time area). The mass-transfer coefficient is a lumped term including
both a tortuosity factor and a diffusion coefficient. Diffusion is the
transport of solutes from an area of high concentration to an area of
low concentration independent of any movement of the media. The
tortuosity of the media limits the exposure between a concentration
gradient. Both dispersion and diffusion will occur during water
transport through soil. The contribution of the dispersion process to
solute mixing is generally of greater magnitude than the diffusion
process, such that the diffusion process is frequently insignificant.
However, in soils that contain immobile-water regions, diffusion is the
only process that transports solutes into, through, and out of the
immobile regions. In such soils the magnitude of the diffusion process
becomes significant.
Parameter Estimation
Adsorption isotherms
The tritium-adsorption isotherms for the three column horizons
and composite gravel samples are presented in Fig. 4-2. A weighted
mean of the slope of the line obtained by plotting the adsorbed versus
solution concentrations of tritium was calculated considering the
depth and gravel content of each horizon. This column adsorption
coefficient, Kd, was applied to Eq. [4-4] with other column parameters
to calculate a retardation factor of 1.05 for both columns. This value
indicates that the tritium is slightly adsorbed to the soil and is
consistent with other values measured for soils of similar mineralogy
(Nkedi-Kizza, 1983; Seyfried and Rao, 1987).