The Comprehensive Environmental Response, Compensation, and Liability Act¹ (CERCLA) does not expressly provide for joint and several liability, but it is well-established that under CERCLA § 107(a)² potentially responsible parties (PRPs) will be jointly and severally liable for the release, or threat of release, of hazardous substances.³ As a general proposition, a PRP may be able to defend against the full application of joint and several liability in a particular case if it can show that harm it caused or for which it is responsible is divisible and reasonably capable of apportionment.⁴ However, no defendant has ever successfully invoked the divisibility defense in any reported decision.

Where hazardous substances are commingled following disposal at a site, evidence that a single PRP's contribution caused a distinct and segregable environmental harm is typically unavailable. The unavailability of this type of evidence is important because the divisibility defense requires a fact-intensive analysis, and because the defendant bears a heavy burden of proof. Consequently, the defense should be unsuccessful in all but the clearest of cases. In addition, proof of divisibility of harm is not a complete defense to liability. A PRP that is able to establish that its harm is divisible is nevertheless liable for its discrete portion of the harm caused by a release for which it is responsible. In this respect, divisibility of harm is only a partial defense to liability for all cleanup costs at a site.