Field measurement of methane and NMOC emissions: Sources, sinks and implications for emissions modeling

Direct field measurements of methane and non-methane organic compound (NMOC) emissions are necessary for the development and validation of realistic landfill emissions models. Such models must address the natural variability of emissions plus appropriate scales for simulation equations in a particular model framework. Field measurements must also include major controlling variables in various temporal and spatial scales. This paper provides an overview of field data collected from in-house projects which is pertinent to the development of realistic emissions models. Using a static closed chamber technique, our observed rates for methane emissions from various controlled monitoring experiments during 1988-1995 ranged from 0.003 to more than 1000 g m{sup -2} day{sup -1}. Field data to date indicate that the major variables affecting methane emission rates are the presence or absence of pumped gas recovery wells, the relative proximity of pumping wells to the point of monitoring, physical properties of cover soils relating to their aeration status, and rates of methane oxidation by indigenous methanotrophs. In addition, at both proximal (near well) and distal (between well) sites at a northern Illinois landfill with an optimized gas recovery system, field measurements during spring, 1994, and summer-winter, 1995, indicated that there were no net methanemore » emissions. Instead, the cover soil was functioning as a methane sink, with methanotrophic oxidation of both landfill methane and atmospheric methane. Realistic emissions models must consider microbial methane oxidation in cover soils as a natural control mechanism on emissions. Moreover, with respect to NMOC emissions, we report preliminary field data from a second northern Illinois site that indicate significant reductions in concentrations between wellfield gas and soil gas at 25 cm in landfill cover materials.« less