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

Appendix G
partitioning coefficient, and Henry's Law constant, as well as certain soil data including
temperature and organic carbon fraction. These theoretical relationships are well defined and
have been widely verified by computer modelling and physical experimentation.
(b) Mass Transfer to Building
Contaminant mass transfer to a building located above the area of contamination is considered in
three parts: migration through the soil, migration through the floor slab, and dilution in the building
air. However, equations governing each are solved simultaneously to satisfy conditions of
continuity. Mass transfer is analyzed under steady state conditions, although the effect of a
depleting source can be accommodated.
Migration of soil vapour from the contaminated zone to the foundation is considered to occur by
diffusion under a concentration gradient (Environment Canada, 1990), as described by equation
(5) in Table G.1. Migration through the foundation system occurs both by diffusion through the
concrete floor slab (Environment Canada, 1990) and by pressure-driven flow or advection
through cracks and joints in and around the floor slab (Conway and Boutwell, 1987). The latter
effect is dominant during the winter when the temperature difference across the building envelope
induces a negative pressure differential. Migration through the foundation is governed by
equations (3) through (7). Finally, contaminants entering the building are diluted by the flow of
fresh air through the building (equation (8)).
The model is capable of considering layered soil systems, including foundation base courses.
Soil parameters required include soil type, moisture content, porosity (and effective porosity
where applicable) and organic carbon fraction. Foundation details that must be given or
assumed include floor slab thickness, crack width, and frequency. Building parameters include
area, height, foundation construction details, air exchange rate, and indoor temperature.
Contaminant-specific properties pertinent to vapour transport include diffusivity in addition to
those required for the phase partitioning calculation.
Use of the vapour transport model results in the prediction of the contaminant concentration in
the building air, corresponding to a given subsurface contaminant concentration. The building air
concentration is then used in combination with the known or assumed receptor characteristics to
determine the projected rate of intake corresponding to the source concentration.
162