The study was carried out for the DoE by Environmental Resources Ltd (ERL). It was the first official attempt to assess the effectiveness of different gas control systems since a series of gas migration incidents in the mid-1980s prompted the DoE to issue a Waste Management Paper (WMP) on landfill gas in 1989.
The first part of the study was a questionnaire survey of landfill operators. ERL received little response from private sector operators. Returns from a sample of local authorities revealed a total of 210 sites with gas control measures. Of these, 57% rely on passive gas venting trenches at the site perimeter. Pumped systems had been installed at 41% of the sites. The remaining 2% have cut-off barriers.
The preponderance of venting trenches suggests that many operators have yet to implement the advice given in a revised version of the WMP, issued in 1991, that "single systems of control are unlikely to be adequate at most landfills."
The survey findings were used by ERL to select 22 landfills for detailed investigation. This included a review of existing gas monitoring data and, at some of the sites, the installation of probes or boreholes to provide supplementary information on gas quality and migration beyond the site boundaries. The control systems were treated as being inadequate if methane concentrations beyond the site boundary exceeded 1% by volume - the trigger level at which improved controls are recommended by the WMP.
The main findings of the investigations were:
Well spacings of no more than 40 metres are recommended by the WMP. But five of the eight sites studied had inter-well intervals of 40 metres or more, and three of these provided inadequate control. One deep landfill with wells 25-30 metres apart also had inadequate gas control because too little methane was being extracted from individual wells.
Insufficient methane yields were being achieved at several other sites. One reason for this, according to ERL, is that many sites lack the instrumentation needed to enable the operators to understand the performance of their gas extraction systems. Many had also failed to take the "essential" step of measuring gas flow rates in individual wells so as to verify the adequacy of the well lay-out and the system design.
The main influence on the adequacy of venting trenches was their depth in relation to the depth of the landfill. The WMP recommends that trenches should extend to the full depth of the wastes, but this was fully achieved at only three of the 11 sites. ERL's investigations showed that adequate gas control is unlikely to be achieved unless a trench is at least 70% as deep as the landfill, and preferably runs to the full depth of the wastes.
The nature of the material used to fill the trenches also proved to be important. Inadequate control over gas migration, even at a site where the trench ran to the full depth of the wastes, was achieved where materials such as rubble or fine gravel had been used as backfill, reducing the gas permeability of the trenches.
Another factor limiting the effectiveness of venting trenches was "blinding" of the backfill by vegetation and refuse dumped on the surface. Conversely, trenches which had been covered and equipped with gas vents, as recommended by the WMP, suffered no impairment in performance.
At two sites plastic membranes had been installed on the outer side of the trenches. No gas migration off-site was detected, indicating that they can enhance the effectiveness of these systems.
Of the three sites studied by ERL, one has a bentonite wall beyond a venting trench. This has been effective to date in preventing gas migration, but ERL warns that for this to be sustained the bentonite will have to be kept moist to prevent cracking and shrinkage.
At a second site with a gas pumping system, jet grouting had been used to form a bentonite cut-off wall as a back-up to prevent the migration of gas towards nearby houses. The gas wells are too far apart to prevent migration, and the gas appears to be passing through the grout curtain as well, albeit at a reduced rate.
The only wholly effective cut-off barrier identified by ERL was a bentonite slurry trench incorporating a plastic membrane, although the long-term performance of this, too, needs to be monitored.
Overall, the study does not inspire confidence in the gas controls achieved at many landfills. Neither does it suggest that all operators are zealously following the DoE's advice that improved measures should be introduced when methane levels above 1% by volume are detected off-site. The research findings are likely to be incorporated in a further revision of the WMP, although ERL has recommended further studies of the performance of current horizontal gas extraction systems and an improved methodology for predicting optimum well spacings in pumping schemes.