Measuring up for ground gas readings

Ground gas concentrations can now be gauged more accurately in both the pre and post development of landfill sites. Julian Butler looks at the benefits of continuous monitoring.

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 landfill sites. However, a new generation of continuous monitoring instruments means that users can now monitor gas concentrations over time in both the pre and post development of landfill 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. They can monitor methane, carbon dioxide, oxygen, borehole pressure and atmospheric pressure on conventional risk sites such as ex-landfills.

There are also options to include hydrogen sulphide, carbon monoxide and volatile organic compounds (VOCs) using photo ionisation detector (PID) technology, and even a water-level transducer to measure water table movement. This enables 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.

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 in the past this has lead to developments occurring on sites that still posed some risk.

Risk assessment
One local authority in Southern England is using this type of instrumentation on a housing development built in the mid 1990s 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 that 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 that would not pose any risk to residents.

Time sensitive
As the instrumentation is continuously in position and monitoring occurs at regular intervals, it is possible to establish with little doubt if there is any risk. With conventional spot monitoring it is possible to completely miss gas fluxes, thus time-series data becomes invaluable as it removes the lacking variable in the current approach - time!

Using this technique it is also possible to characterise boreholes. This approach had been suggested in older guidance, but due to the lack of workable technology became less used in the industry. Purging a borehole using nitrogen and then fitting the instrumentation on a high-resolution time history (i.e. as little as every three minutes) it is possible to chart the recovery of the borehole - the time in which the borehole refills with gas.

This allows developers and consultants to establish if gas levels previously found were due to a site truly gassing or due to a build-up of pressure over time because of water table/pressure movements and no venting on the borehole. This means a developer can easily chart if gas levels on site could be dealt with using the floor void beneath properties.

New standards
This new technology does not exclude all spot monitoring requirements and it can be used in conjunction with existing instrumentation when measuring flow rates. The latest hand-held ground gas portable measuring instruments now incorporate additional features that meet the latest, most stringent MCERTs equipment standard. These latest requirements demand cross sensitivity tests to hydrogen sulphide as well as greater linearity, greater repeatability and less temperature variation.

It is also known that the presence of hydrocarbons is potentially going to give erroneous or over-range methane readings, so the latest instrumentation now incorporate a hexane measurement channel. If the methane level is measured as open range or abnormally high, the hexane indicator can confirm hydrocarbon interference. This reading is a simple crude indicator and is also useful for checking if any remedial work undertaken has been successful.

A separate instrument - normally a PID - is usually used to measure VOCs. Small quantities of methane are not detected by a PID but because methane attenuates or 'soaks up' a VOC signal in larger quantities found in boreholes it is possible to measure grossly erroneous readings on your PID. To help counter this, the latest portable ground-gas instruments have a PID compensation value that can be used to accurately correct your PID reading when up to 10% methane is present in a borehole.

Julian Butler is national sales manager of Shawcity

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