Today's final regulation amends the ``Guidelines Establishing Test Procedures for the Analysis of Pollutants'' under section 304(h) of the Clean Water Act to approve EPA Method 1613 for determination of tetra-through octa-chlorinated, 2,3,7,8-substituted, dibenzo-p-dioxins and dibenzofurans (CDDs/CDFs) by high resolution gas chromatography (HRGC) coupled with high resolution mass spectrometry (HRMS). This regulation makes available at 40 CFR part 136 an additional, more sensitive test procedure for CDDs/CDFs. Method 1613 is the most sensitive analytical test procedure approved under the Clean Water Act for the analysis of CDDs/CDFs because it measures into the low part- per-quadrillion (ppq) range. Use of approved test procedures is required whenever the discharge constituent specified is required to be measured for: a National Pollutant Discharge Elimination System (NPDES) permit application; discharge monitoring reports; state certification; and other requests from the permitting authority for quantitative or qualitative effluent data. Use of approved test procedures also is required for the expression of pollutant amounts, characteristics, or properties in effluent limitations guidelines and standards of performance and pretreatment standards, unless otherwise specifically noted or defined.