 Differential diffusion and gas solubility (varying strongly with temperature and pH) further complicates this behaviour.Tunnelling effects, whereby large items (including monitoring boreholes) create bypass shortcuts into the interior of the landfill, can extend this variability to greater depths in localised zones.Such phenomena can give the impression that bioactivity and gas composition is changing much more radically and rapidly than is actually the case, and any series of isolated time-point measurements is likely to be unreliable due to this variance.