the total area of the facility the railroads owned; (2) the percentage of time the railroads leased their parcel to the
facility operator; and (3) an estimate of the percentage of contamination at the facility attributable to the types of
hazardous substances present on the railroads’ parcel. The court then adjusted its calculation by adding a fifty–
[20] Contrary to PCS’s assertion, the district court’s denial of apportionment here is entirely consistent with Bur–
lington Northern. To be sure, Burlington Northern demonstrates that a “reasonable basis for apportionment” need
not be mathematically precise, and may be based on the “simplest of considerations,” e.g., time and land area. See
556 U.S. at 617–18, 129 S.Ct. 1870; see also NCR Corp., 688 F.3d at 842. However, Burlington Northern neither
mandates these “simplest of considerations,” nor establishes their presumptive propriety in every case. See NCR
fact-intensive, site-specific determination. See NCR Corp., 688 F.3d at 841–42.
Here the district court undertook precisely such a fact-intensive, site-specific determination. Weighing the facts
in this case, the court held that any reasonable basis for apportionment must include reliable evidence as to both the
initial disposals of hazardous substances and any secondary disposals that occurred over time. Indeed, the court dis-
Indeed, as Burlington Northern confirmed, in the face of uncertain causation of harm, “ ‘courts have refused to
make an arbitrary apportionment for its own sake.’ ” 556 U.S. at 614–15, 129 S.Ct. 1870 (quoting Restatement (Sec–
ond) of Torts § 433A cmt. i (1963–64)). An arbitrary apportionment is just what the district court refused to make, as
any apportionment without adequate evidence as to the harm caused by secondary disposals necessarily would have