Hi All,
I'm around a week behind on my Jiscmail strings and just picked up the interesting topic of vapour proof membranes. Chris and Steve are both right and depends on the actual volatiles or range of volatiles being considered.
If I can sidestep the toxicity or additive issues (Yep, one heck of a step!), one of my main concerns with use of membranes for vapours focuses on the integrity of membrane installation. If we take the lab permeability results at face value for the membranes, fine, but you only need a small hole, tear or bad weld in just the wrong place and the membrane may as well not be there. For bulk gases this is different as a few holes may still be fine as were looking at percentage by volume, whereas parts per million or lower for volatiles.
Therefore I would recommend that other levels of management/protection inbuilt, preferably starting with reduction at source, then distancing the receptor and then dilution (clear void) beneath the building, then membrane. The protection measures should be fully verified and in the case of the membrane, professionally installed with follow on CQA to ensure (as much as possible) no damage comes to it due activities from later trades (plumbers for example).
I too think this is an important part of protection that appears to be ignored far too frequently!
John Naylor
Technical Director
Mobile: 07856 244 224
Ground-Gas Solutions Ltd, Williams House
Manchester Science Park, Lloyd Street North, Manchester, M15 6SE
Telephone: 0161 232 7465
Visit our website: www.ground-gassolutions.co.uk
Before you print - THINK!
-----Original Message-----
From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of Steve Wilson
Sent: 04 August 2010 16:38
To: [log in to unmask]
Subject: Re: Contamination Depth & Indoor Vapour Levels
Chris
Think we are talking cross purposes or I did not make myself clear.
My point is that the allowable indoor concentration is very low compared to the rate at which vapours can pass through membranes. Therefore you cannot assume the membrane is impermeable.
This is different to the bulk gases where the permeability of the membrane is much lower (orders of magnitude) than the rate at which gas can pass through a membrane so we can safely assume it is effectively impermeable.
Just something we should all be thinking about .
Steve Wilson
Sent using BlackBerry® from Orange
-----Original Message-----
From: Chris Dainton <[log in to unmask]>
Sender: Contaminated Land Management Discussion List <[log in to unmask]>
Date: Wed, 4 Aug 2010 15:59:23
To: <[log in to unmask]>
Reply-To: Chris Dainton <[log in to unmask]>
Subject: Re: Contamination Depth & Indoor Vapour Levels
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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