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Guidelines on remediation of contaminated sites

Appendix 5.2
Convection model for gas invasion into surrounding buildings

This Appendix describes a model for landfill-gas transport into buildings placed outside of landfill area producing gas. The model, which calculates the convection contribution, is based on a worst-case scenario, where the entire quantity of gas being transported enters the building /1/. As such, this convection model is simpler than the model for convection contribution which is described in Appendix 5.3.

Calculation assumptions and formulae

A landfill with soil gas concentration Cp is situated at a distance x from a building, as shown in Figure 1. Gas transport is assumed to take place solely in the upper unsaturated zone. The landfill has a positive pressure Ps in relation to pressure at the building. Atmospheric pressure is Patm.

Figure 1
Diagrammatic presentation of model for transport of landfill gas to surrounding buildings.

In this model, the extent of the landfill is assumed to be much greater than the distance to the building. The following applies to the situation of equilibrium:

(1)

where:

a = the ratio between inside-air concentration of the building and soil gas concentration of the landfill
k = air permeability (m2)
Ps = landfill positive pressure (Pa)
m = dynamic viscosity (kg/m ž s)
x = distance (m)
l = ceiling height (m)
L = air renewal (s-1)

The period elapsing before establishment of pressure gradient tssp (Equation 2) and equilibrium concentration tssc (Equation 3) is given by

(2)
(3)

where: Ea = air porosity (unitless)

Patm = atmospheric pressure (Pa)

Data basis

The calculation formulae (equations 1-3) state the necessary data. Empirical data for input data are listed in Table 1. For constructional data, see Table 1 in Appendix 5.3.

Table 1
Empirical data for transport model data.

Air permeability, k

m2

Clayey till 1)

Fine sand

Coarse sand

10-13

10-12

2 ž 10-11

Positive pressure in the site, Ps

Pa

Typical values measured

0 – 2,000

Dynamic viscosity, m

kg/m× s

Air

Methane

Carbon dioxide

1.8 ž 10-5

1.1 ž 10-5

1.5 ž 10-5

Air porosity, Ea

unitless

Loam

Sandy loam

Clay

Sand

0.1

0.1

0.1

0.3

1) Estimated value. See also Table 2 and 3 in Appendix 5.8.

References

/1/

Little, J.C., Daisey, J.M. and Nazaroff, W.W: Transport of Subsurface Contaminants into Buildings. An Exposure Pathway for Volatile Organics. Environmental Science and Technology. Vol. 26. No. 11, 1992, p. 2,058 – 2,066.
[Tilbage]

 

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