1,000-micron range, and 92.5 percent was associated with the 0.5-to-10-micron particulate
matter.
Subsequent to Operation Redwing, surface-water samples were collected off Site Elmer in
Eniwetok Lagoon on 1 September 1956.
An analysis of this water on 1 June 1957 indicated 187
dis/min Ce'4, 160 dis/min Ru'®, 52 dis/min Cs’, less than 15 dis/min Mn*4, less than 30
dis/min Zn*5, and 80 y/min of K*per liter of sea water.
During the interval between 28 November and 11 December 1956, a series of cores and
bottom samples were obtained in the vicinity of 11° 18’N, 162°57’E. An analysis of the water
obtained from over the surface of 12 cores indicated the presence of 105 dis/min of the rare
earths, 26 dis/min of the alkali earths, 4 dis/min of the alkali metals, 90 dis/min of Ru‘,
7.5 dis/min Zr, 12 dis/min Zn**, and 690 dis/min K“ per liter of bottom water on 20 July 1957
Most of this contamination was associated with the finely divided sedimentary matter stirred up .
—_——_—
from the disturbed surface of the cores.
In August 1956, the M/V Horizon undertook an equatorial Pacific expedition (Equapac), dur}
which water and plankton were collected and assayed for radioactivity. Along the entire track
from 11°N to 11°S at 164°E, and 5°S to 11°N at 157°E, radioactivity was detected in the plankton,
Fission-product radioactivity was detected in the surface water and at 500 meters’ depth between
3° and 11°N,
Only a trace amount of activity was detected south of the equator.
The concentra.
tion of Ce“, Rul) Rul’ Zr98) Mn54 Zn®, Cod’, Co®, Co® isotopes in 1 gram of plankton
(drained wet weight) over the activity of 1 gram of sea water ranged from 2 x 10° to 150 x 10°,
with the average concentration factor of 2 x 10‘ for 28 Equapac stations. The radioactivity of
gross zooplankton offers a more easily measurable index of oceanic contamination than the timeconsuming analysis of sea water. Figure 5.5 presents the activity per wet gram of zooplankton,
or the approximate activity per 20 liters of surface water.
5.6.3 Suspended Particulate Matter. Filtration of sea-water samples through 0.5 micron
Millipore filters removed from 25 to 75 percent of the radioactivity. Duplicate samples from
the same Station varied by a factor of two. Figure 5.6 gives the gamma spectra of the soluble
and particulate fractions of a Tewa surface water sample.
In general, the activity associated
with the particulate matter was greater in the surface water than in the deeper water.
Radio-
autographs of the suspended surface matter showed inhomogeneous “hot spots” corresponding
to the larger diatoms, dinoflagellates, and radiolaria.
Scavenging the sea water from all depths with Fe(OH), carrier removed from 60 to 75 percent
of the activity.
Zirconium, manganese, and the rare earths were almost quantitatively removed.
The percentage of ruthenium recovered varied greatly from sample to sample.
Natural K* and
the long-lived cesium and strontium isotopes are not recovered byfiltration.
Filtration through a series of graded SS ultra filters at 1,000 psi showed that 25 to 50 percent
of the activity was retained with particulate matter greater than 0.5 microns. Several percent
of the radioactivity was retained in each size range down to 0.01 microns. Zr*®, Ru?and Ce!#
were identified in the 3-to-0.75-micron range. Ruthenium and cerium were identified in the
0.75-to-0.5-micron region. Ruthenium and manganese were identified on all the remaining fil-
ters between 0.5 and 0.01 microns.
Table 5.6 shows a typical distribution of radioactivity with particle size between 3 and 0.01
microns for a Navajo sample of surface water and 50-meter water. Filtration through a series
of three 0.5 micron Millipore filters retained 33 percent of the activity on the top filter, 0.03
percent on the middle filter, and 0.03 percent on the bottom filter. At H+75 hours the top filter
exhibited a 4-day half life, whereas the middle and bottom filters showed about a 95-day half
life, with Mn*‘ being the most prominent isotope.
5.6.4 Lagoon Sediments.
Prior to Shot Cherokee, 21 May 1956, the predominant isotopes
detected in the Bikini Lagoon sediments were Ce’ ranging from 1 x 10° (dis/min)/gm in the
Southeastern section to 40 x 10° (dis/min)/gm in the northwestern area, and Ru‘contributing
,
about a fourth as much activity as the cerium. Traces of Sb’, Mn, and Zn® were also found
in the sediments.
In several bottom samples obtained near Sites Charlie and Oboe, Sr*”, Cs'*',
116