TM99-05 Validation of U.S. Environmental Protection Agency (USEPA) method 365.1 to replace U.S. Geological Survey (USGS) TWRI methods I-2607-90 and I-4607-90 for determination of phosphorus in the concentration range from 0.004 to 0.200 mg-P/L.

This technical memorandum announces replacement of USGS low-level, persulfate-digestion methods for determining total and dissolved phosphorus in the concentration range from 0.004 to 0.200 mg-P/L with a more rugged USEPA persulfate-digestion method 365.1 (see table 3), effective January 1, 1999. Customers should request this method for samples they know or suspect to have phosphorus concentrations between 0.004 and 0.200 mg-P/L. Customers should continue to request the USGS Kjeldahl-phosphorus method for samples containing phosphorus concentrations greater than 0.200 mg-P/L. If the customer requests the EPA method and the sample concentration is determined to be greater than 0.200 mg-P/L, the NWQL will switch to the USGS Kjeldahlphosphorus method and report values from that method. If samples requesting the USGS Kjeldahlphosphorus method have phosphorus values less than 0.200 mg-P/L, the NWQL will not switch to the EPA method. The EPA method should not be used as a sample screening method on a routine basis.