UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WSG 173
Date Signed: March 9, 2004
MEMORANDUM
SUBJECT: Compliance Calculations Under the Lead and Copper Rule
FROM: Cynthia Dougherty, Director
Office of Ground Water and Drinking Water
TO: Jane Downing, Associate Director for Drinking Water Policy
EPA Region I
Region I asked the Office of Ground Water and Drinking Water to give an opinion on wether a
90th percentile lead level can be calculated even if the minimum required number of samples are
not taken by a water system (or by any one or more of a series of consecutive systems where
otherwise applicable monitoring requirements have been modified under 40 C.F.R.§141.29)
during a specific monitoring period. Yes, the 90th percentile level can and should still be
determined. If the systems does not take the required number of samples, it would incur a
monitoring and reporting violation. In addition, if the 90th percentile of the samples actually
taken is above the action level, then the system would have an exceedance, and would be
triggered into further requirements.
The Lead and Copper Rule regulations do not require a minimum number of samples to calculate
the 90th percentile. Rather 40 C.F.R. § 141.80(c)(3) says, "The 90th percentile lead and copper
levels shall be computed as follows:
(i) The results of all lead and copper samples taken during a monitoring period shall be
placed in ascending order from the sample with the lowest concentration to the sample
with the highest concentration. Each sampling result shall be assigned a number,
ascending by single integers beginning with the number 1 for the sample with the lowest
contaminant level. The number assigned to the sample with the highest contaminant
level shall be equal to the total number of samples taken.
(ii) The number of samples taken during the monitoring period shall be multiplied by 0.9.
(iii)The contaminant concentration in the numbered sample yielded by the calculation in
paragraph (c)(3)(ii) is the 90th percentile contaminant level.
The regulations state that the 90th percentile level is calculated based on "all the samples taken
during a monitoring period" and do not state a requirement that the minimum required number or
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WSG 173
samples must have been collected in order to calculate the 90th percentile level. This makes
sense because, otherwise, a watery system could conceivably avoid an action level exceedance
triggering important public health protection steps, simply by its own failure to take the
minimum required number of samples.
The Agency's intent is to prevent any masking of exceedance or violations due to a system's
own failure to conduct proper monitoring. For example, to address this issue, language was
added to the 2001 Arsenic Rule to clarify compliance calculations for chronic contaminants. This
language states that "if a system fails to collect the required number of samples, compliance will
be based on the number of samples collected." Although this language is drawn from the Arsenic
Rule, the same concept applies to the Lead and Copper Rule and is consistent with the above-
quoted language from 40 C.F.R. § 141.80(c)(3).
We recognize, however, that several guidance documents issued by EPA have been written to
indicate that the 90th percentile cannot be calculated if less than the minimum number of samples
were collected. While this may make sense when applied to the smallest systems that collect
only 5 sampless per sampling period - since taking fewer than 5 samples could simply be up
actions - this concern does not exist for systems that take more than 5 samples. Moreover, the
view stated in these guidance documents is simply inconsistent with the language and intent of
the regulations, as discussed above, and we intend to rescind any guidance inconsistent with the
terms of this memorandum. In the meantime, of course, legal requirements are governed by the
terms of the regulations rather than guidance materials and, therefore, a 90th percentile level can
be calculated even if the system has collected fewer than the "minimum required number of
samples."
I hope this answers your question. If you have other questions, please feel free to contanct me or
Ron Bergman, Associate Chief of the Drinking Water Branch, at 202-564-3823.
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