Quantifying the influence of intra-aggregate concentration gradients on solute transport

• Published in: Soil Science Society of America journal Vol. 63; no. 4; pp. 759 - 767
• Main Authors: Cote, C.M, Bristow, K.L, Ross, P.J
• Format: Journal Article
• Language: English
• Published: Madison Soil Science Society 01.07.1999; Soil Science Society of America; American Society of Agronomy
• Subjects: Agronomy. Soil science and plant productions; alternating wet and dry conditions; Biological and medical sciences; concentration; diffusion; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; gradients; leaching; macropores; mathematical models; movement in soil; Physical properties; Physics; Physics, chemistry, biochemistry and biology of agricultural and forest soils; preferential flow; soil aggregates; Soil science; Soils; solutes; Surficial geology; transport processes; Water and solute dynamics; Physical chemistry; Model study; Solutes; Exchange reaction; Modeling; Diffusion process; Lixiviation; Soil science; Mathematical model; Soil solution; Preferential flow; Quantitative analysis; Concentration gradient; Laboratory study; Soil column; SOIL TRANSPORT PROCESSES; Property of soil; Soil structure; Porous medium flow; Chemical transport; Applied mathematics; Concentration effect; Aggregate; Physicochemical properties; Applied physics
• ISSN: 0361-5995; 1435-0661
• DOI: 10.2136/sssaj1999.634759x

The physical nonequilibrium of solute concentration resulting from preferential flow of soil water has often led to models where the soil is partitioned into two regions: preferential flow paths, where solute transport occurs mainly by advection, and the remaining region, where significant solute transport occurs through diffusive exchange with the flow paths. These two-region models commonly ignore concentration gradients within the regions. Our objective was to develop a simple model to assess the influence of concentration gradients on solute transport and to compare model results with experiments conducted on structured materials. The model calculates the distribution of solutes in a single spherical aggregate surrounded by preferential flow paths and subjected to alternating boundary conditions representing either an exchange of solutes between the two regions (a wet period) or no exchange but redistribution of solutes within the aggregate (a dry period). The key parameter in the model is the aggregate radius, which defines the diffusive time scales. We conducted intermittent leaching experiments on a column of packed porous spheres and on a large (300 mm long by 216 mm diameter) undisturbed field soil core to test the validity of the model and its application to field soils. Alternating wet and dry periods enhanced leaching by up to 20% for this soil, which was consistent with the model's prediction, given a fitted equivalent aggregate radius of 1.8 cm. If similar results are obtained for other soils, use of alternating wet and dry periods could improve management of solutes, for example in salinity control and in soil remediation.