7
milk concentretions of any NTIS event and, as noted earlier, its ist
appeared in the PMN milk et Helena, Montana as well.
The SWRHL collected
ilk samples from 154 locations in the western U. S. for this event with
sive sampling concentrated in seven states.
The farthest distance
st in milk >30 pCi/l was found at Miles Clty, Montana, approximately
900 miles from the test site.
The date of this sample preceded that found
at Helena in the PMN from the same event.
"Siovron or the fifteenunscheavlea releases of radioactivity from underground tests at the NTS produced no detectable zis in the local NTS milk
samples.
The Pin Stripe event on. April 25, 1966, resulted in a milk concen-
tration of 4,800 pci/l at a distance of 60 miles.
to 70 pci/l in a sample obtained at 550 miles.
Concentrations decreased
Aside from Pin Stripe, the >
highest concentration of qist in the SWRHL milk network from an unscheduled
release of radioactivity from an underground test (130 pCi/1) occurred from
the June 16, 1965 event.
Actually, at this time, fallout from a Lop Nor
nuclear test deposited radioactivity over the United States and the assignment
of the origin of the radioiodine in milk is ambiguous (23).
‘The same
- eonfusion on the source of radioiog@ine in milk existed a year later for
the June 8, 1966 rocket test (24).
Why did the PMN not reflect the presence of radioiodine when it was
seen in the local SWRHL network?
The probable explanations are both
meteorological and non-meteorological.
In the latter category one notes
that the PMN composites milk from farms dispersed over hundreds to thousands
of square miles.
Milk from ?Parms with detectable concentrations of ist
can be diluted by milk from other farms in the milkshed with no radioiodine,
* the composite qist being too small to detect.
This contrasts with the
SWRHL network where, for the most part, individual farrsor dairies are
sampled.
The clouds from some of the atmospheric releases in Table 3 moved
northward in the cold hal? of the year when cows were not on pasture.
probaoly more important are the several meteorological reasons.
But
A cloud
of radioiodine dilutes as it moves downwind of its source due to both
horizontal and vertical turbulent mixing and to removal processes.
The
dilution caused by atmospheric diffusion, on the average, decreases the
peak concentration in the cloud at the rate of roughly the square of time.” The
peak concentration et 5 hours would thorefore be reduced by a factor or
25 one day later. The rate of loss due to upteke of zis by the soil and