Table 3.
.
-240
Pu in Soil
Mean Concentr ations (nCi/g) of 239
Ratios by
oil
tion/S
Vegeta
and
and Vegetation
13
Strata in Area GMX-5 and Area
Vegetation, (Means)
Soil
”?
(Means) ‘2? Meagured’® “Predicted
Strata
il
(b).
gad
Vegetat tfon{So
Predicted
Measured
Area GMX-5
5
0.084
0.0083
3
4.5
0.26
0.036
0.1
0.4
1.1
2.4
14.0
0.0052
0.013
0.17
0.077
0.28
be
0.059
0.73
1
2
7.3
4
0.0092
0.064
0.31
0.0099
0.13
0.0075
0.055
0.16
0.075
aren
0-6 5
0.34
0.050
0.0
0.23
0.052
Area 13
1
2
3
4
5
6
(a)
From Romney et al.
(b)
yy,
y,
oN
0.0067
0.017
0.058
0.14
0.28
1.36
0.15
0.14
0.44
0.069
o.10
0.078
QO. ae
OM
Xu
oN
03
oN
Q.
(1975).
= 0.13 C 3 0.79 for Area GMX-5
89 for Area 13
7 0.07 C, 0.
in Columns 2 and 3.
the strata means
Both equations are based on
Assuming a steady-state between vegetation and soil, the vegetation/soil ratio
can be expressed in the parameters of Equations (13), (14), and (15) as follows:
Yye/Cg = VaFLAA,
(16)
where, Wy =Ay + Ag ti, is the effective decay rate coefficient for
Plutonium bearing soil particles externally deposited on vegetation,
As noted in the section on afr, the deposition velocity is a function of
particle size (Equation (4)}.
In the soils of Area 13 (Tamura, 1976), most
of the plutonium is associated with coarse silt (20 to 53 um) and the estimated deposition velocity (V,) for particles of 50 um diameter could be as
high as 20 cm/sec or 1.73 x fos em/day.
The plant interception factor (F_} determined by Miller and Lee (1966) for
freshly deposited volcanic dust ¥so0 to 100 um) was 47.4 cm?/g for dry exposure
conditions.
For predictive purposes, Anspaugh (1974) has suggested that a mass loading
factor (i) of 100 ug/m3 (1g-}0 g/cm?) be used.
This is the amount of dust
one would expect to find in the GMX area (Shinn and Anspaugh, 1975) when the
wind velocity averages about 1.4 m/sec. (3 mph).
Substituting these values in Equation (16), assuming a vegetation/soil ratio
of 0.1 and solving for , indicates effective half-life of about 8.5 days.
So,
ve . (1.73 x 10° em/day)(47.4 em?/g) (19-10 g/em>)
1n(2)/8.5
Cs"
= 0,10,
This exercise proves nothing.
It merely demonstrates that Equation (16) might
be adequate to explain the high vegetation/soil ratios observed at NTS.
Shinn
and Anspaugh (1975) have demonstrated that mass loading (L_}) increases with
wind velocity.
The effective half-life may decrease with wind velocity.
Sehmel (1975) has shown that deposition velocity (V,) decreases as particle
size decreases for d_ >lpm.
Zf small particles are more readily retained by
vegetation than larger particles, the plant interception factor and the effec~tive half-life may increase as particle size decreases.
Variations of the
factors of Equation (16) as functions of particle size and wind velocity may
account for the range of vegetation/soil ratios implied by Equation (11), but
there are still too many unknowns to develop a descriptive, dynamic model for
the vegetation compartment.