Using micromorphology to understand the rewetting mechanisms in milled peat

Milled peat, an important energy resource in Ireland, is produced by scarifying the surface of a peat bog and allowing the resultant particulate material to air dry prior to storage in large stockpiles on the peat bogs. The storage period can vary from 6 months to several years, during which time the stockpiles are subject to atmospheric precipitation which causes rewetting and a subsequent reduction in calorific and monetary value. A study was undertaken to characterise the pore structure of stockpiles and identify relationships between structure and propensity to rewet. Recent research examining rewetting mechanisms in milled peat stockpiles has focused mainly on macroscale physical data. With respect to the importance of the structure of a porous medium for water movement and storage, a characterisation of the pore and particulate structure of milled peat in stockpile was conducted using micromorphological techniques. Samples with water contents up to 0.6 g g−1 were impregnated in the field. Thin sections produced in days as opposed to months (no period of chemical dehydration was required) were used to quantify macroporous structures using image analysis. Results illustrated that a significant hydraulic discontinuity existed between the surface and subsurface layers of stockpiles, a factor not identified by laboratory physical analysis. This endorsed the current hypothesis in that the milled peats of different genetic origin had different pore architectures which had a significant effect on the active rewetting mechanisms. The visualisation of water flow pathways using dye tracers suggested that in addition to the wetting front water movement, by-pass flow was an active rewetting mechanism.