Hi Steve
I think we are talking cross purposes.
The allowable indoor air concentration can be calculated directly from the TDI/ID and has nothing to do with what's in the ground. My point being, that the 'allowable' CLEA indoor air hydrocarbon concentrations aren't necessarily that 'low'.
For child receptor in residential setting, they can be quickly estimated using an averaged AC1-6 13 kg body weight and 12 m3/day.
So for an example TPH fraction of Aliphatic 10-12 (typical diesel fraction) with inhalation TDI of 0.29 mg/kg-bw/day, the allowable indoor air concentration will be between 0.15 and 0.3 mg/m3 depending on how you deal with background exposure (outputs rounded).
Using another typical estimation method using a 20 kg child at 10 m3day, would give higher numbers at 0.3 to 0.6 mg/m3.
Which could well be above the odour threshold.....
The CLEA model (standard Residential scenario, 1% SOM, sandy loam, default fixed building Qs and 50% rule for background), gives 93 mg/kg as the soil target for this fraction via this pathway (obviously a pretty unexciting soil concentration in the real world, even with the fudge factor of 10).
However, this gives a whopping soil source vapour concentration of c. 4,300 mg/m3 (due to fudge factor of 10 and the Kaw for this fraction), which is way above the predicted Csat-Vapour of 2,200 mg/m3, gives rise to 0.15 mg/m3 in the building.
That’s a CLEA pathway attenuation factor of c. 29,000 from a source 0.5m below foundations.
But according to the standard CLEA approach, this fraction alone could not be a risk via vapour inhalation as the maximum indoor air concentration (with soils at any concentration) could generate would be around half the allowable indoor air concentrations, so vapour protection in the CLEA world would not be required.
Use that at your peril in the real world!
Hopefully this will illustrate the some of the pitfalls of the indoor air CLEA model....
Chris Dainton
Peak Environmental Solutions
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