Saturated Hydraulic Conductivity as Affected by the Macropore Continuity Inferred From CT Scan Data.

The prominent contribution of macropores to water flow and solute transport points to the need for thorough characterization of their void structure. Large soil columns need to be studied to infer topological properties of macropores. We have sampled and studied undisturbed columns (7.5 cm ID, 20 cm length) of the Taylor soil from a grassed floodplain in Franklin county, PA. The FlashCT - 420 kV system (HYTEC Inc.) was used for X-ray CT scanning. The FlashCT-DAQ, the FlashCT-DPS, and the FlashCT-VIZ software was used for reconstruction. A MatLab® software with GUI for processing, analyzing and visualizing of 3D X-ray CT scans was developed and used to binarize 1480 cross-section images for each of the columns. The resolution of about 100 microns was sufficient to distinguish macropores. The high-connectivity macropore space was reconstructed from the imagery. The columns were first cut in half, then in four, and finally in eight equal parts. The saturated hydraulic conductivity was measured for the whole columns and for all column parts first in their original spatial orientation, and then for the same samples turned upside down. Saturated hydraulic conductivity increased with decrease in the column length. Pinched-out macropores within the column were found that affected the hydraulic conductivity of samples of different sizes. The overall large macroporosity did not translate into large hydraulic conductivity of the column. We discuss observed scale effects on hydraulic conductivity, the correspondence between the CT data and hydraulic properties of soil, and the effect of the flow direction on the hydraulic conductivity. Introducing a new parameter - the through macroporosity - seems to be useful to quantify the observed effects. The 3D CT scan data highlight and eliminate substantial pitfalls in characterization of the soil void space structure that may arise from using of the 2D information about macroporosity.