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18
III, Results for 1955 Chicago Milkshed Samples
A.
1955 Chicago Milkshed Soils
Table l summarizes the results obtained for the Chicago Milkshed soils
collected Septenber 29 and 30, 1955,
The available calcium in grams per square
foot was computed from Beltsville analyses of available calcium per unit weight
of sample and total weight and area of sample collected.
Strontium-90 analyses
were made by the Chicago Sunshine Laboratory.
The samples were taken from the same fields of farms sampled in 1953 and
again in 1954.
Farms #3, #6 and #7 had been plowed recently.
For each of these,
15 plugs, 3.5 inches in diameter, were taken to a depth slightly greater than
apparent plow depth,
On farm #7, two sets of samples were taken from alternately
spaced holes to provide a check on the reliability of sampling recently plowed
fields.
The agreement in the results of the duplicate samples is very striking,
The results in dpm per square foot apply to the available strontium-90
(i.e. » extractable in normal neutral aymonium acetate) and not necessarily the
total strontium-90 deposited per square foot.
Although the Chicago rain data
indicate that the strontium-90 is deposited in soluble form, it is recalled
that 1953 soil samples fused with NagC03 following successive NH)AC and HCl
extraction showed appre clable residual strontium-90,
The average for the first six farms listed in Table
is 970 + 180 dpm
available strontium-90 per square foot, corresponding to about 12 millicuries
per square mile for this area.
The September 30, 1953 average for these same
six soils was approximately S millicuries per square mile,
The increase of
7 millicuries per square mile for the two year period is in surprisingly good
agreement with the total deposition of strontium=90 in Chicago rains over the
same time interval (see Figure hl),