Yes, and no. The decision process can be very involved, and there are avenues for legal recourse, so it is difficult to be black and white in describing the finality of the EPA’s decision. Superfund issues are often decided by federal courts.
If the EPA directs the responsible party to clean up the waste in a certain fashion, the responsible party is bound by law to comply. If the EPA determines that no cleanup is necessary or feasible, the responsible party is released from responsibility and cannot be held accountable in the future.
Superfund, however, does provide for legal recourse. If the responsible party does not agree with the EPA’s decision, the responsible party can refuse to comply. The result of this refusal is generally a lawsuit brought by the EPA to force the responsible party to act according to the EPA’s decision. Conversely, if another party, for example a state government or public interest group, disagrees with the EPA’s decision, that party can sue the EPA for violating its mission to protect the environment and hold accountable responsible parties.