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

Appendix G
2.2 Contaminant Vapour Transport
Volatile organic compounds in the subsurface may exist in pure (immiscible) form or in the dissolved,
adsorbed, and vapour phases. The distribution of a contaminant between phases depends on the
contaminant concentration, soil type, moisture content, porosity and organic carbon fraction and is
governed by specific physical properties of the contaminant. Contaminant vapours present in the
unsaturated zone above the groundwater table, or soil gas, can migrate by diffusion under a
concentration gradient or by advection under a pressure gradient. Vapours may be released to the
outside air at the ground surface or may enter underground structures (e.g., utilities) or buildings. The
latter poses the greatest health risk, since buildings are often underpressured due to heating, and the
degree of dilution in the indoor air of a building is less than that outdoors. Once in the building, the
vapours may represent a potential risk to occupants of the building.
3.0 Methodology
3.1 Introduction
To quantify the relationship between source concentration and point-of-exposure concentration resulting
from the vapour transport and inhalation pathway, a dilution factor has been defined as the ratio of the
contaminant vapour concentration in the soil to the contaminant concentration in the indoor building air.
The relationship between the contaminant concentration in the soil or groundwater and that in the soil
vapour is dominantly contaminant-specific, and has not been incorporated into the dilution factor.
The relationship between soil vapour concentration and indoor air concentration is a complex function of
a number of variables and parameters related to soil conditions, depth to contamination, building
foundation and mechanical system details, meteorological conditions, and contaminant properties.
Recognizing this, the study has not attempted to develop a unique dilution factor applicable in all cases.
Instead, dilution factors have been developed as a function of a number of key parameters including
depth and soil type, and those variables having less influence on the vapour transport process have been
treated probabilistically or standardized for those parameters with minimal influence.
The process involved conducting a probabilistic analysis using a proprietary vapour transport model.
The approach is described in further detail in Sections 3.2 and 3.3.
3.2 Vapour Transport Modelling
A vapour transport and exposure model which predicts the indoor air concentration based on the
contaminant concentration in the appropriate subsurface medium was developed (OAEI 1994). The
model was used in the development of the Alberta MUST guidelines (Alberta Environment, 1991), and
has undergone independent peer review. The vapour transport analysis is divided into two parts: phase
partitioning in the soil and mass transfer to the building.
(a) Phase Partitioning
Using the measured contaminant concentration in soil or groundwater (as appropriate) the
equilibrium soil gas concentration in the unsaturated zone is determined using accepted phase
partitioning relationships. The relationships are described by equations (1) and (2) in Table G.1.
The calculation requires contaminant properties including molecular weight, organic carbon
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