(1)    Any thermal effluent limitation proposed by the department may be modified by it in accordance with s. 283.63, if the owner or operator of the point source which is the subject of the proposed limitation demonstrates to the satisfaction of the department that the proposed limitation is more stringent than necessary to assure the protection and propagation of a balanced indigenous population of shellfish, fish and wildlife in and on the body of water into which the discharge is made.

   (2)   If a point source with a discharge having a thermal component is modified, the point source shall not be subject to any more stringent effluent limitation with respect to the thermal component of its discharge during either the 10-year period beginning on the date of completion of the modification or the period of depreciation or amortization of the facility for the purpose of section 167 or 169 of the internal revenue code, whichever ends first, if all of the following apply:

      (a)    The modification of the point source commenced after October 18, 1972.

      (b)    The point source, as modified, meets the most stringent effluent limitation established under s. 283.13.

      (c)    The limitation under par. (b) assures protection and propagation of a balanced indigenous population of shellfish, fish, and wildlife in and on the water into which the discharge is made.