which identified the most probable dam failure scenarios and eliminated full breaches as probable scenarios, draft meeting notes from technical meetings in November show that the EPA doubled down on its request for the Army Corps to conduct a full breach analysis.
The EPA voiced concern that most dam factors are due to human factors rather than design. The EPA also cited a U.S. Fish & Wildlife decision to require a full breach analysis for a copper mine in Arizona based on public concern.