B7Cs PASSAGE FROM PRECIPITATION TO MILK
705
Table 1— FORAGE AND RAINFALL ACTIVITIES DURING 1963 AND 1964 AND
CORRESPONDING DEPOSITION COEFFICIENTS FOR 8'%Csxs
_
Feed
Harvest
date
Alfalfa
hay
Alfalfa
hay
Alfalfa
hay
June 15,
1963
July 20,
1963
Aug. 30,
1963
Corn
Sept. 15,
silage
Alfalfa
hay
Alfalfa
hay
Alfalfa
hay
Mean
Yield,
g m?
Rainfall,
in.
forage 8'Cs Forage
Rainfall*
Forage—
rainfall
activity,
pe ‘kg
87Cs5,
pe/m?
deposition,
pe /m?
deposition
coefficient
340
4.38
4828
1642
3424
0.48
225
0.21
2357
530
754
0.70
210
0.46
1092
229
1079
0.21
1963
June 15,
1964
1013
4.71
1670
1690
5239
0.32
340
4.12
4070
1384
4174
0.33
1964
Sept. 10,
1964
225
0.35
680
153
1147
0.13
210
0.66
380
80
889
0.09
July 20,
*Integrated vaiues for growth period.
meter of first, second, and third cuttings of alfalfa hay in 1963-1964
and also of corn silage for 1963. First-cutting hay was harvested about
June 15, 1963, during or after the period of heaviest fallout. This
cutting showed the maximum 13"Cs activities of the cumulative deposition during the growing period. The deposition coefficient for firstcutting hay was much higher than for second- and third-cutting hay,
except for the second cutting in 1963. A possible explanation is related
to the type and amount of rainfall during the three growing periods.
During the first-cutting growing period, during the spring, there were
heavy, frequent rains. During the second-cutting growing period, the
rainfall consisted of many rains of low total rainfall, It is reasonable
to assume that the uptake of '*’Cs is greater for this type of rain than
that for heavy rains, which can wash off '*'Cs initially deposited.
During the corresponding period of 1964, when the total rainfall was
greater but fell in fewer rains, there was a significant decrease in the
deposition coefficient.
Measurement of activity in pastures is, in general, an undesirable
way by which to quantify fallout deposition. Grazing animals selectively
remove some fraction of the vegetation daily while the remainder continues to grow until it reaches maturity, at which time it is preferentially not eaten. Pastures change rapidly ingrowthrate and to some extent in botanical composition, depending upon available moisture and
_ environmental temperature. All these factors contribute to the difficulty of calculating a precise and useful deposition coefficient.