Relations between macropore network characteristics and the degree of preferential solute transport

Abstract

Abstract. The characteristics of the soil macropore network determine the potential for fast transport of agrochemicals and contaminants through the soil. The objective of this study was to examine the relationships between macropore network characteristics, hydraulic properties and state variables and measures of preferential transport. Experiments were carried out under near-saturated conditions on undisturbed columns sampled from four agricultural topsoils of contrasting texture and structure. Macropore network characteristics were computed from 3-D X-ray tomography images of the soil pore system. Non-reactive solute transport experiments were carried out at five steady-state water flow rates from 2 to 12 mm h−1. The degree of preferential transport was evaluated by the normalised 5% solute arrival time and the apparent dispersivity calculated from the resulting breakthrough curves. Near-saturated hydraulic conductivities were measured on the same samples using a tension disc infiltrometer placed on top of the columns. Results showed that many of the macropore network characteristics were inter-correlated. For example, large macroporosities were associated with larger specific macropore surface areas and better local connectivity of the macropore network. Generally, an increased flow rate resulted in earlier solute breakthrough and a shifting of the arrival of peak concentration towards smaller drained volumes. Columns with smaller macroporosities, poorer local connectivity of the macropore network and smaller near-saturated hydraulic conductivities exhibited a greater degree of preferential transport. This can be explained by the fact that, with only two exceptions, global (i.e. sample scale) continuity of the macropore network was still preserved at low macroporosities. Thus, for any given flow rate, pores of larger diameter were actively conducting solute in soils of smaller near-saturated hydraulic conductivity. This was associated with larger local transport velocities and, hence, less time for equilibration between the macropores and the surrounding matrix which made the transport more preferential. Conversely, the large specific macropore surface area and well-connected macropore networks associated with columns with large macroporosities limit the degree of preferential transport because they increase the diffusive flux between macropores and the soil matrix and they increase the near-saturated hydraulic conductivity. The normalised 5% arrival times were most strongly correlated with the estimated hydraulic state variables (e.g. with the degree of saturation in the macropores R2 = 0.589), since these combine into one measure the effects of irrigation rate and the near-saturated hydraulic conductivity function, which in turn implicitly depends on the volume, size distribution, global continuity, local connectivity and tortuosity of the macropore network.

Prediction of biopore- and matrix-dominated flow from X-ray CT-derived macropore network characteristics

“…Application of X-ray computed tomography (CT) provides emerging alternative means for estimating subsurface macropore flow and transport (Wildenschild and Sheppard, 2013). Over the last decade, numerous studies related to the characterization of macropore structure (i.e., macroporosity, macropore size distribution, volume, surface area, and tortuosity) have been conducted with X-ray CT for different land use and management systems (Katuwal et al, 2015;Larsbo et al, 2014;Hu et al, 2014;Vogel et al, 2010;Luo et al, 2010). However, to date there have been only very few published studies on quantitatively relating macropore network characteristics to the observations of macropore flow.…”

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

“…These influences of 3-D soil structure on local processes can be studies using 3-D X-ray imaging. Larsbo et al (2014) used 3-D X-ray scanning to investigate the influence of macropore network characteristics on preferential solute transport. They found that soils with a well-connected and large macroporosity decrease preferential solute transport due to increased diffusion and exchange with the soil matrix.…”

Hydropedology and the Societal Challenge of Realizing the 2015 United Nations Sustainable Development Goals

“…Staining techniques may be more effective (e.g., Bouma and Dekker, 1978). In contrast, breakthrough curves demonstrate the effects of bypass flow, but this process is still difficult to express in terms of predictive parameters that can be validated by field measurements (e.g., Bouma and Anderson, 1977;Larsbo et al, 2014).…”

“…Flow processes are complex, and volumetric estimates of the effective pore volumes involved are difficult because the volumes are small and macropore continuity determines the real flow patterns. Applying modern nuclear magnetic resonance spectroscopy and X-ray tomography techniques shows intriguing patterns of macropores but cannot address the continuity aspect either (Schjønning et al, 2013;Larsbo et al, 2014). Staining techniques may be more effective (e.g., Bouma and Dekker, 1978).…”