This regulatory action amends "Guidelines Establishing Test Procedures for the Analysis of Pollutants" at 40 CFR part 136 by approving test procedures (analytical methods) for use by testing laboratories and others for water monitoring. EPA regulations require the use of these methods where measurements of waste constituents are required in applications for National Pollutant Discharge Elimination System (NPDES) permits or for reports required under NPDES permits. These methods must be used for measurements required for an application for an EPA or State NPDES permit, for reports submitted by NPDES dischargers, and for certifications issued by States under section 401 of the Clean Water Act. The regulation also revises, clarifies, and corrects errors and ambiguities in existing methods and the water monitoring regulations.