use one sample or two samples per month for Cryptosporidium
LT2ESWTR has separate
requirements for one sample per month versus two samples per month?
of the new concepts incorporated into the LT2ESWTR is to link treatment
requirements in the rule to results from Cryptosporidium
source water monitoring. In developing the LT2ESWTR source water Cryptosporidium monitoring requirements,
USEPA and the FACA
stakeholders had to weigh two competing factors. On the one hand, collecting
samples for a long time period spanning several months or years is desired so
that events representing fluctuations in water quality conditions have a
greater chance of being captured during the monitoring period. On the other
hand, if the source water is characterized by a high Cryptosporidium risk (i.e., Bin 2, 3, or 4), this needs to be
established as soon as possible so that corrective action can be initiated.
Therefore, the compromise solution was to require collection of at least two
years of data in order to limit the length of time prior to initiation of
corrective action, while collecting sufficient data to capture events during
multiple seasons (eight seasons in a two year study).
during preparation of the LT2ESWTR indicated that the overall
mean for 48 or more samples had an error rate of <2% for both false
positives (facility assigned to bin level that is too high) and false negatives
(facility assigned to bin level that is too low). The overall mean of 24
samples had a false negative rate that was too high (6%), but using the LT2ESWTR
approved approach (highest average of any 12 consecutive months in the 24 month
period) produced a more acceptable false negative error rate (<2%), though
false positive rate is higher with this approach (5.3 versus 2.8 %). Therefore,
the LT2ESWTR requires overall mean to be used for 48 or more samples, but
requires maximum average for any 12 month period when 24-47 samples are used.
is bin assignment determined for systems with fewer than 48 samples? How is it
different for systems with 48 or more samples?
When 24-47 samples are
collected, the average is determined for each month, then
these monthly average values are averaged for each 12 month period. The largest
of 12 month averages is used to establish bin assignment when the number of
samples is less than 48. When 48 or more samples are collected, the overall
average of all of these samples is used to establish bin assignment.
would a utility benefit from collection of two samples instead of one? Doesn’t
it cost more to analyze more samples?
LT2ESWTR compliance monitoring
for 48 samples is expected to cost about $25,000 (including filters), versus
$13,000 for 24 samples. Therefore, analyzing two samples a month for 24 months
will cost about $12,000 more than analyzing only one sample per month. However,
most utilities would probably consider this cost difference a worthwhile
investment given the consequences of being improperly assigned
to a bin level that is too high (false positive). Using LT2ESWTR information on false
positive and negative error rates, for the $12,000 investment of increasing
from 24 to 48 total samples, a utility would marginally improve the false
negative error rate (1.7 to 1.4%), but would markedly improve the false
positive error rate (5.3 to 1.7%). Consequently, a utility could potentially
avoid unnecessary expenditures by collecting enough samples to make sure they
are assigned to the appropriate bin.