Imaging and quantification of preferential solute transport in soil macropores

Abstract

Despite significant advances during the last decades, there are still many processes related to nonequilibrium flow and transport in macroporous soil that are far from completely understood. The use of X-rays for imaging time-lapse 3-D solute transport has a large potential to help advance the knowledge in this field. We visualized the transport of potassium iodide (20 g iodide l 21 H 2 O) through a small undisturbed soil column (height 3.8 cm, diameter 6.8 cm) under steady state hydraulic conditions using an industrial Xray scanner. In addition, the electrical conductivity was measured in the effluent solution during the experiment. We attained a series of seventeen 3-D difference images which we related to iodide concentrations using a linear calibration relationship. The solute transport through the soil mainly took place in two cylindrical macropores, by-passing more than 90% of the bulk soil volume during the entire experiment. From these macropores the solute diffused into the surrounding soil matrix. We illustrated the properties of the investigated solute transport by comparing it to a 1-D convective-dispersive transport and by calculating the temporal evolution of the dilution index. We furthermore showed that the tracer diffusion from one of the macropores into the surrounding soil matrix could not be exactly fitted with the cylindrical diffusion equation. We believe that similar studies will help establish links between soil structure and solute transport processes and lead to improvements in models for solute transport through undisturbed soil.

Quantitative imaging of the 3-D distribution of cation adsorption sites in undisturbed soil

“…1) were used as the target grey values for the greyscale standardization (21 418 and 16 225 respectively). All other grey values were scaled accordingly by the linear relationship between the target grey values and the initial grey values of the corresponding image according to Koestel and Larsbo (2014). A 3-D unsharp mask with 1 pixel radius was applied to all images in order to increase sharpness.…”

“…Roscoat et al (2014) proposed a method to visualize biofilms in porous media using chloronaphthalene as a contrast agent. Koestel and Larsbo (2014) used iodide to increase the X-ray photon attenuation of water when studying water flow in an undisturbed soil column. A suitable contrast agent should contain element(s) with a higher atomic number than other common elements present in the soil.…”

“…The mean grey values of the two solutions were subtracted to obtain the maximal contrast in grey values (γ SAT = 2637) corresponding to the density contrast between the KCl and BaCl 2 solutions. The BaCl 2 mass (m j,d in mg) was then calculated according to Koestel and Larsbo (2014) using Eq. (1).…”

Ecohydrological particle model based on representative domains

“…Some orientation is given by findings of Hincapié and Germann (2010) and Moebius and Or (2012). Promising techniques like time-lapse X-ray or µCT tomography just emerge to be applied (Koestel and Larsbo, 2014;Schlüter et al, 2016). Yet there is consensus that macroporematrix interaction depends on the matric head, the wetting of the macropore wall (Klaus et al, 2013) and is optionally affected 10 by organic coatings which may act hydrophobic (Jarvis, 2007;Rogasik et al, 2014).…”

Winter hydrology and soil erosion processes in an agricultural catchment in Norway

“…However, improvements in the image resolution of X-ray tomography (XRT) scanners and an increase in computation capacity in the past decade have made it possible to study soils at the pore scale without destroying the samples (De Kock et al, 2015). This option of using XRT for quantitative investigations of soil structure has resulted in a number of recent studies relating water flow and solute transport properties to macropore network characteristics derived from XRT images (Luo et al, 2008(Luo et al, , 2010a(Luo et al, , 2010bGarbout et al, 2013a;Koestel and Larsbo, 2014;Katuwal et al, 2015;Zhang et al, 2015).…”