The Environmental Protection Agency (EPA) is finalizing minor amendments to its Clean Water Act (CWA) regulations to codify that under the National Pollutant Discharge Elimination System (NPDES) program, permit applicants must use "sufficiently sensitive" analytical test methods when completing an NPDES permit application and the Director must prescribe that only "sufficiently sensitive" methods be used for analyses of pollutants or pollutant parameters under an NPDES permit. The final rule is based on requirements in the CWA and clarifies existing EPA regulations. It also codifies existing EPA guidance on the use of "sufficiently sensitive" analytical methods with respect to measurement of mercury and extends the approach outlined in that guidance to the NPDES program more generally. Specifically, EPA is modifying existing NPDES application, compliance monitoring, and analytical methods regulations. The amendments in this rulemaking affect only chemical-specific methods; they do not apply to the Whole Effluent Toxicity (WET) methods or their use.