Image processing-based study of soil porosity and its effect on water movement through Andosol intact columns B. Prado a,f, *, C. Duwig b , J. Ma ´ rquez c , P. Delmas d , P. Morales e , J. James d , J. Etchevers f a Universidad Nacional Auto ´noma de Me ´xico, Instituto de Geologı´a, Ciudad Universitaria, 04510, Mexico, D.F., Mexico b UMR 5564 LTHE/IRD, BP 53, 38041 Grenoble Cedex 9, France c CCADET, UNAM, A.P 70-186, 04510 D.F., Mexico d Department of Computer Science, The University of Auckland, Private Bag 92019, New Zealand e Laboratorio de Geoquı´mica Isoto ´pica, Instituto de Geologı´a, UNAM, D.F., Mexico f Colegio de Postgraduados, Laboratorio de Fertilidad de Suelo, CP 56230 Montecillo, Mexico 1. Introduction Solute transport through soil has become a key research topic since contamination of groundwater sources has been observed worldwide. The Valle de Bravo watershed in Mexico needs to be protected from further contamination as it provides 10% of the drinking water for the 20-million inhabitants of Mexico City. Water and solute transport through soil is a complex process that can be directly related to the pore network. Both soil porosity and other soil characteristics such as structure and texture affect transport processes (Strock et al., 2001). Literature on soil structure characteristics and its relationship with solute transport is ambiguous. Seyfriend and Rao (1987) reported that solute dispersion is related to the soil structure and its water content. Bejat et al. (2000) observed that in an unsaturated soil, there is a linear relationship between the soil water content and the dispersion of a non-reactive solute, as well as between the dispersion and the pore water velocity. They did not find a direct relationship between the soil’s structural properties and hydro- dynamic dispersion. The pore network, which depends on the soil structure, plays a decisive role in water and solute movement through soil. Walker and Trudgill (1983) found significant correlations between geometric variables describing soil porosity and solute transport parameters. For example, the dispersivity Agricultural Water Management 96 (2009) 1377–1386 ARTICLE INFO Article history: Received 15 September 2008 Accepted 12 April 2009 Available online 30 May 2009 Keywords: Macroporosity Image analysis Displacement experiment ABSTRACT The soil pore network and marcoporosity are important factors affecting water and solute transport. The transfer of contaminants to water resources is of particular importance in the Valle de Bravo watershed as it provides 10% of the drinking water for the 20 million inhabitants of Mexico City. This watershed is composed mainly of Andosols with unique mineralogical and physical characteristics. Soil porosity is usually examined on thin sections, using various image analysis techniques. We propose a novel methodology combining image analysis and a displacement experiment to study relationships between soil structure and water tracer transport parameters. H 2 18 O displacement experiments were conducted through intact soil columns sampled at three depths from a representative cultivated Andosol profile. The soil structure and pore characteristics were obtained by image analysis on thin sections obtained from each column at the end of the displacement experiment. The total 2D porosity (for pores larger than 50 mm) varied from 80% of the total section area in the topsoil to around 60% in the subsoil. Tubular pores were the most abundant in the soil profile, but ploughing of the topsoil had destroyed sections of these pores and replaced them with packing pores. Water transport in the intact subsoil columns was always in physical non-equilibrium, showing the existence of preferential flow pathways. In the topsoil, one column out of three showed no preferential flow, demonstrating that soil ploughing also homogenised pore connections. Pore connectivity was larger in the ploughed topsoil than in their deeper soil horizon counterparts. Our methodology offers a 2D quantitative characterisation of the macroporous network at 50 mm resolution and the determination of water transport parameters on the same intact soil samples. 3D characterisation of soil porosity using X-ray computed tomography (CT) gives a better picture of pore connection but usually has lower spatial resolution and a larger cost. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author at: Universidad Nacional Auto ´ noma de Me ´ xico, Instituto de Geologı ´a, Ciudad Universitaria, 04510, Mexico, D.F., Mexico. Tel.: +52 56 22 42 86x159. E-mail address: bprado@geologia.unam.mx (B. Prado). Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat 0378-3774/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.agwat.2009.04.012