For over a decade, the U.S. Environmental Protection Agency's (EPA's) "mixture rule" clarified the status under the Resource Conservation and Recovery Act¹ (RCRA) of mixtures containing listed hazardous waste and nonhazardous solid waste.² But the rule's recent vacatur and repromulgation have created a significant gap in the rule's coverage. Because the U.S. Court of Appeals for the D.C. Circuit vacated it ab initio, the rule does not cover conduct that occurred before EPA's 1992 repromulgation. To resolve disputes over the management of mixed solid and hazardous waste that occurred before the repromulgation,lawyers must determine the status of such mixtures in the absence of a mixture rule.

The D.C. Circuit did not address this issue when it vacated the rule because it based its decision on purely procedural grounds. But since then, EPA and the courts have addressed this issue several times. Even before the vacatur, EPA and the U.S. Department of Energy (DOE) addressed the status of hazardous waste mixtures to which the rule does not apply.