Soil gas monitoring

Several soil-vapor monitoring techniques are currendy being used to define areas of volatile organic chemical contamination. These procedures usually involve the collection of representative samples of the soil gas for analysis of indicator compounds. Maps marked with concentration contours of these indicator compounds can be used to identify potential sources to delineate the contaminated area. Indicator compounds (usually the more volatile compounds) are selected for each specific situation. For gasoline contamination, the compounds are usually benzene, toluene, ethylbenzene, and total xylene (BTEX). In the case of a fuel oil spill, the most commonly used indicator is naphthalene. Some laboratories have adapted the laboratory procedures used for quality analysis of wellhead condensate (i.e., normal paraffins) to include light-end (<8 carbons) molecular analysis. 

Laney, D. F., 1988, Hydrocarbon Recovery as Remediation of Vadose Zone Soil/Gas Contamination In Proceedings of the National Water Well Association Second National Outdoor Conference on Aquifer Restoration, Groundwater Monitoring and Geophysical Methods, Vol. Ill, Las Vegas, NV, May, pp. 1147-1171. 

Diem D, Kerfoot H, Ross B. 1988. Field evaluation of a soil-gas analysis method for detection of subsurface diesel fuel contamination Second national outdoor action conference on aquifer restoration, ground water monitoring and geophysical methods. National Water Well Association Dublin, OH. 

Moseley, C. L., and M. R. Meyer, Petroleum Contamination of an Elementary School A Case History Involving Air, Soil-Gas, and Groundwater Monitoring, Environ. Sci. Technol, 26, 185-192 (1992). 

Faults and fractures can act as pathways for the migration of He derived from the crust and mantle, and hence are possible secondary sources for anomalies located during exploration surveys. The potential use of He surveys to locate deep faults, and any associated mineralisation, has been discussed in the Russian literature in particular. Faults in seismically-active zones similarly act as channelways for the escape of gases, and He is one of a number of components of soil gas and groundwater that are monitored to determine whether variations in concentration can be used in the prediction of earthquakes. 

The transport of contaminants in soil gas and groundwater represent major pathways of contaminant exposure to humans or sensitive ecosystems. The vadose zone treatment methods discussed previously exert control over further contaminant release to soil gas when applied and monitored carefully. 

Scintillation chamber monitor This instrument is made of a scintillation flask in contact with a photomultiplier. Air is filtered to remove the radon daughters and is drawn continuously through the chamber. For continuous measurements of relatively high concentration of radon, such as in soil gas, an open scintillation cell may be used directly. The open end, usually connected to a tube, is inserted directly into the soil or water. The counting rate of the photomultiplier pulses is monitored continuously and changes proportionally to the radon concentration. 

Figure 3. Typical example of the application of hand-held monitoring technique for the soil gas emission of a landfill site (depth of exploration of the drilling points 1.50 1.80 m visualisation by using SURFER software)...

A more robust method of estimating surface emission rates uses the measured differential pressure in boreholes. Most gas monitoring instruments in use today are capable of recording this parameter. The differential pressure in the well should also be less variable as it should depend less on the radius of influence. It will, however, be influenced by factors such as changing ground-water levels which trap and pressurise air in the top of well. There will also be an effect due to the lag between changing atmospheric pressure and the resultant changes in soil pore pressures. 

A comparison of the results of the gas monitoring with ground conditions can help to identify the main source of gas on a site. For example, it is often useful to group together the results of gas concentration or borehole flow rate for wells with response zones in a particular soil type. 

A site was underlain by 2 m of made ground that was inert soil with very occasional pieces of wood in it. The site was to be developed with a steel framed warehouse building with piled foundations and a reinforced concrete cast in situ suspended floor slab. Historical gas monitoring data was available but did not include flow rates. 

A hole drilled in the ground in order to obtain samples of soil or rock. Permanent gas monitoring weUs can be installed in a borehole... 

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