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 UAOs are unilateral administrative orders, directing companies and others to clean up hazardous waste for which they are responsible.

To use its UAO authority, EPA must first determine 'that there may be an imminent and substantial endangerment to the public health or welfare or the environment because of an actual or threatened release of a hazardous substance from a facility.' If EPA makes such a determination, it must then compile an administrative record and select a response action. Id. § 9613(k)(1). For remedial actions, the longer-term option, CERCLA requires EPA to 'provide for the participation of interested persons, including [PRPs], in the ...