As required under the Clean Water Act section 304(h), the EPA is amending existing regulations to provide regulated entities with more options for measuring pollutants in wastewater. This was a routine action that the Office of Water undertakes every three to five years. This regulatory action amended "Guidelines Establishing Test Procedures for the Analysis of Pollutants" at 40 CFR Part 136 and approved test procedures (analytical methods) for use in testing water for certain constituents. The EPA's regulations require the use of these methods when measurements of waste constituents are required in applications for National Pollutant Discharge Elimination System (NPDES) permits or for reports required under NPDES permits. The regulation also revised, clarified, and corrected errors and ambiguities in existing methods.