644.058.  Water quality standards revised, when — evaluation to be conducted, when. — Notwithstanding the provisions of section 644.026 to the contrary, in promulgating water quality standards, the commission shall only revise water quality standards upon the completion of an assessment by the department finding that there is an environmental need for such revision.  As part of the implementation of any revised water quality standards modifications of twenty-five percent or more, the department shall conduct an evaluation which shall include the environmental and economic impacts of the revised water quality standards and criteria on a subbasin basis.  This evaluation shall be conducted at the eight-digit hydrologic unit code level.  The department shall document these evaluations and use them in making individual site-specific permit decisions.