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

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. 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 m(2)) 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). 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. 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. 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 x 10(8) mol annum(-1) (2,100 t year(-1)) of CH4 and 1.99 x 10(8) 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. 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.