The isotopic compositions of carbon compounds in landfill leachate provide insights into the biodegradation pathways that dominate the different stages of waste decomposition. In this study, the carbon geochemistry of different carbon pools, environmental stable isotopes and compound specific isotope analysis (CSIA) of leachate DOC fractions and gases show distinctions in leachate biogeochemistry and methane production between the young area of active waste emplacement and the old area of historical emplacement at the Trail Road Landfill (TRL). The active area leachate has low DOC concentrations (<200 mg l–1) dominated by fulvic acid (FA = 160 mg l–1), and produces CH4 dominantly by CO2 reduction (D-excess = 20.6‰). Leachate generated in the area of older waste has high DOC (>4770 mg l–1) dominated by FA (4482 mg l–1) and simple fatty acids (acetic =1008 mg l–1 and propionic = 608 mg l–1), and produces CH4 by acetate fermentation pathway (D-excess = 9.8‰). CSIA shows an advanced degradation and a progressive accumulation of 13C of fatty acids in older area leachate. The enriched 13C value of FA (-20‰ and -26‰ for the older and active parts, respectively,) and of LMW DOC (-8‰ and -27‰) as well as of the bulk DOC (-21‰ and -25‰) shows more advanced degradation in the older part of the landfill which is consistent with the shift in the humic/fulvic acid ratios (0.05 and 0.18). The enrichment in 13C of acetate (-12‰) above the 13C of DOC (–21‰) and of the propionic acid (-19‰), in older leachate, suggests that this acetate has not evolved from the simple degradation of larger organic molecules, but by homoacetogenesis from the enriched DIC pool (8‰) and H2 which produce a more enriched 13C of acetate. By contrast, the 13C of the minor acetate in active area (-17‰) indicating that CO2 reducing bacteria must be the primary consumers of H2 which is resulted in enriched 13CDIC (10‰) and depleted 13CCH4 (-58‰).

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