Effect of solute application method on preferential transport of solutes in soil

Abstract

In soils containing macropores, both water and solutes can move preferentially, bypassing much of the soil matrix. The objective of this study was to examine the effect of solute application method on preferential solute transport in soil materials containing differing amounts of macroporosity. Transport experiments were conducted with three undisturbed soil columns (18 cm diameter, 33–35 cm long) that contained extensive networks of macropores. The macroporosity of the soil columns was first characterized by measuring the saturated hydraulic conductivity and drained porosity, and by conducting saturated, steady-fluid-flow miscible displacement experiments in which a 0.05 N CaCl 2 solution displaced a 0.01 N CaSO 4 solution. These measurements indicated extensive macroporosity and a range in macroporosity among the three columns. Two transport experiments were then conducted with each of the three columns. The initial moisture condition of the soil in both experiments was a gravity-drained profile. In the first transport experiment, a 200-ml pulse of 0.05 N CaCl 2 solution was ponded on the surface of each of the three columns and allowed to infiltrate. A solution of 0.01 N CaSO 4 was then immediately ponded on the soil surface. In the second transport experiment, after leaching the excess chloride remaining in the columns from the first experiment, 40 ml of a fivefold more concentrated CaCl 2 solution (0.25 N CaCl 2) was uniformly dripped on the soil surface of the same three columns (no ponding), allowed to redistribute for 15 min, and then leached with a steady flow of 0.01 N CaSO 4 solution. The resulting breakthrough curves for the two solute application methods were distinctly different, indicating a sensitivity of the transport process to solute application method, or solute boundary condition. In addition, the curves showed that the sensitivity to application method decreased with decreasing soil macroporosity. An analysis of the breakthrough curves is presented to explain the experimental results obtained. Two factors are important in understanding the results. The initial water status of soil macropores, saturated or drained, is important in determining the degree of dispersion that takes place in individual macropores and thus in the soil as a whole. Second, mass transfer of solute into the soil matrix as a consequence of both diffusion and solution absorption from macropores is important in causing dispersion of the solute. The implications of these findings for conducting solute transport experiments are discussed.