Methane emission to the atmosphere from landfills in the Canary Islands

<p>Methane (CH₄) is an important greenhouse gas, and is increasing in the atmosphere by 0.6% (10 ppb) each year. Important sources of this gas are landfills; in fact more than 10% of the total anthropogenic emissions of CH₄ are originated in them by anaerobic degradation of organic matter. Even after years of being closed, a significant amount of landfill gas can be released to the atmosphere through its surface as diffuse or fugitive degassing.</p><p>Many landfills currently report their CH₄ emissions to the atmosphere using model-based methods, which are based on the rate of production of CH₄, the oxidation rate of CH₄ and the amount of CH₄ recovered (Bingemer and Crutzen, 1987). This approach often involves large uncertainties due to inaccuracies of input data and many assumptions in the estimation. In fact, the estimated CH₄ emissions from landfills in the Canary Islands published by the Spanish National Emission and Pollutant Sources Registration (PRTR-Spain) seem to be overestimated due to the use of protocols and analytical methodologies based on mathematical models. For this reason, direct measurements to estimate CH₄ emissions in landfills are essential to reduce this uncertainty.</p><p>In order to estimate the CH₄ emissions to the atmosphere from landfills in the Canary Islands, 34 surveys have been performed since 1999 to the present. Each survey implies hundreds of CO₂and CH₄ efflux measurements covering the landfill surface area. Surface landfill CO₂ efflux measurements were carried out at each sampling site by means of a portable non-dispersive infrared spectrophotometer (NDIR) model LICOR Li800 following the accumulation chamber method. Samples of landfill gases were taken in the gas accumulated in the chamber and CO₂ and CH₄ were analyzed using a double channel VARIAN 4900 micro-GC. The CH₄ efflux measurement was computed combining CO₂ efflux and CH₄/CO₂ ratio. To quantify the diffuse or fugitive CO₂ and CH₄ emission, gas efflux contour maps were constructed using sequential Gaussian simulation (sGs) as interpolation method. Considering that (a) there are 6 controlled landfills in the Canary Islands, (b) the average area of the 34 studied cells is 0.15 km² and (c) the mean value of the CH₄ emission estimated for the studied cells range between 6.2 and 7.2 kt km^-2 y^-1, the estimated CH₄ emission to the atmosphere from landfills in the Canary Islands showed a range of 5.7-6.7 kt y^-1 (mean value of 6.2 kt y^-1). On the contrary, and for the same period of time, the PRTR-Spain estimates the CH₄ emission in the order of 6.4-16.4 kt y^-1 (mean value of 9.2 kt y^-1), nearly 46% more than our estimated value. This result demonstrates the need to perform direct measurements to estimate the surface fugitive emission of CH₄ from landfills.</p><p><em>Bingemer, H. G., and P. J. Crutzen (1987), J. Geophys. Res. 92, 2182-2187.</em></p>