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emanation rates from underlying soils, transport pathways into the home, and the
amount of air exchange with outdoor air. Arnold
Uranium mill tailings are a potentially significant source of radon gas contamina-
tion, if they are not sufficiently isolated from the environment. Uranium is extracted
from the ore in the milling process, but the decay products of uranium, including
radium, remain in high concentrations within the mill tailings. Consequently, the mill
tailings are a long-term source of 222 Rn from their 226 Ra content.

20.2 RADON TRANSPORT IN SOILS
Radon transport in soils begins with the release of radon into the gas phase present in
the soil. Because radium is present in the solid phase of mineral grains, radon must
be released following decay of its parent. This process may be enhanced by crystal-
lographic dislocations resulting from alpha recoil following decay of the radium. The
efficiency at which radon is released by soils is often expressed as the radon emana-
tion coefficient, which is simply the fraction of radon released to the gas phase. The
radon emanation coefficient for uranium mill tailings typically varies from 0.1 to 0.4
(Rogers et al., 1984).
Migration of radon in geologic media can occur by both diffusion and advec-
tion. Although advective gas migration may be significant in some geologic settings,
strong gradients in gas pressure are generally not present under ambient conditions
in the shallow subsurface. However, advective barometric pumping of radon gas into
dwellings may be a factor in the case of this exposure mechanism. Diffusion is the
dominant process for radon migration in most subsurface environments, particularly
in landfill systems specifically designed for the disposal of uranium mill tailings.
Theequationforsteady-state, one-dimensional, verticaldiffusivetransportofradon
in soils is:
2
d C
D − λC + Q = 0
dz 2
where D is the effective diffusion coefficient, C is the concentration of 222 Rn (activ-
ity/volume), λ is the decay constant of 222 Rn (2.1 × 10 −6 −1 ), and Q is the source
s
term for 222 Rn. The source term for radon in the soil can be estimated by the equation:
Rρ b λE
Q =
φ

where R is the specific activity of 226 Ra in the soil, ρ b is the dry bulk density of the
soil, E is the radon emanation coefficient, and φ is the porosity.
Radon gas is moderately soluble in water that is present in variably saturated soils
of the vadose zone. Because the diffusion coefficient in the liquid phase is much
lower than in the gas phase, diffusion of radon is dominated by migration in the gas
phase. However, partitioning of radon into the relatively immobile liquid phase may
inhibit diffusive migration somewhat.