Tritiated water, released at a point in a uniform, relatively dry soil
, diffuses in both the liquid and vapor phases. A model which accords
well with published data indicates that the flux density of tritium in
the liquid exceeds that in the vapor phase provided the water content
is greater than approximately 20% of the total soil porosity. Thus tr
itium redistribution should be modeled recognizing transfer ''in paral
lel'' in both phases. The diffusion equation cast in spherical coordin
ates, and taking into account radioactive decay, then provides a basis
for design of field experiments and predicts the long-term fate of tr
itiated water released in these experiments. We calculate the evolutio
n of profiles of tritium concentration, within and external to the sph
ere of released solution, assuming the initial concentration to be uni
form. We also predict the speed and the envelope of the tritium maximu
m as it advances and attenuates in the soil. We briefly discuss effect
s of variation in the volume fractions of soil water and air on the ef
fective diffusion coefficient of tritium in soil and comment on possib
le effects of convective transfer by moving water and air.