counting of evaporated water samples was used for comparative evaluation of total activity and
of chemical group activities.
The average values of four surface water samples collected along the Horizon track 28 May
to 1 June 1956 and analyzed 1 June 1957, assayed 8,300 (dis/min)/liter Ce!*, 1,250 (dis/min)/
liter Ru!*, none of the Cs'3”, less than 400 (dis/min)/liter Ma®‘, and a trace of Zn®*. A depth
of 80 meters at the samestations gave 5,000 (dis/min)/liter Ce‘, 2,250 (dis/min\/liter Rul,
500 (dis/min)/liter of Cs'**, 300 (dis/min)/liter Mn*, and about 200 (dis/min)/liter of Zn*®,
At 80 meters’ depth, 33 percent of cerium, 8 percent of ruthenium, 45 percent of manganese,
and 100 percent of the zinc were removed from the water by filtration through a 0.5 microa
Millipore filter.
No significant variation of activity or isotopic concentration was observed with
depth above the thermocline. The activity below the thermocline was insufficient for radiochemical analysis.
Shot Flathead, 12 June 1956, produced widespread oceanic contamination presisting until the
next event in the area northwest of Bikini Atoll between 11° to 14°N and 163° to 166°E. Variations
a me eae caer
of surface water activity ranged from 10? to 3 x 10° relative counts per minute at 49 stations be-
tween 30 June and 7 July 1956. A typical surface water sample, collected 15 June 1956 at 12°
24'N, 164° 30’ E and analyzed 15 June 1957 showed the presence of 3,800 dis/min Ce'4*, about
600 dis/‘min Ru’? 2,560 dis/min Ru!*, 390 dis/min Cs", and 1,400 dis/min Zr” per liter of
sea water. Mn**, Sb! Zn®5 and Co® were not detected above the 100 to 200 (dis/min)/liter
lower limit of detectability forthis sample.
The vertical distribution of total activity northwest of Bikini Atoll at H+400 to H+600 hours
was constant or decreased to the thermocline, reached a minimum at 200 or 300 meters, and
increased to a maximum of one to four times the surface activity at a depth of 500 to 800 meters.
—_— oe ee
In general, the point of minimum radioactivity coincided with the salinity minimum. This maximum pool of deep water contamination coincides with the deep water area of contamination found
northwest of Bikini Atoll in April 1956 prior to Operation Redwing. Figure 3.4 shows the relative variation of radioactivity with depth at three stations between 12° and 13°N at 164° to 165°E.
These are compared to the average April 1956 values in the same general areas.
Shot Navajo, 11 July 1956, produced from 1 x 10° to 3 x 10’ (counts/min)/liter of surface
water along the Horizon track west of Bikini Atoll at H+2 and H+3 days.
Vertical water pro-
files indicated uniform contamination to the thermocline below which the gross activity increased
2 to 3 fold reaching a maximum between 500 and 800 meters. The deep water activity, exhibiting
the same order of magnitude as observed in a pre-Navajo survey and having a half life three to
five times greater than the 70-hour half life observed in the surface water at H+60 hours, probably originated prior to Shot Navajo, which might be expected also to contribute to the subthermocline contamination.
The analyses of water samples at various depths for three stations west of Bikini Atoll are
shown in Table 5.3.
The values are reported as of 1 July 1957.
A substantial portion, about 37
percent of the total gamma-emitting isotopes, are radioisotopes not ordinarily associated with
fission products, with Mn* contributing about 30 percent of the total activity.
Shot Tewa, 21 July 1956, produced approximately the same level of oceanic contamination
west of Bikini as the previous Navajo test. A sample of surface water collected 22 July 1956
at 12° 05’N, 165° 16’ E assayed 16 x 10° (y/min)/liter at H+54 hours. The sedimentfiltered
from the same volume of water through a 0.5 micron filter counted 4.8 x 10° (y/min)/liter at
H+54 hours. The analysis of this sample, presented in Table 5.4 shows the presence of Ce!4,
Ru} Rul® Cs!37 and Zr* after 10 months of decay. Mn* is present only to the extent of 3
percent of the total gamma emitters, as compared to 30 percent in the Navajo samples. The
Mn*4 may have originated in a previous test.
Bikini Lagoon water contamination between shots appeared to be largely associated with suspended particulate matter, with the greatest concentration of activity located near the floor of
the lagoon. An analysis of bottom water is shown in Table 5.5. Ona volume basis, 43 percent
of the activity of one bottom water sample collected on 29 May 1956 at 11° 30’ 02” E was associated with the suspended particulate matter in the 0.5-to-10-micron range. Ona weight basis,
0.2 percent of the activity was dissolved in the water, 7 percent was associated with the 10-to115