Movement of solutes associated with intermittent soil-water flow .I. Tritium and bromide

Abstract

The movement of tritiated water and bromide through columns of repacked silt loam was examined in the laboratory. A pulse (5 mm) of a dilute solution of potassium bromide in tritiated water was applied, left for 3 or 10 days, and then leached further down the column of soil with 30 mm of distilled water. Twelve days after the solute pulse was applied, the columns were sectioned, and the distributions of water, tritiated water and bromide were measured. The bromide moved only slightly further than the tritiated water, indicating little anion exclusion. The bromide peaks were much sharper than those for tritiated water. This reflects the slower diffusion of bromide during the periods of several days between leaching and the termination of the experiment, and the importance of molecular diffusion relative to hydrodynamic dispersion. Given the soil hydraulic properties, the behaviour of water, tritiated water and bromide could be simulated by coupling the water flow equations with the convection-dispersion equation, and by using solute diffusivity and dispersivity values from the literature. A significant assumption was which cation was mostly convected with, and hence diffused with, the bromide. The use of the diffusion coefficient for calcium bromide rather than potassium bromide resulted in a better description of the observed bromide profiles in the soil.