In two major Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) opinions, United States v. Atlantic Research, Inc. and Burlington Northern & Santa Fe R.R. v. United States, the U.S. Supreme Court provided long-sought guidance for parties litigating hazardous waste cleanup issues under CERCLA.

On May 4, 2009, the U.S. Supreme Court handed down its decision in Burlington Northern & Santa Fe Railway Co. v. United States. The decision is of major significance with respect to two areas of Superfund jurisprudence--"arranger" liability, and divisibility or apportionment of harm. This Article is concerned only with the latter issue and, moreover, only with one specific element of that issue.