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Chapter 20: Radon Transport
The effective diffusion coefficient for radon transport in soils is a function of soil
type, compaction, and moisture content. Finer grained and more compacted soils have
generally lower values of the diffusion coefficient. The effective diffusion coefficient
of radon in the gas phase decreases significantly with increasing moisture content of
the soil, being on average about two orders of magnitude lower as the liquid saturation
approaches 90% (Rogers et al., 1984). The variability in measured values of the
effective diffusion coefficient also increases dramatically with increasing moisture
content of the soil.
The moisture content of soils effects diffusive radon transport in two offsetting
ways. As noted above, the effective diffusion coefficient is significantly lower at
higher moisture contents. The volume of the gas phase also decreases with increasing
moisture content, leading to an increase in the concentration of radon in the gas phase
of the soil. This increased radon concentration and higher concentration gradients
relative to the essentially zero concentration in the atmosphere above the soil, tends
to enhance the diffusive radon migration. As stated above, these two effects tend to
counteract one another, but the relatively higher dependency of effective diffusion
on moisture content dominates. Consequently, net radon diffusion decreases with
increasing moisture content of the soil.

20.3 RADON MIGRATION FROM URANIUM MILL TAILINGS

The disposal of uranium mill tailings is regulated to limit the environmental expo-
sure of human populations to radon gas. The federal standard (EPA 40 CFR 192)
states that the average 222 Rn flux to the atmosphere at the landfill cover shall be
2
less than 20 pCi/m · s. Landfill cover systems for uranium mill tailings are typically
designed with a layer of compacted clay as a barrier to diffusive radon migration.
Design requirements for the thickness of the clay layer are generally calculated
based on the estimated values of material properties (e.g., effective diffusion coeffi-
cient), the radium content of the mill tailings, and other design features in the landfill
cover.
Uncertainties in the disposal system for uranium mill tailings may have an impact
on the assessments of landfill cover performance and on the risks associated with
a particular cover design. Uncertainties exist in the parameter values for various
components of the disposal system. In addition, considerable uncertainty exists in
future climatic conditions and potential degradation of the disposal system.
A risk-based probabilistic performance assessment of the uranium mill tailings
disposal site at Monticello, Utah was conducted with regard to radon migration (Ho
et al., 2002). The landfill cover at this site consists of a 24 inch clay layer above
the tailings, overlain by a 60 mil high-density polyethylene liner, a 12 inch sand
drainage layer, and 66 inches of soil. This analysis considered uncertainty in the radon
emanation rate of the tailings, the effective diffusion coefficients in multiple layers
of the system, and the moisture content in the layers above the tailings. Scenarios for
present and future conditions were constructed, in which uncertainty in infiltration
and moisture content was considerably higher under future conditions that could be