Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 13 10 2009 10.5194/hess-13-1993-2009 http://www.hydrol-earth-syst-sci.net/13/1993/2009/ http://www.hydrol-earth-syst-sci.net/13/1993/2009/hess-13-1993-2009.html http://www.hydrol-earth-syst-sci.net/13/1993/2009/hess-13-1993-2009.pdf 1993 2002 2009-10-28 Examining the effect of pore size distribution and shape on flow through unsaturated peat using computed tomography F. Rezanezhad W. L. Quinton J. S. Price D. Elrick T. R. Elliot R. J. Heck Cold Regions Research Centre, Wilfrid Laurier University, Waterloo, Canada Department of Geography, University of Waterloo, Waterloo, Canada Department of Land Resource Science, University of Guelph, Guelph, Canada The hydraulic conductivity of unsaturated peat soil is controlled by the air-filled porosity, pore size and geometric distribution as well as other physical properties of peat materials. This study investigates how the size and shape of pores affects the flow of water through peat soils. In this study we used X-ray Computed Tomography (CT), at 45 μm resolution under 5 specific soil-water pressure head levels to provide 3-D, high-resolution images that were used to detect the inner pore structure of peat samples under a changing water regime. Pore structure and configuration were found to be irregular, which affected the rate of water transmission through peat soils. The 3-D analysis suggested that pore distribution is dominated by a single large pore-space. At low pressure head, this single large air-filled pore imparted a more effective flowpath compared to smaller pores. Smaller pores were disconnected and the flowpath was more tortuous than in the single large air-filled pore, and their contribution to flow was negligible when the single large pore was active. We quantify the pore structure of peat soil that affects the hydraulic conductivity in the unsaturated condition, and demonstrate the validity of our estimation of peat unsaturated hydraulic conductivity by making a comparison with a standard permeameter-based method. Estimates of unsaturated hydraulic conductivities were made for the purpose of testing the sensitivity of pore shape and geometry parameters on the hydraulic properties of peats and how to evaluate the structure of the peat and its affects on parameterization. We also studied the ability to quantify these factors for different soil moisture contents in order to define how the factors controlling the shape coefficient vary with changes in soil water pressure head. The relation between measured and estimated unsaturated hydraulic conductivity at various heads shows that rapid initial drainage, that changes the air-filled pore properties, creates a sharp decline in hydraulic conductivity. This is because the large pores readily lose water, the peat rapidly becomes less conductive and the flow path among pores, more tortuous. Beckwith, C. W., Baird, A. J., and Heathwaite, A. L.: Anisotropy and depth related heterogeneity of hydraulic conductivity in a bog peat. I: laboratory measurements, Hydrol. Process., 17, 89–101, 2003. Berryman, J. G. and Blair, S. C.: Kozeny-Carman relations and image processing methods for estimating Darcy's constant, J. Appl. Phys., 63, 2221–2228, 1987. Boelter, D. H.: Methods for analysing the hydrological characteristics of organic soils in marsh-ridden areas, in: Hydrology of Marsh-Ridden Areas, Proceedings of IASH Symposium Minsk, 1972, IASH, UNESCO, Paris, 161–169, 1976. Carman, P. C.: Fluid flow through Granular Beds, Trans. Inst. Chem. Eng., 15, 150–166, 1937. Carman, P. C.: Flow of Gases through Porous Media, Butterworths Scientific Publications, London, 1956. Carrier, W. D.: Goodbye, Hazen; Hello, Kozeny-Carman, J. Geotech. Geoenviron., 129, 11, 1054–1056, 2003. Dullien, F. A. L.: Porous media- flow transport and pore structure, 2nd Edn., Academic Press, New York, 1979. Elliot, T. R. and Heck, R. J.: A comparison between 2D vs 3D thresholding of X-ray CT imagery, Can. J. Soil Sci., 84, 4, 405–412, 2007. Elrick, D. E. and Bowman, D. H.: Note on an improved apparatus for soil moisture flow measurements, Soil Sci. Soc. Am. Proc., 28, 450–453, 1964. Elrick, D. E. and Reynolds, W. D.: Infiltration from constant-head well permeameters and infiltrometers, in: Advances in Measurement of Soil Physical Properties: