Lacunarity and Fractal Analyses of Soil Macropores and Preferential Transport Using Micro-X-Ray Computed Tomography

Quantification of soil macropore networks and solute transport patterns is important to enhance our understanding of preferential flow in structured soils. We investigated soil macropore structure and solute transport dynamics in an intact soil column of 76-mm diameter and 265-mm length. Five positions (each with a thickness of 10.7 mm) in the soil column were scanned with a voxel resolution of 78.1 by 78.1 by 86.7 mu m using micro-x-ray computed tomography. The scanning was done at three stages after the soil column was satiated: (i) before tracer (KI) introduction, (ii) 6 min after tracer introduction, and (iii) 78 min after tracer introduction. The macropore network and tracer distribution were reconstructed at the five scanned positions. Relative lacunarity functions (RLFs) and pore fractal dimensions, in both two and three dimensions, were calculated. Distinct macropore characteristics and flow patterns were observed at the five positions. The biopores were active in solute transport because of their high continuity and low tortuosity. Positive logarithmic trends were found between the fractal dimension and the volume percentage of the macropores and tracer distribution. Generally, the lacunarity function reflected the size distribution of macropores and the spatial pattern of flow and transport. The RLFs indicated that the tracer distributions exhibited more self-similarity than the macropore networks and that the representative elementary volume had not been reached for the three-dimensional macropore networks within the size range investigated (cube size <= 63). Lacunarity has diagnostic value in characterizing soil macropore structure and flow pattern and may be coupled with the fractal dimension to better describe and model soil structural properties.