17Feb / 2015
Continuous ground-gas monitoring is a popular method for certain situations (see previous article on ‘when to use continuous ground-gas monitoring’).
This article tries to answer the question: what does continuous really mean in the context of ground-gas monitoring?
The Oxford English dictionary defines continuous as: Forming an unbroken whole; without interruption.
Traditionally, the contaminated land industry has relied on ground-gas monitoring results collected at a frequency of 1-2 weeks. It can be shown that data collected at these frequencies can miss the full variability in what is being measured. A comparison of weekly monitoring data (plotted on a time series graph) with continuous monitoring data demonstrates this.
Data plotted above is collected with GasClam, an unmanned, in-borehole ground-gas monitoring instrument. Unmanned ground-gas monitoring instruments can constantly pump, running a continuous flow of gases across sensors. However, the output from these sensors must be recorded and stored for user analysis and interpretation. An infinite number of data points would be collected unless they are separated by a time interval. Therefore in practice, an instrument monitoring ground-gas concentrations must log these concentrations at discrete time intervals. The optimum time interval must be sought; too short and instrument lifespan will be compromised; too long and you risk missing variability in the measured parameter.
Providing the frequency of measurements is high enough to detect the full variability in ground-gas concentrations, we can be confident that the data collected doesn’t miss anything. Previous research has shown that ground-gas concentrations are highly variable and driven by changes in the environment, such as atmospheric pressure, temperature and groundwater fluctuations. However, these variations do not commonly occur in time intervals below 60 minutes. This research resulted in the publication of CL:AIRE RB13 which defines ‘continuous monitoring’ as monitoring at a frequency which matches or exceeds the frequency of change in the measured parameter.
In addition, not only does continuous monitoring allow you to identify the full variability of ground-gas at a particular location, it can provide confidence that the worst case conditions have been captured. More on that in a future article.
So, to summarise:
- In order to identify the full variability of ground-gas on a site, use continuous monitoring.
- Continuous monitoring can be defined as: A frequency which matches or exceeds the frequency of change in the measured parameter.
If you have any comments, please do post them below.