Measuring and interpreting gas flow from landfill boreholes

Current landfill gas measurement practice for measuring the gas composition is well understood, and there are standard procedures and instruments available to analyse the gas. It is also current recognised practice to measure the gas 'flow' from boreholes to monitor the production of gas in a landfill site. If care is taken to make the measurement in the same way each time, changes in the 'flow' can be a useful indicator that something has changed within the landfill and may indicate the need for further investigation. However, using these 'flow' readings as an absolute measure of the gas generation within the landfill will lead to misleading results, as explained below. A capped landfill, with sealed boreholes, will build up a static pressure, Ps. Although this pressure is related to the rate of gas generation it is also heavily influenced by other parameters, such as the gas permeability and thickness of the cover, and of course any gas escape routes that may exist through the cover. Thus the pressure in the borehole (and thus landfill) cannot be used as an indicator of gas generation rate. Flow diagram 1The equilibrium situation within a landfill is shown in Fig 1a. Here the gas generation within the landfill is balanced against the loss of gas. Under these conditions there will be no gas flow into or out of the borehole and the gas pressure in the borehole, PB, will equal the gas pressure in the landfill, PL. When a flow measuring device is connected to the borehole and the tap opened the situation changes. If the flow device has no restriction to flow, as the tap is opened the pressure in the borehole will force the gas out until the borehole is at or near atmospheric pressure In removing the gas we have upset the equilibrium of the landfill. With the tap open and the system open to atmosphere we have a borehole at near atmospheric pressure within a landfill at higher pressure. The low pressure borehole will act as a sink for the gas in the landfill which is still at the elevated pressure of the landfill. We will thus get a flow of gas into the borehole from the surrounding landfill, as shown in the fig 1b.

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