Bringing Theory into Practice, edited by: Topp, G. C., Reynolds W. D., and Green, R. E., Soil Science Society of America, Madison, WI, Special Publication No. 30, 1–24, 1992. Gardner, W. H.: Water Content, in: Methods of Soil Analysis: Physical and Mineralogical Methods, edited by: Kiute, A., Agronomy Series 9 (Part 1), Soil Science Society of America, Madison, Wisconsin, 493–544, 1986. Hazen, A.: Discussion of "Dams on sand formations" by A C Koenig., Transactions of the American Society of Civil Engineers, 73, 199–203, 1911. Hoag, R. S. and Price, J. S.: A field-scale, natural gradient solute transport experiment in peat at a Newfoundland blanket bog, J. Hydrol., 172, 171–184, 1995. Hoag, R. S. and Price, J. S.: The effects of matrix diffusion on solute transport and retardation in undisturbed peat in laboratory columns, J. Contam. Hydrol., 28, 193–205, 1997. Hubbert, M. K.: Darcy's law and the field equations of the flow of underground fluids, Petroleum Transaction, Amer. Inst. Min. Mandal Eng., 207, 222–239, 1956. Kozeny, J.: Über Kapillare Leitung Des Wassers in Boden, Sitzungsber. Akad. Wiss. Wien, Math. Naturwiss. Kl., Abt. 2a, 136, 271–306, 1927. Kettridge, N. and Binley, A.: X-ray computed tomography of peat soils: measuring gas content and peat structure, Hydrol. Process., 22, 4827–4837, 2008. Lewitt, R. M.: Reconstruction algorithms: Transform methods, in: Proc. IEEE, 71, 390–408, 1983. Marshall, T. J.: A relation between permeability and size distribution of pores, J. Soil Sci., 9, 1–8, 1958. Milne, D. M., Pakalnis, R. C., and Lunder, P. J.: Approach to the quantification of hangingwall behaviour, Trans. Inst. Min. \mboxMetall., 105, A69–A74, 1996. Price, J. S., Whittington, P. N., Elrick, D. E, Strack, M., Brunet, N., and Faux, E.: A Method to Determine Unsaturated Hydraulic Conductivity in Living and Undecomposed Sphagnum Moss, Soil Sci. Soc. Am. J., 15, 487–491, 2008. Quinton, W. L., Gray, D. M., and Marsh, P.: Subsurface drainage from hummock-covered hillslope in the Arctic tundra, J. Hydrol., 237, 113–125, 2000. Quinton, W. L. and Hayashi, M.: The flow and storage of water in the wetland-dominated central Mackenzie Rriver basin: Recent advances and future directions, in: Prediction in ungauged basins: Approaches for Canada's cold regions, edited by: Spence, C., Pomeroy, J. W., and Pietroniro, A., Canadian Water Resources Association, 45–66, 2005. Quinton, W. L., Hayashi, M., and Carey, S. K.: Peat Hydraulic Conductivity in Cold Regions and its Relation to Pore Size and Geometry, Hydrol. Process., 22, 2829–2837, 2008. Quinton, W. L., Elliot, T., Price, J. S., Rezanezhad, F., and Heck, R.: Measuring Physical and Hydraulic Properties of Peat from X-ray Tomography, Geoderma, 153, 269–277, 2009. Rasband, W.: ImageJ, Image Manipulation Software. National Institute of Health, Bethesda, MD, USA, available at: http://rsb.info.nih.gov/ij/, 2005. Salem, H. S. and Chilingarian, G. V.: Determination of specific surface area and mean grain size from well-log data and their influence on the physical behavior of offshore reservoirs, J. Petrol. Sci. Eng., 22, 241–252, 1999. Schlueter, E. M., Zimmerman, R. W., Witherspoon, P. A., and Cook, N. G. W.: The fractal dimension of pores in sedimentary rocks and its influence on permeability, Eng. Geol., 48, 199–215, 1997. Taud, H., Martinez-Angeles, R., Parrot, J. F., and Hernandez-Escobedo, L.: Porosity estimation method by X-ray computed tomography, J. Petrol. Sci. Eng., 47, 209–217, 2005. Verry, E. S. and Boelter, D. H.: Peatland hydrology, in: Wetland Functions and Values: the State of Our Understanding, Proceedings of the National Symposium on Wetlands, Lake Buena Vista, Florida, November 7–10, edited by: Greeson, P., American Water Research Association: Minneapolis, MN, 389–402, 1978. Vogel, H. J.: Morphological determination of pore connectivity as a function of pore size using serial sections, Eur. J. Soil Sci., 48, 365–377, 1997. Wadell, H.: Volume, Shape and Roundness of Quartz Particles, J. Geol., 43, 250–280, 1935. Wang, J., Milne, D., Wegner, L., and Reeves, M.: Numerical evaluation of the effects of stress and excavation surface geometry on the zone of relaxation around open stope hanging walls, Int. J. Rock Mech. Min., 44, 2, 289–298, 2007. White, F. M.: Viscous Fluid Flow, McGraw-Hill, New York, 1974.