California Code, Health and Safety Code - HSC § 25299.91

(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:

(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.