Saturday, 15 July 2006
136-16

Synchrotron Computed Microtomography for Assessing Changes in Porosity as Influenced by Compaction.

Clark J. Gantzer1, Stephen Anderson1, and Shmuel Assouline2. (1) Dept of Soil, Environmental and Atmospheric Sciences, Univ of Missouri-Columbia, 302 ABNR Natural Resources Building, Columbia, MO 65211, (2) Inst. of Soil, Water and Environmental Sciences, A.R.O. - Volcani Center, Bet Dagan, Israel

Compaction degrades soil structure by decreasing pores affecting water, heat and gas exchange; root penetration; and crop production. Traditional methods for measurement of soil structure are unable to describe intact soil micromorphology. These methods limit accurate prediction of soil water relations. Recent applications using the synchrotron for Computed MicroTomography (CMT) have shown that CMT is useful for quantifying 10-micron resolution heterogeneities in porous media. This work proposes to use CMT methods to quantify soil conditions of pore connectivity, tortuosity, and pore size as altered by compaction, and relate these measurements to soil water relations and to predict hydraulic conductivity. Soil used for the study included compacted and non-compacted repacked soil cores of Hamra from Israel. Air-dry soil cores were scanned at the GeoSoilEnviroCARS sector at the Advanced Photon Source for x-ray computed microtomography at the Argonne National Laboratory. Data was collected on the APS bending magnet Sector 13. Soil sample cores 5- by 5-mm were studied. Skeletonization algorithms in the 3DMA-Rock software were used to extract pore structure. Spatial distributions for pore path length and coordination number, pore throat size and nodal pore volume were obtained for 183mm3 volumes with porosities ranging from 33- 44%. Soil cores were imaged at a 10-micron resolution. Spatial distributions were computed using a three dimensional medial axis analysis of the void space in the image. A newly developed aggressive throat computation was used to find throat and pore partitioning needed for soil because of relatively high porosity. Results show that the coordination number (C) distribution measured from the medial axis were reasonably well fit to an exponential relation P(C)=10-C/C0, where P is the probability, and C0 is the characteristic coordination number. Data for the characteristic area (A), were also reasonably well fit by the relation P(A)=10-A/A0, where A0 is the characteristic area. Results indicate that compression preferentially affects the largest pores, reducing them in size. When compaction reduced porosity from 44% to 33%, the average pore volume was reduced by 30%, and the average pore-throat area was reduced by 26%. Compaction increased the shortest paths interface tortuosity by about 2%. Assessment of quantitative morphology as affected by soil structure alterations induced by compaction shows promise. CMT-measured pore properties were correlated with hydraulic properties. Techniques developed with this study will benefit agriculture with new tools for assessing changes in soil pores that affect hydraulic properties. Information will also provide new management assessment methods to assist in amelioration of soil compaction.

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