Hi Paul
Have you looked at CIRIA C669. This guidance suggests 'worst case' scenario conditions during which monitoring should be undertaken:
When groundwater/tide levels are high and during a variety of weather conditions such as rainfall, frost and dry weather. During periods of low and rapidly falling barometric pressure from 1010mb or 1020mb.
The guidance says: "investigations concerned with soil gas are required to provide monitoring data sufficient to allow prediction of worst case conditions enabling the confident assessment of risk and gas protection design."
The guidance also suggests that a small datatset is likely to provide insufficient confidence.
Kind regards
Ruth
Ruth Willcox
Environmental Protection Officer (Land Quality)
Plymouth City Council
Civic Centre
Plymouth
Devon
PL1 2AA
Tel: 01752 304147
Fax: 01752 226314
Email: [log in to unmask]
-----Original Message-----
From: Contaminated Land Management Discussion List [mailto:[log in to unmask]] On Behalf Of Paul Burns
Sent: Friday, September 16, 2011 11:08 AM
To: [log in to unmask]
Subject: Gas monitoring in saturated ground
I have recently had a site investigation report submitted in support of a planning application. It is for a small site where 1 house is to be built on a plot of land adjoining a former landfill site. High levels of methane are present in the landfill although gas generation is low. Groundwater flow is towards the development site and leachate has been found to contain dissolved methane. The published geology shows alluvial deposits, associated with a former stream, to be present overlying mudstone. The SI on the development site has found several metres of made ground that includes bands of 'organic sands'. The groundwater is very shallow and the whole of the slotted section of the standpipes are within the saturated zone. When the wells are purged of water they recharge within minutes. Gas monitoring has been undertaken immediately following the purging of the wells. CH4 and CO2 levels are up to 4% and 10% respectively. Flow rates up to about 10 l/hr have been measured although this is attributed to the rapid inflow of water into the wells. The gas regime is suggested to be Amber 1. My question is how likely are the gas monitoring results reported to be representative of the gas regime at the site?
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