Environmental Engineering Reference
In-Depth Information
11.3.4 Simulating the Lower MCL for Lead for Ontario
We now proceed to estimate the probable MCL given 6-h stagnation simulation
results:
11.3.4.1 Hypothetical Experiment 1
Assume 231 samples of a total of 1,352 were indeed exceeding 10
g/L: this is
approximately 17.5 percent of data. Assume further that this proportion of samples
above an MCL is representative of the population.
Then for the data for Ontario under the 30-min Stagnation protocol, what is the
MCL level given these assumptions?
Order Ontario data under 30-min Stagnation for lead from lowest to highest;
remove top 17.5 percent; assume that top 17.5 percent of samples are above MCL
as in 6 h stagnation. This results in a cut-off point of 2.8
μ
g/L. Samples above this
value can be seen as above the hypothetical MCL. This is our
μ
first estimate of what
the Maximum Allowable Contamination or MCL by Lead should be, IF we must
use the 30-min Stagnation protocol.
11.3.4.2 Hypothetical Experiment 2
Next, use statistics to obtain minimum and maximum lead below 10
μ
g/L under 30-
min Stagnation protocol that would become greater than 10
μ
g/L after 6-h stag-
nation as shown in Table 11.2 .
Most restrictive (most risk averse) MCL estimate: Sample with minimum value
for lead under 30 min stagnation protocol but which would be above 10
μ
g/L after a
stagnation time of 6 h is 2.7
g/L.
Moderately risk averse MCL estimate: (1) Half of samples had lead values below
μ
5
g/L before adjustments were made, and (2) Average lead value for samples were
μ
5.4
g/L before stagnation adjustment. This is a workable average.
Very
μ
oriented, and possibly risky MCL estimate: Samples with
maximum lead value of 9.9
status quo
g/L would be the new
cut off,
given the samples that
μ
Table 11.2 Statistics for
samples below 10 μ g/L before
adjustments but above 10 μ g/L
after adjustments
Min
2.7
Max
9.9
Median
5.0
Average
5.4
No. of samples
179
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