California Code, Health and Safety Code - HSC § 78770

(a) “Priority tier one” shall include any site that the department determines, using the criteria described in Section 78760, meets any of the following conditions:

(b) “Priority tier two” shall include any site that poses a substantial but less immediate threat to public health or safety or the environment and any site that will require a response action, but presents only a limited and defined threat to human health or safety or the environment. Priority tier two may contain sites previously listed in priority tier one if the department determines that direct threats to human health or safety have been removed and if physical deterioration of the site has been stabilized so that threats to the environment are not significantly increasing.