be 10° 7m.
presumably caused by the migration of
However, the total inte~
the initial surface-deposited material
grated air activity (from t = 0 to o& for
into the soil,
239 py would be changed only by
Attempts to use the resuspension
wm
AX 104 [ exp (- 0, 693t/45d) dt
factor approach to derive acceptable
Oo
levels of soil surface coniamination have
included this "attenuation factor" as a
exp (- 0, 693t/24, 400y) dt
oO
simple exponential function with half-
times of 35 or 45 days”? 4 .
eo
+axio!?
_= 6.5AX10°+1,3AX10°%,
-3
-3
There are
major uncertainties in such a formulation,
which is an increase of 20%, and more
however,
importantly, cannot be accumulated dur-
The longest study of this de-
crease with time extended to only11 mo
ing an individual's life span,
Because the functional nature of the
following the initial deposition®, which
is extremely short compared to the half-
decrease in resuspension rate with time
life of a radionuclide such as 239 pu,
cannot be confidently extrapolated, pre-
There are also published reports which
viously published models should not be
indicate on experimental and theoretical
applied to the reoccupation of areas many
bases that the decrease with time will
years after the contaminating event.
The resuspension-factor approach can
not be adequately represented by a single
exponential function, but that the rate of
be applied in an approximate way, how-
decrease itself will also decrease with
ever, if resuspension factors are used
Fortunately, the exact nature
time!? 6
which were derived from measurements
of this time dependence is not critical in
over aged sources,
determining the integrated exposure from
relevant data are unpublished results
the time of initial deposition due to the
from current resuspension experiments
fairly well-documented rapid decrease at
at the GMX site in Area 5 of the Nevada
early times,
Test Site.
However, it is obviously
Perhaps the most
The 239 Pu at this location
the controlling factor for questions con-
was deposited following 22 high-explosive
cerning the reoccupation of areas many
detonations during the period from
years after the contaminating event.
December 1954 to February 1956,
As an illustration, the most conserva-
Measurements of resuspended air activity
tive published model (Kathren?) may be
levels at this site curing 1971-1973
used to calculate a resuspension rate for
appear to be the only available data con-
material 15 yr aiter deposition:
cerning resuspension of 2395, from a
K-10
-4
m
-n
-..
_
ony ( 0.693 X liv X 365d
‘
45d
Vv
\
source of this age,
Data from two types or measurements
}
~ 10° 74/m,
are available and can be used to derive
average resuspension factors,
The first
If, however, the resuspension rate
tvpe of measurement? was accomplished
asymptotically approached some finite
by placing five high-volume cascade
value 19° of the original, then the resus-
impactors © within the most highly con-
pension rate 15 yr later would obviously
taminated area, and running them for
I-17