"Some Superfund response actions and RCRA corrective actions do not focus on cleaning up a medium that is already contaminated to a particular goal, but rather, focus on keeping a certain amount of contaminants from entering a medium. In such a case, the medium is indirectly positively affected, but the objective of the response action is not to attain a particular standard for the medium, but rather a standard for how much to keep out of the medium. It is not appropriate to count the volume of the entire medium, and other methods are needed for the measure. Examples of these are mining drainage diversion and mining waste drainage treatment remedies, and vapor intrusion remedies."
Source: 12/12/03 "Final Methodology for Estimating Superfund and RCRA Corrective Action Case Conclusion Data Sheet (CCDS) Environmental Benefits", p. 8 of the guidance.