States are required to develop and adopt anti-degradation policies and implementation procedures pursuant to federal regulations [1] to protect existing in stream water quality and water uses. States are requiring National Pollutant Discharge Elimination System (NPDES) dischargers from new sources, which could result in lowering of existing water quality, to perform an anti-degradation demonstration as part of the NPDES permit application process. These evaluations are appropriate where a proposed discharge may have a significant adverse impact on water quality. However, they are an unnecessary burden on both the permit applicant and the reviewing agency where the proposed discharge is minor relative to existing water quality. Therefore, de minimis evaluations provide a technical approach that States can use to avoid the need for, or limit the scope of, anti-degradation demonstrations where the impact at the proposed discharge is minor.

This paper provides a de minimis evaluation approach for addressing anti-degradation as part of an NPDES permit application process. The approach, which ascertains what conditions would or would not constitute a “de minimis lowering of water quality,” was recently used in support of a new combined-cycle power plant in one of the central states.

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