is liable for any of the harm, as an “arranger.” A. Apportionment of Liability 1. Standard of Review and Burden of Proof Because we have not heretofore faced a CERCLA apportionment issue directly, there is no Ninth Circuit precedent concerning the standard of appellate review for such an issue. Three circuits have addressed the question, and two separate approaches have emerged. The Fifth and Eighth Circuits look first to whether there is a reasonable basis for apportioning the harm, an inquiry they consider a question of law reviewed de novo. See Hercules, 247 F.3d at 718-19; Bell Petroleum, 3 F.3d at 896, 902. These two circuits then examine, as a question of fact reviewed under the clearly erroneous standard, precisely how damages are to be divided. See Hercules, 247 F.3d at 718 (<HOLDING>); Bell Petroleum, 3 F.3d at 896 (same). In
holding that estoppel was a question of fact holding it is a question of fact holding that apportionment of cercla liability is  a matter of federal common law holding that actual apportionment of damages is a question of fact holding that generally the question of waiver and estoppel is a question of fact