286
RUSSELL
Sr® per km? would initially lead on average to about 1.6 pe Sr®°/gCa in
plants; with the passage of time the deeper penetration of strontium 90 in
soil would cause this value for shallow rooted crops to decrease by about
one-third (65). The analysis of the results of surveys of world-wide fallout
have lead to broadly similar conclusions; so also did the field experiments in
Sweden (2).
Relative importance of direct contamination and absorption from the
soil in the contamination of plants with world-wide fallout.—The first
convincing evidence, on a wide scale, of the importance of direct contamina-
tion was provided by the comparison of the ratios of strontium 89 to 90 in
rain with that in milk during the years 1957 and 1958. Because the half life
of strontium 89 is oniy 7 weeks, its ratio to strontium 90 in any sample of
fission products decreases by a factor of 2 in this period. Information on
the rapidity with which strontium 90 is transferred to diet could therefore
be obtained. In North America and in Britain it was found that the ratio in
milk was half or more of that in fallout during the summer (27). The
average delay in the transfer of strontium 90 to milk was thus between one
and two months. Later, when the suspension of weapons trials caused the
rate of fallout to decrease throughout the Northern Hemisphere, though
the cumulative total still rose, marked decreases in the levels of strontium 90
in foodstuffs demonstrated the importance of direct contamination in the
earlier period (2). It is estimated that during 1958 and 1959 the contamination of milk in the United Kingdom was mainly due to the direct contamination of pastures with the recent deposit (66) and the situation appears to
have been similar in the Netherlands (67). In the United States about half
the strontium 90 which entered milk in the summer of these years has been
attributed to direct contamination (68).
Attempts have been made to derive proportionality factors which relate
the levels of strontium 90 in agricultural products to the rate of fallout and
to the cumulative deposit in the soil (2, 65 to 71). Any such calculation is
subject to obvious limitations; changes in weather and seasonal factors
influence the entry of strontium 90 into plants. With milk, which has been
particularly considered from this viewpoint, additional complications are
introduced by the seasonal grazing pattern of cattle and the consumption
of stored food in winter. Further uncertainties result from, the imprecision
of the survey data on which calculations are based. The effects of these
limitations are likely to be considerably reduced when estimates are made
for the average situation over an entire year or longer in a large area
where extensive surveys have been carried out. Calculations which satisfy
these criteria have been shown to be of considerable practical value for predicting the probable long-term consequences of environmental contamination (2).
CaESIuUM 137
Caesium 137 in plants——Caesium is readily absorbed by plants and like
potassium it is freely redistributed within them. However, it is well established that the absorption of caesium is not related to that of potassium ir
~
wm
ogee
ee at
ee SacitesCienaSahel
‘
*