The International Information Center for Geotechnical Engineers

Landfill Gas Monitoring Systems - Soil Gas Monitoring

 3.2 Soil Gas Monitoring

Soil gas monitoring is the measurement of concentrations of gases in the subsurface. Methane is the major landfill gas targeted by soil gas monitoring, because methane is the major cause of landfill explosion and fire incidence.

Soil gas samples are collected using temporary probes or permanent monitoring wells. During sampling, the air pressure and water table in the soil must be known well by the technicians, because this two factors will significantly influence the mitigation of  soil gas. Figure 5 shows three different kinds of tips of monitoring probes. 


Figure 6-1Figure 6-2Figure 6-3

Figure 5. Three kinds of tips (From left to right) (a) Original Gas Vapor Probe Kit, (b) Dedicated GVP Tip, (c) Retract-A-Tip GVP Tip (source: AMS, Inc.)


Nowadays, various soil-gas well designs exist. The main documents dedicated to soil-gas sampling (ASTM D 5314-92; 011, VDI 3865-2; 014, ISO 10381-7; 013) do not precisely define how they should be implemented. (CityChlor, 2013). In the following part, the basic installation procedure of both temporary and permanent soil-gas monitoring wells are introduced, and then some sampling methods which can be used in soil-gas monitoring are discussed.


3.2.1 Temporary soil-gas wells

The installation of temporary soil-gas wells are usually used for perfoming a grab sampling, so it can't be used for long time monitoring or real time tracking of soil gas. Temporary wells, compared with permanent wells, are rapid to set up, which minimize the disturbance to the soil gas. So sampling can be carried out once the wells are installed, we don't need to wait for the "stabilization".

Among the different temporary soil-gas well designs, two main methods are used: rod and direct push systems.

Rod method, as shown in figure 6, firstly a rod is driven into the ground, then a specific casing with screened interval on its bottom is advanced into the borehole. The casing is linked to a specific sampling system (see 3.3). This method is only used for sampling at a shallow depth because of the rod can't be drilled to deep soil.


Figure 8

Figure 6. Rod method (INERIS)


For the direct push method, fistly, drill a borehole using a Percussion Hammer (Figure 7). Then push a rod equipped with a steel tip to the target depth. After that, the rod is withdrawn to expose target interval, through which the soil-gas will be collected by a pump.

Figure 8 shows two kinds of sampling devices. The drive point of the expendable tip samplers is left in the ground. When sampling ends, the whole system is pulled out from the ground and a new drive point is mounted before taking another sample at greater depth or at another location. In contrast, The driving point of retractable probe can be gathered again with sampling tool, so the system don't need to be pull out and can advance to greater depth for another sample.

 percussion hammer

Figure 7. Percussion Hammer (Source: Geoprobe)

direct push

Figure 8 Direct push method (CityChlor Technical Report: Direct Push Technologies, 2013, 05)


3.2.2 Permanent soil-gas well installations

Usually, permanent soil-gas wells are drilling wells fitted with one screened interval. It is used to regularly monitors the soil gas at low to medium depth. Its diameter is generally from 20 mm to 80 mm and its depth is anywhere from 80 cm to several meters (depending on the water table, source, and plume locations, as well as the objectives of the study) (CityChlor, 2013).

As shown in figure 9, a solid tube assembled with a screened interval is placed in the borehole. The screened interval is located in the middle of the sand pack filled in the borehole. Then dry granular bentonite should be placed above the sand pack. From the top of dry bentonite to the surface, fill the borehole with hydrated bentonite. The function of the bentonite layer is to seal the screened interval from the surface ambient air intrusion. At last, there should be a weatherproof for the borehole, which is placed at the surface, to prevent water and ambient air intrusion. It is obtained by sufficient thickness of low permeability material, such as concrete.

This is a design of permanent wells for a simple single-depth monitoring, there are also many other more complicated designs for multi-depth soil gas monitoring.


Figure 9. Permanent soil gas well (Waste Management, 2003)


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