Measuring physical and hydraulic properties of peat from X-ray tomography

Organic soils cover a large percentage of the Canadian land mass in the form of peatlands (14%) and other organic terrains, including arctic and alpine tundra and taiga (25%). Sphagnum mosses are the dominant peat-forming species in all of these terrains. Understanding the processes by which this material conducts, retains and redistributes moisture and energy is crucial to the successful development of physically-based methods of measuring and modelling their hydrological response. Sphagnum-peat was sampled from Scotty Creek, in the wetland-dominated zone of discontinuous permafrost near Fort Simpson, Northwest Territories, Canada. Physical properties of the peat samples were measured from standard methods, and X-ray tomographic images of sub-samples were obtained at 5 levels of volumetric moisture content between ∼ 0.4 and 0.7. The volumetric moisture content measured using the tomographic images closely approximated the gravimetric measurements. Properties of the pore spaces were measured in both two and three dimensions. The active porosities measured in 2D and 3D were very similar, and matched closely with values reported in the literature. Specific surface area (3D), however, was larger than the specific perimeter (2D). The 2D analysis suggests a relatively gradual transition from small to large pores; while in 3D, the distribution is dominated by a single large pore space, whose volume and surface area are 3-orders of magnitude larger than the next largest pore, and > 99% of the total inter-particle pore volume. This single, large pore space contains the flow network. This study elucidated the volume and configuration of this network for discrete ranges of soil water pressure that typically occur in the field. The flow network diminished from 17.6% to 9.0% of a sample volume as its volumetric moisture content lowered from 0.65 to 0.42. This volume loss was accommodated by the thinning and disaggregation of moisture films, although as the sample lost moisture, the flow network maintained a relatively even spatial distribution.