As used in this article, the following terms have the following meaning:

1. (a) “Commingled plume” means the condition that exists when groundwater contaminated with petroleum from two or more discrete unauthorized releases have mixed or encroached upon one another to the extent that the corrective action performed on one plume will necessarily affect the other. A commingled plume does not include either of the following:

   1. (1) Contaminated groundwater plumes resulting from unauthorized releases or discharges from a single site.
   2. (2) Soil contamination, unless it can be demonstrated that the contaminated soil is an immediate threat to groundwater.
2. (b) “Contributing site” means a site on which an unauthorized release or discharge of waste has occurred or is occurring and has impacted or threatens to impact groundwater.