Making sense of ground gas with continuous monitoring

Dr Steven Goodman of monitoring expert Shaw City appraises the limitations with many existing ground gas monitoring programmes, and the opportunities that now exist to better understand what’s happening, through the use of continuous monitoring instruments

The GasClam gas monitor can provide up to three months of continuous data collection.

A ground gas risk assessment is often undertaken at the outset of a construction site’s development. This is usually required by the local planning authority and subsequently undertaken by a developer or their contractor. Spot monitoring visits can continue for up to 12 months or more, if methane is found at over 5% by volume.

Such a monitoring programme has to determine the actual ground gas regime and how it is likely to change in the future. In other words, it is not just a screening exercise. At present, it is most commonly carried out via discrete periodic static measurements of ground gas concentrations, from which a ground gas regime is inferred. However, this discrete-measurements approach is inherently flawed, being unable to accurately  measure   ground-gas  concentrations  and  ground-gas  fluxes. Neither  are  measured  directly  and  both  are  likely  to  vary over time.

Measurement  is  indirect  since  ground-gas  concentration  is  inferred  from  periodic sampling  of  gas  accumulated  within  a  borehole  and  flux  is  then  inferred  from  these borehole  gas  concentrations. The  unit  of  flux  is  volume/time,  therefore  it  cannot  be determined without time series data. With  the  ability  to  collect  time  series  data,  an  improved  measurement  of  flux  can  be made  and  temporal  variability in ground gas concentrations can  be  quantified  and  accounted  for.

This  will  provide an improved understanding  of  the processes under observation,  thereby  reducing  the  uncertainty  implicit in using  measurements  that  are  indirect. Contaminated  land  and  landfill  industry  regulators  recognise  the  need  for  more representative  data  but  cost  has  prevented  the  collection  of  continuous  records  of ground-gas  measurements.

However, technologies are appearing which permit continuous ground gas monitoring, such as the GasClam landfill gas monitor (pictured, right), available from monitoring expert Shaw City.

Uncertainties with the conventional approach

The  current  approach  relies  on  making discrete  measurements  of  concentration  from  which representative  ground  gas  concentrations  and  gas  migration  potential  can be  inferred. However,  as  system  data  is  poorly  resolved  temporally,  uncertainties  in  these inferences  remain  large. For  example,  the  frequency  of  variation  in  gas concentration  may  be  higher  than  the  sampling  frequency,  in  which  case measurement  will  not  be  representative.

The Frog-4000 provides organic compound analysis in water, air and soil. It can be used as a portable GC PID when carrying out environmental tests in the field.

Continuous  monitoring  can overcome this mismatch. Importantly,  time-series  data  can also  reveal  that  the  frequency  of  variation  in  gas concentration  is  highly  variable.

Linking gas concentration and  other environmental variables

With higher time-axis resolution of not only gas concentration but also other environmental variables it becomes possible to observe their inter-relationships more clearly. This in turn makes it easier to predict how gas concentration will change as other parameters change. Atmospheric pressure is considered to be a strong driving force for gas migration (Wilson et al, 2008). In general it is assumed that concentrations are higher when pressure is low and vice versa. Existing guidance (e.g. CIRIA Report 665) recommends collecting at least one spot sample below 1000mbar in falling pressure. However, continuous ground gas measurements reveal the arbitrary nature of the 1000mbar limit: Concentration levels change continuously with variations in atmospheric pressure, rather than displaying a clear dependency on the absolute atmospheric pressure.

Furthermore, under some conditions this assumed relationship between pressure and concentration doesn’t exist – an inverse relationship between the two has even been observed.

So, the  ability  to  monitor  environmental  parameters  and gas concentration  simultaneously can provide  an  understanding  of  the  processes  contributing  to  ground-gas  production and  migration. Initial  results  suggest  that  the  relationship  between  environmental parameters  and  this gas concentration  are  complex  and  currently  poorly  understood.

As further understanding of the underlying processes is unearthed, it will be possible to build   more  representative conceptual  models.  This  will also benefit  risk  assessment,  which  is  currently based  on  a patchwork of inferences  made about  worst-case  conditions  determined  by periodic measurements  of  gas  concentration.

Organic compounds risk

In many situations it is also important to consider the risk of organics contamination in ground gas in the form of volatile organic compounds (VOCs). These can easily be ignored or misidentified due to ground gas monitors mistakenly identifying them as methane (CH4). This can occur as a result of infrared absorbance measurements detecting the methyl groups present on most organic compounds (as well as methane).

Photo-ionisation detectors (PIDs) are often used to identify the presence of organic compounds by spot-sampling the ground gas. This can be very useful as an initial screening exercise if methane levels are genuinely below approximately 4% by volume. Any concentration close to this renders a PID impotent due to methane absorbing the UV output of the PID lamp. Hence, VOC analysis has to be undertaken, either through grab sampling or laboratory analysis.

Continuous on-site analysis of VOCs is becoming more practical. One instrument that is being used here is the Frog 4000 from Defiant Technologies (pictured, above). This unit allows for VOC analysis to laboratory standards. It can be used for both qualification and quantification of organic vapours. Target analytes can be separated and detected in less than five minutes.

The promise of continuous monitoring

Continuous  gas-monitoring  data  has  revealed  several  potential  flaws  with  existing monitoring  methodologies. Ground-gas regimes appear to vary, depending on the conditions present on
the site. So it can be difficult to identify a sampling frequency that will allow measurements to capture the variability in gas concentrations.

New varieties of instrument that permit on-site field analysis of VOCs in boreholes hold forth the promise of gaining a much fuller understanding of the nature of ground gas emissions and the potential problems presented by organic compounds, with their potential to contaminate groundwater and soils.

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