UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
                                                                              WSG20
                                                             Date Signed: April 6, 1981
MEMORANDUM

SUBJECT:   Procedures for Rounding-Off Analytical Data to Determine Compliance with
             Maximum Contaminant Levels Present in NIPDWR

FROM:      Joseph A. Cotruvo, Ph.D., Director
             Criteria and Standards Division, ODW (WH-550)

TO:          Water Supply Representatives, Regions I-X, and holders of the Water Supply
             Guidance Series

All Maximum Contaminant Levels (MCL) contained in the National Interim Primary Drinking
Water Regulations are expressed in the number of significant digits permitted by the precision
and accuracy of the specified analytical procedure(s). Data reported to the State or EPA should be
in a form containing the same number of significant digits as the MCL. In calculating data for
compliance purposes, it is necessary  to round-off by dropping the digits that are not significant.
The last significant digit should be increased by one unit if the digit dropped is 5, 6, 7, 8 or 9. If
the digit is 0, 1, 2, 3, or 4,  do not alter the preceding number.

For example, if the monthly mean for coliform bacteria is 1.4999, the reported result should be 1
(one). A result of 3.50 should be rounded to 4 (four).

Chemical and radiological data may be treated in like manner. Analytical results for mercury of
0.0016 would round off to 0.002 while 5.4 pCi/1 of combined radium-226 and radium-228 would
round down to 5 pCi/1.

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