UPTAKE OF RADIOACTIVE NUCLIDES
=LL
mean delay period in the transfer of strontium 90 from rain to milk was
the deposit
| weeks the major part of
h water.
wit
g
hin
was
be removed by
sometimes only 1 to 2 months (see p. 286) and that permanent pastures were
subject to considerably greater direct contamination than annual crops.
under nor» the major cause of loss
Despite the large discrimination factor against strontium relative to calcium
(33) have
ily one. Moorby & Squire
ch have
whi
mantities of strontium 89
in its passage to milk the ratio of strontium 90 to calcium in milk has generally differed little from that in the vegetable component of diet (2). The
same conclusion was indicated by the sampling of grass in a number of
if the plants
s can become air-borne
first called)
stem-base entry as it was
t that in hill
7 to account for the fac
to the bones
y
‘onsiderably more readil
in the lown
tha
es
growing hill pastur
enhanced
of
ms
ter
in
t be explained
entrapment of
ow calcium status; the
explanation
eared to be the probable
prostrate
of
g
nent of “mat,” consistin
es. The
tur
pas
hill
the
haracteristic of
art of
orp
maj
the
t
tha
ons
‘th observati
yin
erl g soil.
at zone and not in the und
plant-base
for
remain readily available
ld-wide
wor
of
rs
yea
since, in the early
e hill
som
m
fro
als
eri
mat
in biological
eriExp
),
the cumulative deposit (34
the
for
ce
den
considerably stronger evi
ton
dle
Mid
s.
nce
| under these circumsta
from various
3) as a spray to turves
hin a relatively
insplanted to boxes. Wit
lands swards
n 90 on herbage from low
ie
of distinguishing between them lies in the fact that in lowland, as opposed
to upland, pastures strontium 90 does not remain readily accessible for ab-
ee bee
Kingdom. The
ttention in the United
areas. Such observations do not, however, answer the question of whether
foliar or plant-base absorption is the operative mechanism. The difficulty
sorption in the plant-base zone for long periods; thus the expected time
course of the two mechanismsis relatively similar. Some evidence suggestive of plant-base uptake in lowland pastures was obtained in 1958 and
1959; for example the degree to which some lowland pastures were directly
contaminated from world-wide fallout appears to increase with age and the
consequent development of a surface mat (37). However, until relatively
recently it remained possible that foliar uptake might be the major mechanism of direct contamination. The statistical studies of Bartlett & Mercer
(38) now show that this is most unlikely. They examined the correlation
between the ratios of strontium 90 to calcium observed in milk produced
throughout England and Wales during the summers of 1958 to 1960, and
the extent of fallout in the month preceding the mean date of sampling
(f,), one month earlier (f,} and two months earlier (f,). The correlation
coefficients between the observed levels in milk and the deposits in months
z Ste! akin ott. tT
particles
activity is contained in fine
ks after
wee
3
to
2
l
unti
ot take place
ditions are
chat losses under these con
cuticle.
nt-base entry
ortance of foliar and pla
f,, f,, f; were respectively 0.83, 0.94 and 0.86. It was found also that the
best linear relationship between the level in milk (m) and these quantities
was given by the equation:
m= 4.7 + 6.61 f:
No significant advantage was gained by including other terms. A “lag” of at
least a month between the deposition of strontium 90 and its maximum
soil. In contrast,
£ absorption from the
effect on the contamination of milk was thus established. This delay could
not be attributed to the metabolic process of the cow since the delay between
the ingestion of strontium 90 and the occurrence of the highest levels in
many years a
id pastures showed for
ly
inhibited entire
re and this could not be
pared relationships
milk is about one week (39). The significance of the lag period is apparent
"Newbould (36) com
ination of pas} in soil and the contam
ontium 89 was
str
the extent to which
oratory conlab
er
und
s by grass grown
contamination iS
t the extent of direct
hill areas. Other
to plant-base uptake, in
when the rate at which direct contamination is lost from leaves is remembered; after 7 to 8 weeks some 90 per cent of the original deposit is likely
to have been removed (31). Thus, if the pastures had been contaminated
mainly by simple foliar retention the lag demonstrated by Bartlett & Mercer
(38) could not have occurred; it is, however, readily compatible with plant-
base absorption. The analysis of results for some local milk sources in the
United Kingdom again demonstrated this lag phenomenon (38, 40) and it
ium 90 enters by
om whereby stront
es has proved
tur
pas
d
- growing lowlan
is, therefore, probable that plant-base contamination is the dominant mech-
rate of fallout was
dence that, when the
anism whereby recently deposited fallout enters pastures of all types in that
country. Comparable data are not available for other areas and quantita-
of entry and the
on was the major route
a
wee er
“poo cow . pee
1ETE
~ar
>z
TY
+
yoy
ot
281
.
Weee neem en ee wT
ee ere
wee ee ee
fe ree reer en rer,