contamination resulting from dry deposition and/or wet deposition, with
subsequent calculation of retention times based on soluble or readily
leachable components found in fallout particles such as '?!1, 137¢,) and
°sy. Resuspension data from the NTS provides a basis for much of the
information we have concerning the interception of plutonium contaminated
soil particles and their retention.
The behavior of the transuranic elements in the environment and their
potential for transfer in the food chain has been the subject of extensive
study over the past 25 years.
Although there is a general understanding
of many problems concerning atmospheric transport, and the behavior of
plutonium in specific ecosystems (NTS, Rocky Flats, etc.), little is known
of the controlling mechanisms’ influence on the bioavailability of plutonium
and the other transuranics, and their subsequent transfers along the food
web to man. With the current stratospheric depletion of fallout plutonium
(Bennett, 1976), the inhalation route to man is greatly reduced.
This
would then suggest that the major sources of transuranic elements would be
from resuspension of fallout contaminated soils on a global basis, resuspension from highly contaminated local sources, accident situations, and lowlevel releases from nuclear facilities.
A frequent practice in present radiological safety estimations is to
discount foliar sorption and emphasize the soil to root pathway for food
chain entry of transuranic and other radiocelements (Vaughan ef al.,
1976). Typical dose assessment codes assume a rapidly declining exponential
loss of material from leaves (Soldat, 1971).
This is certainly not a
general situation.
Tt does not apply to the behavior of plutonium aerosols
to be described herein, and probably applies only to very large particles
and to certain gaseous radioelements like iodine (Markee, 1971). The
relatively greater importance of the foliar entry pathway compared to root
absorption for worldwide fallout was, of course, recognized long ago
(Chamberlain, 1970; Russell, 1965).
In later studies, for particles
probably of wind resuspended origin, it has been shown that 872% of the
Sst, 81% of the '?’Cs, and 73% of the '**Ce in forage plants was derived
from foltar contamination (Romney ef al,, 1973). This was done by comparing plants grown with and without plastic enclosures at the NTS site.
Currently, the relatively greater importance of foliar to root pathways to
the plant has been incorporated in the LMFBR Final Environmental Statement
(USAEC, 1974).
Despite the inconsistencies in many other dose assessment
codes, risk tends to be minimized because of extremely conservative limits
specified at points where radioelements enter
the human body.
However,
as
a matter of systematic practice, an improved quantitative understanding of
the basic environmental processes is required. This becomes especially
important in situations:
1) where new technology may lead to different
physical (size) and chemtcal characteristics of the source term for release,
especially in nuclear fuel reprocessing plants; and 2) where comparatively
large increases in the handling of radioelements are projected for the
future (ERDA, 1975).
The following discussion will briefly review what is known concerning the
absorption and adsorption potential of foliar surfaces, and attempt to
by plant
describe the fate of transuranic particulates on interception
s and the
foliage, by extrapolation from the behavior of other particulate
limited information on plutonium.
CANOPY INTERCEPTION
correlations of fallout
in the HERMES Model (Soldat, 1971) air to piant
representing the
sr, Cs and Iz have led to an average estimator, 0.25,
surface deposijnterception fraction, i.e., the fraction of each month's
initially retained by
tion from air and from sprinkler irrigation that is
Such a value
.
agricultural vegetation, before leaching by rainfall
to marked variarepresents an operational definition, in practice subject
tion depending on specific circumstances of exposure.
interception has
In field observations, another way of considering canopy
This coefficient
been to calculate the interception coefficient, f.
) was found to vary
(sometimes erroneously termed "interception fraction"
1000-fold in various field situations (Bloom et al., 1974).
2
cm’ /g
£ = d/dt {c\) + c, Vy>
deposition on plant
where d/dt (C_) = instantaneous rate of radioactivity
leaf, pCi/g dry/sec
v
Cy = air concentration, pCi/cm
3
V5 = deposition velocity, cm/sec
n clearly establishes
A range this wide for estimates of initial depositio
foliar
that aerosol polydispersity and other unassayed variables control
Probably chief among
deposition and retention, as measured operationally.
these variables
is the particle size distribution,
since deposfiton
known particle sizes
velocities can be estimated with greater accuracy for
eous
Values determine? for air concentration and instantan
(USAEC, 1974).
generally represented
deposition rates, in past field observations, have
Leaf type and degree of mineralian unknown distribution of particles.
likely to he the
zation may also affect these estimates, but they are
only minor variables.
BOUNDARY-LAYER PARAMETERS AFFECTING DEPOSITION
basis in the
Although particle deposition fs an aerodynamic problem with a
n of particles
physical sciences, in real-world situations, the interactio
a modification of
with a dynamic surface such as a plant canopy “requires
(Slinn,
to data"
predictive formulae which are mostly just empirical fits
e due to
Several of the factors which become difficult to quantitat
1975).
, eddy
their ever-changing nature in the boundary layer include turbulence
333