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influenced by climatic changes. Also, degradation of the polyethylene geomembrane
layer was considered for future conditions. Arnold
The RAECOM software program (Rogers et al., 1984) was used to simulate steady-
state, one-dimensional diffusion of 222 Rn through a five layer cover system at the
Monticello site. The uncertainty analysis was conducted by coupling the RAECOM
code with the FRAMES software program, which supports Monte Carlo analyses
with automated parameter sampling using the Latin Hypercube method (Wyss and
Jorgensen, 1998). Uncertainty distributions were assigned to the parameters for
each of the layers in the landfill system based on variations in site measurements.
A negative correlation was assigned to the sampling of effective diffusion coeffi-
cient and the moisture content, based on the relationship reported in Rogers et al.
(1984).
For the Monte Carlo analyses of uncertainty in 222 Rn flux through the surface of
the landfill cover system, 100 realizations of the diffusion model were run for present
conditions and for future conditions. The results of these simulations are shown as
a cumulative distribution of surface 222 Rn flux in Figure 20.1. For present climatic
conditions and essentially undegraded geomembrane performance, the uncertainty in
simulated 222 Rn flux at the land surface spans more than two orders of magnitude
2
with a maximum value of about 1 pCi/m · s. For future conditions (greater than
100 years), the simulated 222 Rn flux shows a similar span in uncertainty and has
2
a maximum value of 29 pCi/m · s. Overall, these results indicate a high degree of
confidence (greater than 97%) that the landfill cover design at Monticello meets the
2
performance objective of 20 pCi/m · sof 222 Rn flux for both present and future
conditions.

1

0.8
Cumulative probability 0.6 Present Future 40 CFR Part 61.222

0.4

0.2

0
0.01 0.1 1 10 100
2
Radon flux (pCi/m s) ·
Figure 20.1. Cumulative distribution of simulated radon flux from uncertainty analyses of the Monticello
uranium mill tailings cover, for present and future conditions