The department shall assign each site listed pursuant to

Section to one of the following priority tiers for the purpose of informing the public of the relative hazard of listed sites: (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:

(1) The site may pose a known or probable threat to public health or safety through direct human contact.

(2) The site may pose a substantial probability of explosion or a fire or a significant risk due to hazardous air emissions.

(3) The site has a high potential to contaminate or to continue to contaminate groundwater resources that are present or possible future sources of drinking water.

(4) There is a risk that the costs of a response action will increase rapidly or risks to human health or safety or the environment will increase significantly if response action is deferred. (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.