 aExact percentage distribution will vary with the age of the landfill

Most landfills are highly heterogeneous environments, both physically and biologically, and the gas composition sampled can vary radically within a few metres.Near-surface monitoring is additionally vulnerable over short time periods to weather effects.As the atmospheric pressure rises, the rate of gas escape from the landfill is reduced and may even become negative, with the possibility of oxygen incursion into the upper layers (an analogous effect occurs in the composition of water at the mouth of an estuary as the sea tide rises and falls).