1. The department shall use credible data when doing any of the following:

 a. Developing and reviewing any water quality standard.
 b. Developing any statewide water quality inventory or other water assessment report.
 c. Determining whether any water of the state is to be placed on or removed from any section 303(d) list.
 d. Determining whether any water of the state is supporting its designated use or other classification.
 e. Determining any degradation of a water of the state under 40 C.F.R. §131.12.
 f. Establishing a total maximum daily load for any water of the state.
 2. Notwithstanding subsection 1, credible data shall not be required for any section 305(b) report and credible data shall not be required for the establishment of a designated use or other classification of a water of the state.
 3. This section shall not be construed to require credible data as defined in section 455B.171, subsection 4, in order for the department to bring an enforcement action for an illegal discharge.