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An examination of the processes contributing to the production of landfill greenhouse gas (GHG) emissions is required, as the actual level to which waste degrades anaerobically and aerobically beneath covers has not been differentiated. This paper presents a methodology to distinguish between the rate of anaerobic digestion (r), composting (r) and CH oxidation (r) in a landfill environment, by means of a system of mass balances developed for molecular species (CH, CO) and stable carbon isotopes (δC-CO and δC-CH). The technique was applied at two sampling locations on a sloped area of landfill. Four sampling rounds were performed over an 18 month period after a 1.0 m layer of fresh waste and 30-50 cm of silty clay loam had been placed over the area. Static chambers were used to measure the flux of the molecular and isotope species at the surface and soil gas probes were used to collect gas samples at depths of approximately 0.5, 1.0 and 1.5 m. Mass balances were based on the surface flux and the concentration of the molecular and isotopic species at the deepest sampling depth. The sensitivity of calculated rates was considered by randomly varying stoichiometric and isotopic parameters by ±5% to generate at least 500 calculations of r, r and r for each location in each sampling round. The resulting average value of r and r indicated anaerobic digestion and composting were equally dominant at both locations. Average values of r ranged from 9.8 to 44.5 g CO m d over the four sampling rounds, declining monotonically at one site and rising then falling at the other. Average values of r ranged from 10.6 to 45.3 g CO m d. Although the highest average r value occurred in the initial sampling round, all subsequent r values fell between 10 and 20 g CO m d. r had the smallest activity contribution at both sites, with averages ranging from 1.6 to 8.6 g CO m d. This study has demonstrated that for an interim cover, composting and anaerobic digestion of shallow landfill waste can occur simultaneously.

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Modern landfills are often thought of as exclusively anaerobic systems, with landfill gas generation and regulatory emission models typically describing waste degradation as a first order decay process.However, in most instances, observed biogas production from landfills fall short of predictions by these landfill gas models. To improve biogas estimates, a closer examination of the configuration and operation of landfills is required, to identify if a significant portion of waste is degraded aerobically.An improved understanding of aerobic processes will contribute towards more accurate landfill gas estimation and emissions reporting.It was hypothesised that the rate and extent of anaerobic digestion (rAD), CH4 oxidation (rOX), and composting (rCOM) within the soil cover and the fresh waste immediately below the cover could be determined by the combination of stable isotope and molecular mass balances for carbon species (CH4 and CO2). The primary objective of this thesis was to determine the extent of simultaneous insitu aerobic (CH4 oxidation and composting) and anaerobic processes occurring in an operational landfill cell. An 18-month field trial was established on a fresh layer of waste placed on a sloped face at the edge of a landfill cell and covered with an interim soil cover (30-50cm).The evolution of CH4, CO2 and O2 through the waste profile and the surface emissions were monitored by gas samples collected from gas probes and static flux chambers. Stable isotopes ( 2 δH for CH4, 13 δC for CH4 and CO2) were monitored. The developed model consisted of four mass balances for CH4, CO2, δ 13 C-CH4 and δ 13 C-CO2 over a control volume that extended approximately 1.5m below the surface, incorporating the soil cover and the uppermost portion of the waste layer.The model was applied to data collected from two locations over four separate sampling campaigns, each representing a climatic season in Brisbane.It was necessary to conduct companion laboratory work to characterise key isotopic parameters in the model, these being (1) CH4 oxidation fractionation factor for the landfill cover soil, (2) anaerobic digestion fractionation factor for the waste, (3) the composting signature of the landfill cover and waste. The sensitivity of predictions of rAD, rCOM and rOX to the values of these parameters as well as the stoichiometry of the three reaction processes was assessed by randomly varying each parameter by ±5% over 500 simulations for each data set.iii The mass balance model revealed that aerobic activity forms a large proportion of early degradation