Water and Air Redistribution within a Dual Permeability Porous System Investigated Using Neutron Imaging

Abstract

A ponded infiltration experiment was conducted under simultaneous imaging to investigate variations in quasi-saturated hydraulic conductivity a process frequently observed in infiltration experiments in soils with wide grain -size distribution. An artificially prepared heterogeneous sample composed of coarse quartz sand (representing pathways of preferential flow) and fine porous ceramic (representing soil matrix) was investigated. The sample was 34.5mm high and 29.0mm in diameter. Sequences of neutron radiography images (RI) of pixel size 0.045 × 0.045mm were taken at one angle during particular transient phases of the flow process. During quasi-steady state flow stages of the experiment radiography images were acquired in range of angles 0-180° in 0.9° step and. 3D neutron tomograms (TI) were then developed. Using the data a quantitative evaluation of the spatial and temporal distribution of water content within the sample was conducted. For every RI and TI the amount of water in particular pixels and voxels, respectively, was calculated by subtracting the image of dry sample. The accuracy of the water content estimates derived from the images was checked by comparing them to the corresponding gravimetrically determined water content data. Heavy water with equilibrium air saturation was introduced into the sample during two recurrent infiltrations. Thirty five hours later, during second infiltration, the inflow was switched to degassed heavy water in order to remove residual air present in the sample. During the first twelve hours of first infiltration run flow rate through the sample decreased from 3.7cm/hour to 1.0cm/hour at the end of the “steady state flow” stage. The flow rate in second run decreased from 3.6cm/hour to 1.6cm/hour. Comparison of the tomogram of the sample at the beginning and one taken at the end of the steady state flow stage in each run shows an increase of water content in the porous ceramic, while the water content in the coarse sand decreased. On the contrary, during the subsequent infiltration with degassed water the flow rate increased to its maximum value of 10.5cm/hour. The tomograms confirmed removal of the residual air during this stage. Increased water content in the coarse quartz sand was evident on a tomogram made at the end of the degassed water infiltration. The results show that the residual air saturation and its spatial distribution strongly affected the water flow in the quasi-saturated heterogeneous media representing natural soil.