CoZinCo, Inc. (“CoZinCo”) has filed two petitions for reimbursement of response costs pursuant to § 106(b) of the Comprehensive Environmental Response, Compensation, and Liability Act (“CERCLA”), 42 U.S.C. § 9606(b). The petitions concern removal actions ordered by U.S. EPA Region VIII (the “Region”) in response to elevated levels of zinc found in certain residential water supplies in the vicinity of CoZinCo’s zinc sulfate production facility near Salida, Colorado. In April 1994, the Region issued to CoZinCo a unilateral administrative order (“UAO”) under CERCLA section 106(a), 42 U.S.C.