Instantaneous monitoring consists of walking over the surface of the landfill, while carrying a flame ionization detector (FID).
The principal odoriferous compounds are hydrogen sulfide (which is also toxic) and the majority of a population exposed to more than 5 parts per billion will complain (World Health Organisation : WHO (2000) .
The presence of H2 is consistent with thermal inactivation of CO2-reducing microbes, which normally combine all H2 produced by fermentation of organic acids with CO2 to form methane (CH4).
Several techniques have been developed for evaluating whether landfill gas (rather than leachate) is the source of volatile organic compounds (VOCs) in groundwater samples.
Highly soluble VOCs, such as MtBE, diethyl ether, and tetrahydrofuran, are evidence of leachate effects, since they are too water-soluble to migrate in landfill gas.
The presence of highly soluble semi-volatile organic compounds, such as phenols, are also consistent with leachate effects on the sample.
Tunnelling effects, whereby large items (including monitoring boreholes) create bypass shortcuts into the interior of the landfill, can extend this variability to greater depths in localised zones.
Such phenomena can give the impression that bioactivity and gas composition is changing much more radically and rapidly than is actually the case, and any series of isolated time-point measurements is likely to be unreliable due to this variance.
Physical settlement as waste decomposes makes borehole monitoring systems vulnerable to breakage as the weight of the material shifts and fractures equipment.