The composition of carbon pools is subject to seasonal variations and is greatly influenced by the length of time that has elapsed since waste placement. In this study, the carbon geochemistry of different carbon pools (DIC, DOC, CH4, CH3COOH, and humic substances (HS)), environmental stable isotopes (d13C, d18O, and d2H) and compound specific isotope analysis (CSIA) of leachate DOC fractions are used to characterize the carbon composition of leachate from the Trail Road Landfill (TRL) site receiving municipal solid waste (MSW). In this research the isotope analysis on leachate, landfill gases and various leachate DOC components indicate that: 1) microbial processes are more advanced in the degradation of dissolved organic carbon (DOC) in the leachate collected from the older parts of the landfill site (M32); 2) simple fatty acids (e.g. acetic =1008 mg l–1 and propionic = 608 mg l–1, etc.) accumulate in the leachate from older part of the landfill; 3) the acetate fermentation CH4 production pathway dominants in older parts of the landfill. While, in the younger part of the landfill (LPS), CO2 reduction pathway (α13C-CO2-CH4=1.06 and D-excess = 20.6‰) is the more favoured CH4 production pathway; 4) fulvic acid (FA) is the main fraction of leachate DOC and the enriched 13C value of FA and of LMW DOC as well as of the bulk DOC with increasing age of the landfill is due to more advanced degradation in the older part of the landfill.

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