This final regulation amends the ``Guidelines Establishing Test Procedures for the Analysis of Pollutants'' under section 304(h) of the Clean Water Act by adding EPA Method 1631, Revision B: Mercury in Water by Oxidation, Purge and Trap, and Cold Vapor Atomic Fluorescence Spectrometry. EPA Method 1631 measures mercury at the low levels associated with ambient water quality criteria (WQC). EPA has promulgated WQC for mercury at 12 parts-per-trillion (ppt) in the National Toxics Rule, and published a criterion for mercury at 1.3 ppt in the Water Quality Guidance for the Great Lakes System. The version of Method 1631 promulgated today includes changes to the method based on public comments at proposal (63 FR 28867, May 26, 1998). These changes increase measurement reliability at WQC levels. EPA recommends the use of clean sampling and laboratory techniques in conjunction with EPA Method 1631 to preclude contamination at the low ppt levels necessary for mercury determinations. EPA has published guidance documents on sampling and clean rooms for trace metals, including mercury.