Protocol for the Derivation of Environmental and Human ... - CCME

Appendix D
Table D.2
Lower Quantile Values of Cumulative Frequency Distribution for Dilution
Factor
Percentile
Dilution Factor
0 3.28
5 40.26
10 66.04
15 88.39
20 113.28
25 139.73
In this region a quasi-linear relationship exists between the modelled magnitude of dilution and its
expected frequency of occurrence at contaminated sites. A dilution factor of 65 or less is expected to
occur at about 10% of sites conforming to the model assumptions, whereas only 5% of conforming sites
should have dilutions less than about 40.
A 5% probability of accepting an incorrect decision has a long tradition in statistical evaluation of
scientific and engineering data (Steel and Torrie 1980) and, in the absence of catastrophic
consequences of an error, is a reasonable point of departure here. In view of the uncertainty in the
design of the model itself, which by virtue of assumed stratigraphy and contaminant distribution is
probably conservative, a dilution factor of 50 is recommended to protect against excessive leaching of
organic contaminants to groundwater.
3.0 Application of Dilution Factor to Groundwater Check
At equilibrium, the relationship between the contaminant concentration in soil pore water (C ws ) and in
the aquifer (C wa ) is:
C ws = DF × C wa [14]
And a final check on PSQC HH can be performed by substituting equation [14] above into equation [6]
from Appendix C, setting C wa equal to the relevant drinking water guideline, and calculating the critical
soil concentration, Y:
Y = DF × C wa (K d + ? m ) [15]
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