Under the draft guidance, if the new TMDL-derived limit results in an increase in an effective existing limit in a permit, then an antidegradation evaluation is needed. The initial imposition of a water quality-based effluent limit, which include TMDL-derived limits, does not require an antidegradation evaluation as long as the pollutant of concern was previously present in the discharge and the permittee is not proposing an increased load to the receiving water . According to the WDNR, possible exceptions include the initial imposition of a TMDL-derived limit for a discharge to Exceptional and Outstanding Resource Waters, for a bioaccumulative chemical of concern such as mercury when an increased discharge is proposed, and when a change in discharge location is proposed.
With a few exceptions, Wisconsin chapter NR 207 requires an antidegradation evaluation when a new or increased discharge is proposed. Therefore, an antidegradation evaluation is necessary before a TMDL-derived limit, which has been incorporated into a WPDES permit and has become effective, is increased or the TMDL-derived limit replaces a less restrictive effective effluent limit.
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