(

a) Except as provided in subdivision (b), the department shall expend all funds appropriated to the department for any response action pursuant to this part, and shall take all response action pursuant to this part, in conformance with the assignment of sites to priority tiers pursuant to

Section 78770. (

b) The department may expend funds appropriated for a response action and take a response action, without conforming to the listing of sites by tier pursuant to

Section 78770, or at a site that has not been listed pursuant to

Section 78760, if any of the following apply:

(1) The department is monitoring a response action conducted by a responsible party at a site listed pursuant to

Section or at a site that is not listed but is being voluntarily remediated by a responsible party or another person.

(2) The expenditure of funds is necessary to pay for the state share of a response action pursuant to

Section 104(c)(3) of the federal act (42 U.S.C.

Sec. 9604(c)(3)).

(3) The department is assessing, evaluating, and characterizing the nature and extent of a hazardous substance release at a site for which the department has not been able to identify a responsible party, the responsible party is defunct or insolvent, or the responsible party is not in compliance with an order issued, or an enforceable agreement entered into, pursuant to

Section 79055.

(4) The department is carrying out activities pursuant to

Section or subdivision (

b) or (

c) of

Section 79060. (

c) The department may, at any one time, expend funds and take a response action at more than one site on the list established pursuant to

Section 78760. In addition, the department may, at any one time, oversee the performance of any activities conducted by a responsible party on more than one site on the list established pursuant to

Section 78760.