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The antidegradation rule is implemented in Chapter NR 207 of the Wisconsin Administrative Code. The purpose of the rule is to address new or increased discharges to surface waters (river, lakes, drainage ways, wetlands, etc.). The way in which these new or increased discharges are addressed depends on the type, or use designation, of the water body.

In general, the antidegradation rule language is intended to, where appropriate, require a justification of the reasons for new or increased discharges before such discharges can be allowed under Wisconsin's discharge permit program (WPDES).

In Chapter NR 207, new discharges are defined as point sources that did not have a discharge permit as of March 1, 1989. On the other hand, increased discharges are more pollutant-specific, referring to changes in concentrations, levels or loadings (mass) of a particular pollutant that exceeds a limit that is already in a discharge permit.

For some higher quality waters, new or increased discharges are either prohibited or allowed only in extreme and unique situations. In Outstanding Resource Waters (listed in Chapter NR 102, Wis. Adm. Code), new or increased discharges are allowed only if they maintain the existing water quality.

New discharges to Exceptional Resource Waters (also listed in ch. NR 102) are treated similarly to Outstanding Resource Waters if the discharge is not needed to prevent or correct an existing surface or groundwater contamination problem. If the new discharge is needed to prevent or correct any of those problems, or for any increased discharge, it is addressed similar to a more typical fish and aquatic life protection situation.

For waters designated for Fish and Aquatic Life uses (meaning coldwater, warmwater sportfish or warmwater forage fish communities as described in ch. NR 102) and which are not classified as Outstanding or Exceptional Resource Waters (except as noted above, where increased discharges to Exceptional Resource Waters may be evaluated here), the extent or allowance of a new or increased discharge depends on the results of the following demonstrations where applicable:

For increased discharges (those which exceed limits in existing discharge permits), it must be shown that the existing discharge exceeds or approaches the current permit’s limits for the pollutant in question, under the condition that any exceedance occurs while the treatment facility is maintained in good working order and is operated and maintained as efficiently as possible.

If new limits are not needed to regulate the discharge, the discharge permit cannot be changed. If the new or increased discharge results in any lowering of water quality, the discharger must demonstrate to DNR that the discharge accommodates important social or economic development. This may include a showing of increased employment, increased production, avoiding reductions in employment, increased efficiency, an economic or social benefit to the community (including industrial, commercial, or residential growth), or correcting an environmental or public health problem. If the social or economic demonstration cannot be made, no additional lowering of water quality will be permitted.

If the new or increased discharge results in a "significant" lowering of water quality (discharge would exceed 1/3 of the allowable and available capacity of a pollutant in a water body), the discharger must demonstrate whether or not the significant lowering can be prevented in a cost-effective manner or if the significant lowering can be prevented by discharging to a different location.

The resulting discharge limits may vary based on the results of this demonstration in combination with the social/economic demonstration; these typically result in either limits that prevent any lowering of water quality, limits that prevent significant lowering of water quality, or limits based on the allowable lowering of water quality that still meets available water quality standards.

For waters not classified for the fish and aquatic life uses listed earlier (meaning those classified as limited forage fish communities or limited aquatic life waters), the antidegradation demonstrations associated with a new or increased discharge are based on downstream uses where waters are classified as coldwater, warmwater sportfish, warmwater forage fish or Outstanding or Exceptional Resource Waters using the procedures described above.