Frequent Questions

Where can I find the standard rounding procedures or significant figure use conventions when judging compliance with an MCL?

The standard rounding procedures used to determine compliance with an MCL are detailed in EPA Water Supply Guidance Memo 21 Procedures for Rounding-Off Analytical Data to Determine Compliance with Maximum Contaminant Levels Present in NIPWR (April 1981). Laboratories should observe conventions concerning proper use of significant figures in making calculations to avoid the appearance that the data are more precise than the method allows. Conventions for the use of significant digits are discussed in the following EPA publications: Analytical Quality Control in Water and Wastewater Laboratories (EPA 600/4-79-019) and Standard Methods for Examination of Water and Wastewater (Section 1050B, 21st Edition).
Have more questions? Submit a request