Peat as a potential analogue for the long-term evolution in landfills

A survey of the existing studies on peat and its decomposition processes is presented with the aim to characterise the long-term behaviour of peat acc umulating systems. The chemical and physical characteristics of peat togeth er with its accumulation and decay processes have been analysed. Peat is an acidic mixture of dead and decomposed. mainly vegetable, matter formed in boggy areas; it is the youngest and least altered component of th e combustible rocks and is characterised by the lowest content of fixed car bon and the highest content of volatile constituents. Peat is formed by deg eneration processes under exclusion of atmospheric oxygen by the action of water; the speed of formation depends upon the climatic and environmental c onditions. In most peatlands two layers can be characterised: the aerobic acrotelm and the anaerobic catotelm, their relative importance being controlled mainly by the position of the water table. In the acrotelm the aerobic processes a re responsible for the loss of up to 90% of the original mass, Degeneration in the acidic and anaerobic catotelm is still imperfectly characterised ev en though the catotelm is the real site of peat accumulation. Most of the recent literature considers peat as composed of easily degradab le compounds, e.g. polysaccharides. and recalcitrant matter (lignin and com plex aromatics). The lone-term destiny of peat has not been sufficiently ch aracterised: although in a large majority of cases it seems probable that p eat decomposes completely (even though slowly) provided that it is given a sufficiently long residence rime in the catotelm, some cases can still be i nterpreted as examples of simple accumulation. The rates of influx of oxygen and hence the degradation of organic matter i nto both saturated and partially saturated peat have been estimated. The de pletion rate is about 4500 g m(-1) year(-1) for partially saturated peat. T he average depletion rate of the peat for this case will then be such that it will take on the order of 5 to 50 years to degrade half of the organics in a 10 cm partially saturated layer. For the water-saturated case the depl etion rate varies between 8 and 12 g m(-2) year(-1), which is considerably lower than in the partially saturated region. The models used to analyse th e field and laboratory data on generation, diffusion and emission of methan e and carbon dioxide indicate that laboratory data and field observations a gree reasonably well. It is suggested that peat-accumulating ecosystems may be valuable natural a nalogues for the study of the long-term destiny of industrial and municipal solid wastes. Accurate studies of active mires together with an ad hoc rev iew of the existing literature give valuable insights in this problem. Peat lands might then be considered as organic waste deposition experiments last ing up to several thousands years. (C) 2001 Elsevier Science B.V. All right s reserved.