Karen


That sums it up in a nut shell. There are also lots of good comments from everyone else.  I would also add:

There is no problem using Gasclam (or any other high frequency data) with the CIRIA GSVs and it is entirely compatible with the table on frequency of monitoring.  The notes to the table and the text around it are just as important as the values in the table and they are not absolutes.  It all depends on the site conceptual model and the impact any additional gas monitoring will have on the risk assessment.

All of the following are cases we have come across

No gas monitoring is required (as Karen pointed out we have published a risk assessment method that does not necessarily need any gas monitoring).

A single spot sample is all that is required (it recorded 80% methane in a peat layer and we know this will not get any worse)

Spot samples over 3 to 6 months may be sufficient (we know from past experience that this can give a reasonable answer on many sites).

A mixture of high frequency and spot sampling 

High frequency monitoring in more than one well for 12 months or longer.



If you do decide that high frequency data is appropriate it can be used in the design of gas protection design using a reliability approach. 

It is straight forward to determine if you have predicted or measured the worst case on a lot of sites.  It comes back to .....yes the conceptual site model!  If you assess the source, phase of generation, key drivers for worst case (which is not always low or falling atmospheric pressure), distance from source, time since gas source was present, etc it is usually possible to determine whether you are close (or close enough) to worst case.

It also depends on the risk and you can do "what it" scenarios with the risk assessment to see if it is worth collecting more data - eg if you have say 50% methane and have recorded flow rates up 2 l/h and you understand the nature of source and it is low risk - the results give a  GSV of 1l/h ie CS 3.  Even if the gas concentration goes up to 100% it will still be CS3 so there is little point in high frequency monitoring (unless it would worthwhile showing a lower CS).  

I have seen one consultant spend several thousand pounds on high frequency monitoring to reduce a site from CS3 to CS2 - the results was that they saved the client a few hundred quid from a lower membrane specification!  It was not worth it.  Economics are also an important consideration (on a larger site with more buildings the economics may have stacked up).

High frequency monitoring is not a silver bullet that solves everything - you have to weigh up short term high frequency data vs longer term spot data - as John Naylor has pointed out the balance will change on each site.  I have sites where high frequency data has clearly not picked up worse case concentrations and the spot monitoring has. 

Line of evidence is a term taken from VOC risk assessment and originated in the USA.  It means assessing risk using different methods, for VOCs the different lines of evidence would be modelling partitioning and migration (eg J&E model),  measure vapour concentration at source, measure vapour conetnration in pathway, measure at receptor.  For gas it would be model waste degradation and gas migration, measure gas at source (ie in wells) measure in pathway (outside source in the ground or in a void for example), measure at receptor. Measuring gas in a borehole is one line of evidence - in my opinion the high frequ data is doing the same thing and is not a separate line of evidence.


With respect to monitoring inside the building - how are they going to measure the gas. Inside buildings you really need to monitor down to ppm levels to be sure that there is no ingress (ie using an FID).  You would be better off monitoring in the underfloor void if they have one where monitoring to 0.1% would be acceptable.  If you do that make sure they put a specific monitoring point into a dead spot in the void and that it extends far enough in.  When monitoring make sure the sample time is sufficient to draw a completely new sample along the line each time.



Phew!  Well done to anyone who has got this far!




Steve Wilson, Technical Director
on behalf of EPG 

Tel 07971 277869
www.epg-ltd.co.uk

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-----Original Message-----
From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of Karen Thornton
Sent: 01 February 2012 16:31
To: [log in to unmask]
Subject: Re: Continuous Monitoring using Gas Clam

Not wishing to state the obvious; & in acknowledging that full information regarding the scheme (in question) is essentially unknown, I would have believed that the essential starting point really should be to go back to first principles (i.e. walk before you run) 

A full detailed consideration of the conceptual site model is always the starting point. 

Essential questions to enable a robust defensible judgement on how to proceed should really be questions such as: What is the gassing source? Is it actually on site? off the site? What's the likelihood of the pathway existing? What's the likely driver for gas movement (geology & surface covering obviously strongly affects this aspect), What measures have actually been installed, what is the slab type? & are the properties currently occupied? (& so on). 

Steve Wilson produced a paper recently as a means to simplify Gas Risk Assessment & judgements, which might assist in rationalising your judgement on the likely gas risks & hopefully enable a defendable judgement on whether basic gas measures might (or might not) suffice. 

It could be that post construction integrity testing (to assess the seal/permeability contrast of measures installed) could offer the necessary reassurance required or it might be more appropriate to obtain multiple lines of evidence (one example being use of continuous monitoring but supplementing by other means). Again a thorough review of as much information that can be found should hopefully enable a robust defendable judgement on options on how to proceed  

Where properties are actually occupied & where risks are considered plausible; then urgent measures (such as temporary monitors & alarms) should be considered until the risks are appropriately investigated.  

In terms of experience of the gas clam/etc,  NHBC have accepted evidence using such methods but also other approaches/methodologies have proved just a adequate/appropriate. The selection is really based on a considered judgement of ALL available evidence & what method can actually be employed post construction.   


Karen Thornton  
Specialist Environmental Engineer 
NHBC Engineering 


-----Original Message-----
From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of Sara Ball
Sent: 31 January 2012 13:34
To: [log in to unmask]
Subject: Continuous Monitoring using Gas Clam

I have a site where the houses have been put up without the required gas monitoring being undertaken before hand.  I now need to determine whether the gas protection measures incorporated into the dwellings is sufficient.  The developer is proposing to employ a firm who will use the GasClam system to undertake 6 weeks of continuous monitoring in Boreholes to be installed in the gardens.  In addition they will be doing continous monitoring inside the dwellings near a service entry point or other potential weak spot in the construction.

Has anyone had this method submitted to them as the only form of gas monitoring to determine protection measures and what were their experiences?  What would be considered an adequate period of monitoring for a residential site next to an extensive area of made ground?  How would you compare this to the CIRIA/NHBC guidance tables? 

I've already read the literature provided on the companies case studies and the CL:AIRE research bulletin RB13.

Thanks, in advance, for any advice. 

Sara Ball
EHO - Nottingham City Council


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