Landfill gas risk assessment — understanding reasonable worst case conditions

Closed and operating landfills represent a significant source of landfill gases, primarily consisting of methane and carbon dioxide. A commonly adopted approach to assessing the risk of landfill gas migration and risk, is the Gas Screening Value (GSV) approach originally developed by Wilson and Card (1999) and stipulated in various guidelines and standards in the United Kingdom. This approach has also been adopted as the defacto approach to assessing landfill gas risk in Australia. The GSV approach is a simplistic quantitative risk assessment approach which uses methane and carbon dioxide concentrations combined with gas flow rates measured in monitoring bores to derive a screening value which can then be compared to a risk ranking table. Although the risk assessment method is simplistic, unless the site conceptual model is well understood, then a misrepresentation of the risk can occur. It is important to understand the variability in landfill gas distribution, concentration and flow over time and space. This abstract provides data from a case study which illustrates the variability in gas flow over time and resultant changes in risk profile, and hence the need for obtaining robust datasets based on a thorough conceptual site model. (author)