A morphological approach to describe solute movement influenced by bypass flow in a structured clay soil

Abstract

Chloride and methylene blue balance influenced by bypass flow was estimated in an experiment using 15 undisturbed 6 L soil cores taken from a well structured clay soil. Chloride concentration of outflow was 2 to 10 times lower than the inflow concentration of 10 mmol L ⁻¹. Methylene blue concentration was 15 to 500 times lower than the initial concentration. Based on the reduction of the concentration, we estimated separately three retention processes, mass flow, diffusion, and adsorption. Ratio of chloride outflow to diffusion retention or ratio of methylene blue outflow to adsorption retention was strongly related to the geometry of water-conducting macropores, which was quantified by the volumetric fraction of methylene blue stain raised to the power of the fractal dimension of staining pattern. Increase in the volume of water-conducting macropores with low fractal dimension promoted leaching influenced by bypass flow. Using the relationship between the two ratios, the relationship between chloride and methylene blue concentrations could be more accurately estimated.