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The presence of H2S would suggest that there is an amout of carbon present somewhere within the system.  Reduction of sulphate via removal of oxidants will lead to H2S generation...call it what you want anaerobic or anoxic.  H2S is very soluble as Oxygen and Sulphur are similar to some degree due to their position in the periodic table...H2S, H2O.  The source of the H2S could therefore be a little remote from your site.  Look at dissolved H2S in a reducing environment.  This is probable.  Also remember that H2S will adsorb to certain soils and in Oxidising conditions it will react back to sulphate..... so you can understand where it is not being generated/stored.
 

Keiron Finney MSc MRSC MCIWM CChem CSci CEnv CWMan, Grad IOSH

Midlands Technical Specialist (Landfill Gas)

Environment Agency

 

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From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of Steve Wilson
Sent: 20 May 2012 17:48
To: [log in to unmask]
Subject: Re: Gas within boulder clay

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Alex

 

It is not unusual to get gas under pressure in sand layers in Glacial Till.  As you say the confining clay layer is the reason it is under pressure.  You need to take the clay layer into account in the risk assessment.  If the foundations do not penetrate very far into the clay it is not likely to be a particular risk.  If the foundations do penetrate the clay into the layer it may well be a short term release (depends on extent of pocket) that again is not a major risk to the development, but does have H&S implications during construction.  It is relatively easy to demonstrate whether there is a risk or not by simple calculations.

 

I have not come across H2S before but have found methane and carbon dioxide in Glacial Till (and in London Clay).  Given that Glacial Till is sediment deposited by a glacier it is a reasonable assumption that you will get pockets of gas like this occasionally as it can contain organic material.

 

It is also reasonable to find H2S in the Sherwood SST – wells drilled into sandstone often have levels of H2S in the water.  The gas may have been formed biologically or chemically – you should do testing to understand the chemistry of the soil and groundwater around the hole you have found the gas in.  This will help to inform the risk assessment.

 

Steve Wilson, Technical Director

on behalf of EPG

 

Tel 07971 277869

www.epg-ltd.co.uk

 

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From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of Alex Smiles
Sent: 19 May 2012 13:45
To: [log in to unmask]
Subject: Gas within boulder clay

 

Hi

 

We are currently drilling on a site and have encountered a significant pressurised gas ingress.  The ingress occurs at the base of a 5m deep glacial clay layer and the gas in question seems to be h2s

 

The csm does not indicate the presence of mine workings beneath the site and there is no infilled land or landfill sites within 1000m of the site

 

The underlying geology is said to be sherwood sandstone and I can see no obvious source for the ingress, although it is evident the confining glacial clay layer is most likely causing it to be pressurised.

 

I am about to take a reading on the gas monitor using a standpipe we have sealed into the layer however thereafter i am grouting the hole up as the gas is escaping from the tap at a considerable pressure and I am not happy leaving it insitu 

 

Has anyone ever had a similar experience with glacial till ?

 

The joys of weekend working!!! 



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