The latest Ground Gas monitoring innovations
The traditional methodology for ground gas monitoring has been ‘spot monitoring’ – taking event readings at various set times to predict the sub surface gas regime on both on Brownfield and Landfill sites.
This methodology is in general current practice under both the EU Landfill Directive 1999/31/EC which is supported by guidance publications from the Environment Agency and the procedures for Environmental Compliance Gas Monitoring on Land Development Sites which have come about by a different route via the publication of the DEFRA / Environment Agency Contamination Land Report 11 (CLR 11) – ‘Model Procedures for the management of Land Contamination’., a joint NHBC / RSK document entitled ‘Guidance on evaluation of development proposals on sites where methane and carbon dioxide are present’ and the CIRIA Report C665 ‘Assessing risks posed by hazardous ground gases to buildings’ which have culminated in the development of BS8485 ‘Code of Practice for the characterisation of ground gas in Brownfield development’.
All this guidance was based on the technology available at the time. Now with the introduction of a new generation of continuous monitoring instruments, a key breach in the current approach has been filled allowing for the monitoring of gas concentrations over time in both the pre and post development of these sites.
These instruments can be installed in an existing or new borehole on a site and left to continuously monitor gas levels over several months and can monitor Methane (CH4), Carbon Dioxide (CO2), Oxygen (O2), Borehole Pressure (Barometric) and Atmospheric Pressure for use on conventional risk sites; such as ex-landfills. However there are also options to include Hydrogen Sulphide (H2S), Carbon Monoxide (CO), Volatile Organic Compounds (VOCs); using Photo Ionisation Detector (PID) technology and even a water level transducer to measure water table movement. This allows the user to monitor all of these parameters in one unit and most importantly at regular intervals, usually once an hour, over a specific monitoring period.
Continuous Monitoring Instrument being deployed
Into borehole (Shawcity, GasClam)
Using time series data it is possible to more accurately determine the true subsurface gas regime and therefore predict with a much higher level of accuracy how this may change in the future. Using existing methodology it has always been difficult to accurately gauge the risk of sites and has in the past lead to developments occurring on sites which still posed some risk.
One local authority in Southern England is using this type of instrumentation on a housing development built in the mid 1990’s over a former landfill site. A continual monitoring program had been in place using spot sampling technology and a level of gas had been monitored on one borehole suggesting there were some residual gas issues on the site and local properties could be at risk. In a joint approach between the Environment Agency, the local authority and a specialist consultancy it was decided this site needed some further investigation as a matter of urgency. Key monitoring points were identified and the instruments were deployed over a six month period with data retrieved at regular intervals. The quality of the time series data provided enabled the local authority to make key decisions about the site and also revealed that, in fact, gas levels were at safe sustainable levels which would not pose any risk to residents.
This technology is also being used by developers before construction allowing them to create extremely high quality plans and predict with accuracy the point at which existing Brownfield sites can be developed. The existing methodology requires up to 24 months of monitoring using spot sampling on multiple occasions in a month depending on the risk level of the receptor and the site. However time series data has been used to reduce this required monitoring period to as little as one month!