P owns and operates a 'Kentucky Fried Chicken' restaurant on the land. In 1988, P discovered during the course of a construction project that the property was contaminated with petroleum. The County ordered P to attend to the problem, and P spent $ 211,000 removing and disposing of the oil-tainted soil. P owns and operates a 'Kentucky Fried Chicken' restaurant on the land. In 1988, P discovered during the course of a construction project that the property was contaminated with petroleum. The County ordered P to attend to the problem, and P spent $ 211,000 removing and disposing of the oil-tainted soil. Three years later, P brought this suit under the citizen suit provision of RCRA, 90 Stat. 2825, as amended, 42 U.S.C. § 6972(a), seeking to recover these cleanup costs from Ds. P claimed that the contaminated soil was a 'solid waste' covered by RCRA that previously posed an 'imminent and substantial endangerment to health or the environment,' and that Ds were responsible for 'equitable restitution' of P's cleanup costs under § 6972(a) because, as prior owners of the property, they had contributed to the waste's 'past or present handling, storage, treatment, transportation, or disposal.' The District Court held that § 6972(a) does not permit recovery of past cleanup costs and that § 6972(a)(1)(B) does not authorize a cause of action for the remediation of toxic waste that does not pose an 'imminent and substantial endangerment to health or the environment' at the time suit is filed, and dismissed P's complaint. The court of appeals reversed. It held that a district court had authority under § 6972(a) to award restitution of past cleanup costs, and that a private party can proceed with a suit under § 6972(a)(1)(B) upon an allegation that the waste at issue presented an 'imminent and substantial endangerment' at the time it was cleaned up. The court granted certiorari to address the conflict between the Circuits.