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Taken directly from an email sent by the NELAC PT Board December 23, 2005
TOTAL TRIHALOMETHANES: The disinfectant byproduct rule
(40 CFR 131(b)(2)) requires that laboratories successfully analyze 80% of the
individual parameters in a single PT Study for total triholomethanes (TTHM) and
the five haloacetic acids (HAA-5). Since there are four trihalomethanes, this
means that the laboratory must successfully analyze all four individual compounds
in a single PT Study. The federal regulations do not require successful reporting
of the total of the four individual compounds for certification or accreditation.
Because of the manner that the acceptance limits are calculated, it is theoretically
possible to successfully analyze all four individual compounds and the resultant
total would be graded as not-acceptable. Similarly it is possible to obtain a not
acceptable grade for one of the individual trihalomethane compounds and to receive
an acceptable grade for the total trihalomethanes. Therefore, the reporting and
grading of the total trihalomethane concentration becomes redundant and confusing to
both laboratories and accrediting authorities. After consulting with the US EPA
Office of Ground Water and Drinking Water, the PT Board will remove the parameter
Total Trihalomethanes from the list of analytes in the NELAC potable water proficiency
test tables.
ARSENIC: With the imminent implementation on January 23, 2006 of a lower
maximum contaminant level or MCL for Arsenic, the PT Board received a question
regarding the allowable concentration range for arsenic in drinking water PT Studies.
The federal regulations, (40 CFR 141.23 ), require that to be certified or accredited
for arsenic, a laboratory must successfully analyze a PT Study in the range at a
level greater than 5 µg/L with an acceptable range of +/- 30% of the true values.
In order for the NELAC PT tables to be consistent with the federal regulations, the
PT Board will change the concentration range for the PT Studies to 5 – 50 ug/L and
the PTRL will be changed accordingly to 3.5 ug/L. These changes should be implemented
PT Providers as soon as possible, and not later than January 23, 2006, to be in
compliance with the Federal Drinking Water Regulations.
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