Gas emission into the atmosphere from controlled landfills: an example from Legoli landfill (Tuscany, Italy)

Background, aim and scope Landfill gas (LFG) tends to escape from the landfill surface even when LFG collecting systems are installed. Since LFG leaks are generally a noticeable percentage of the total production of LFG, the optimisation of the collection system is a fundamental step for both energy recovery and environmental impact mitigation. In this work, we suggest to take into account the results of direct measurements of gas fluxes at the air-cover interface to achieve this goal. Materials and methods During the last 5 years (2004-2009), 11 soil gas emission surveys have been carried out at the Municipal Solid Waste landfill of Legoli (Peccioli municipality, Pisa Province, Italy) by means of the accumulation chamber method. Direct and simultaneous measurements of CH₄ and CO₂ fluxes from the landfill cover (about 140,000 m²) have been performed to estimate the total output of both gases discharged into the atmosphere. Three different data processing have been applied and compared: Arithmetic mean of raw data (AMRD), sequential Gaussian conditional simulations (SGCS) and turning bands conditional simulations (TBCS). The total amount of LFG (captured and not captured) obtained from processing of direct measurements has been compared with the corresponding outcomes of three different numerical models (LandGEM, IPCC waste model and GasSim). Results Measured fluxes vary from undetectable values (<0.05 mol m⁻² day⁻¹ for CH₄ and <0.02 mol m⁻² day⁻¹ for CO₂) to 246 mol m⁻² day⁻¹ for CH₄ and 275 mol m⁻² day⁻¹ for CO₂. The specific CH₄ and CO₂ fluxes (flux per surface unit) vary from 1.8 to 7.9 mol m⁻² day⁻¹ and from 2.4 to 7.8 mol m⁻² day⁻¹, respectively. Discussion The three different estimation methodologies (AMRD, SGCS and TBCS) used to evaluate the total output of diffused CO₂ and CH₄ fluxes from soil provide similar estimations, whereas there are some mismatches between these results and those of numerical LFG production models. Isoflux maps show a non-uniform spatial distribution, with high-flux zones not always corresponding with high-temperature areas shown by thermographic images. Conclusions The average value estimated over the 5-year period for the Legoli landfill is 245 mol min⁻¹ for CH₄ and 379 mol min⁻¹ for CO₂, whereas the volume percentage of CH₄ in the total gas discharged into the atmosphere varies from 29% to 51%, with a mean value of 39%. The estimated yearly emissions from the landfill cover is about 1.29 × 10⁸ mol annum⁻¹ (2,100 t year⁻¹) of CH₄ and 1.99 × 10⁸ mol annum⁻¹ (8,800 t year⁻¹) of CO₂. Considering that the CH₄ global warming potential is 63 times greater than that of CO₂ (20 a time horizon, Lashof and Ahuja 1990), the emission of methane corresponds to 130,000 t annum⁻¹ of CO₂. Recommendations and perspectives The importance of these studies is to provide data for the worldwide inventory of CH₄ and CO₂ emissions from landfills, with the ultimate aim of determining the contribution of waste disposal to global warming. This kind of studies could be extended to other gas species, like the volatile organic compounds.