Good morning all,

I am looking at a site where development of a commercial facility is proposed. The consultant has commissioned gas monitoring across the site and 9 boreholes have been monitored 6 times over a 5 month period.

The consultant has calculated Gas Screening Values for each measurement taken, i.e. concentration (%) x flow (l/h). Each result has been calculated to be zero, and therefore the site has been concluded to be Characteristic Situation 1.

However, elevated concentrations of CO2 were recorded at a few locations during the rounds of monitoring (up to a maximum of 17.1%), although when they were recorded, flow was found to be zero, resulting in a GSV of zero. Similarly flow was recorded at the site up to a maximum of 5.7l/h, but when flow was recorded, CO2 was found to be 0%, again resulting in a GSV of zero.

CIRIA C659 states:

"The assessor should be confident that the gas monitoring results are representative of the likely worse case gas regime on a site..." (Section 8.3)


"For example, monitoring data giving a maximum flowrate of 3.5l/h and a maximum concentration of 4.0% methane would have a GSV of 0.14l/h" (Section 8.3.1)

My question is, should assessors be taking each monitoring result as a standalone, i.e. only multiply flows and concentrations measured at the same borehole on the same monitoring visit? Or should we be taking the absolute worst case scenario, i.e. the highest recorded flow and highest recorded concentration at any borehole on any monitoring visit?

In my case this worst case scenario would be 5.7 x 17.1 = 97.47l/h. This gives us CS6 - very high risk, which seems a bit absurd for a site which has been agricultural since 1850s but is overlain by made ground likely sourced from nearby road construction. In general flows across the site are low or zero, and CO2 concentrations are low, save for a couple of elevated results.

Any help would be greatly appreciated!


Neil Brown
Contaminated Land Officer

Development Planning
Development and Regulatory Services
West Lothian Council
County Buildings
West Lothian EH49 7EZ
Tel. 01506 775263
Fax. 01506 775265
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