40 CFR 27.30 - The hearing and burden of proof.
(a) The presiding officer shall conduct a hearing on the record in order to determine whether the defendant is liable for a civil penalty or assessment under § 27.3 and, if so, the appropriate amount of any such civil penalty or assessment considering any aggravating or mitigating factors.
(b) The Authority shall prove defendant's liability and any aggravating factors by a preponderance of the evidence.
(c) The defendant shall prove any affirmative defenses and any mitigating factors by a preponderance of the evidence.
(d) The hearing shall be open to the public unless otherwise ordered by the presiding officer for good cause shown.