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

Appendix G
at the outset that, rather than incorporating the variations associated with these parameters into the
determination of the dilution factor, the dilution factor would be determined as a function of depth for
two standardized soil types, specifically a typical sandy (coarse-grained) soil and a clayey (fine-grained)
soil. Variations in soil properties within these categories were considered along with other variables in
the probabilistic analysis.
The probabilistic analysis comprised two parts. First, a sensitivity analysis was conducted in which all
input variables except depth to contamination were assigned probability distributions, and were allowed
to vary during execution of a Monte Carlo simulation of the vapour transport. This was to identify the
parameters having the greatest influence on the dilution factor. A series of simulations were then
conducted, allowing the values of only the most significant parameters to be varied, to develop the
dilution factors as a function of depth and soil type.
4.0 Results of Analysis
4.1 Input Parameters
A listing of the input parameters, together with the probability distributions and values assigned to the
parameters, is presented in Table G.2. Since the nature of the actual distributions is not generally
known, most variables have been assigned a triangular distribution defined by minimum, maximum, and
modal (most probable) values. These values are based on those used in the development of the
probabilistically-derived guidelines for the Alberta MUST guidelines, and represent assumed or typical
conditions. The soil parameters are considered representative of typical Alberta deposits of finegrained
and coarse-grained soils. Building details reflect typical residential construction conditions.
4.2 Sensitivity Analysis
A sensitivity analysis was carried out in which all relevant variables were assigned probability
distributions as defined in Table G.2. A form of Monte Carlo simulation of the vapour transport analysis
was carried out using Crystal Ball Version 3.0 (Decisioneering) for one soil type at a nominal depth to
contamination, to identify the input parameters to which the dilution factor is most sensitive. The results
of the sensitivity analysis are presented in Table G.2. The parameters having the greatest influence on
dilution factor are:
• foundation crack spacing,
• air diffusivity of contaminant,
• building air exchange rate,
• floor slab thickness, and
• building height.
Certain soil parameters (moisture content, porosity) are normally significant, but in the sensitivity analysis
were kept within reasonably narrow ranges by prescribing a specific soil type. Depth to contamination
is also a significant variable; the nominal depth assumed in the sensitivity analysis was allowed to vary by
±0.25 m.
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