Decompostion Phase of Landfill Leachate

I'm looking for information on what laboratory tests could be performed on landfill leachate or gas to determine what phase of degradation the waste is currently at (phase of decomposition).  I have come across information on testing the leachate for BOD/COD ratio and the gas for methane / CO2 ratios. I would be grateful if someone could recommend an analytical test or preferably some papers/literature on the hydrochemistry of leachate through the lifetime of a landfill site. The waste is a made up of construction/demolition, household food, wood etc. The landfill was capped with impermeable layer approximately 12 years ago. (I am currently assessing the risk to groundwater and surface water at the site which is underlined with a discontinuous layer of peat).