Clive,
I don't think any cover system can be relied upon forever. Using an engineered cover system such as what you are suggesting with a fixed textile/membrane, layers of compacted aggregate etc may be ok in the short term but can you really rely on it to remain as it was installed forever?, and who checks if this is the case? Plus these covers can often cause other problems i have found, including drainage, and the grass/plants above them can suffer.
If you do not agree with the 600mm limit included in the BRE guidance, then only consider covers up to <600mm acceptable. If i see a report which states 600mm from the BRE guidance, it can often be because there are high concentrations of contamination and the BRE guidance is beyond its upper limit, and that's where i think caution is advised. Up to that depth, i would rather expect complete intermixing and account for it, rather than design for it to never occur and have to rely on an engineered cover lasting for hundreds of years.
Although the EA may have not encouraged the use of this guidance due to waste dilution issues, I would expect their main concern would be that contamination can remain in the ground, under the cover and the EA, more often than not, will be looking at other receptors than human health. If you are worried about risks to groundwater, what use is having 534mm of soil put on top of the contamination, which is understandable.
Marcus
Marcus Bell
Scientific Officer: Environmental Services
East Cambridgeshire District Council
The Grange, Ely, Cambs, CB7 4EE
TEL: 01353 665555 DD 616463
FAX: 01353 616223
-----Original Message-----
From: Contaminated Land Management Discussion List
[mailto:[log in to unmask]]On Behalf Of Clive
Williams
Sent: 25 August 2010 13:53
To: [log in to unmask]
Subject: Re: BRE: BR 465
The document assumes that mixing of the contaminated soil and the clean cover occurs and uses a dilution principal to determine how clean the clean cover needs to be in order to dilute the resultant soil once mixing has been completed - which kind of sounds like a waste treatment exercise, possibly. An earlier posting has discussed how you can reduce the cover thickness required by making the cover cleaner, which is kind of counter intuitive to anyone thinking that the purpose of the cover is to protect from what is underneath, whereas the cover is actually controlling your epxosure to an "acceptable" degree. Its not really a cover system in that cover implies permanance (to my mind).
The fact that on every ocassion I have used it I get 600mm tends me to think that it isn't particularly sophisticated; that's not to decry it as it is only a simple weighted averages spreadsheet after all, its a blunt instrument.
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