Fallout deposition m the Marshall Islands @ HL Beck er a
In a 2004 report to Congress (DCEG 2004), stmplistic estimates of deposition across the Marshall Islands
were made and used to estrmate the maximum doses to
people living on the inhabited atolls In this work, the
estimates of fallout that were used im the 2004 report
have been re-analyzed and mproved In the 2004 report

to Congress,thetotal activity of '"’Cs deposited per umt

area of ground on eachatoll of the Marshall Islands was
imferred from soil samples collected m 1991-1994 by
Simon and Graham (1997) The calculations assumed

that all of the total ‘Cs present after correctmg for

global fallout was deposited in 1954 as the result of the
Bravo test The Bravo deposition was then estimated by
subtractmg an estimated contribution from “global fall-

out” from the maximum '°7Cs measured m sor] sampled

m 1991-1994 at each atoll and then decay-correcong
back to the time of deposition Theactivity ratio of other
radionuclides was then estrmated from published values
of activity ratios for Bravo debris (Hicks 1982) and the
estimated fallout transit trme (h), called trme-of-arrival
(TOA), for Bravo fallout to reach eachatoll

The methodology used m 2004 for derrvmg deposition estimates was not completely realistic for several


1 The estimation of '*?Cs m Marshall Islands sols at the

time of deposition was based on back-correcting
contemporary measurements only for radioactive decay and did not take mto account the contmualloss of

“Cs from the upper layers of soil due to downward


estimateof the average mventory at undisturbed sites,
some, but not all, of the underestimation due to losses

from the expected deposition was compensated for,
4 The assumption thatall fallout was a result of the
Bravo test biased some of the 2004 deposition estmates toward Igher values than were likely because
some tests had substantially longer fallout TOAs(as
great as 6—8 d compared to 5-40 h) Also, as shown
later im this paper, a substantial fraction ofthe fallout
m some of the northern atolls was from tests other
than Bravo, and m the most southern atolls most ofthe

fallout was from tests other than Bravo

Our new estimatesoffallout are much more detailed
as they take mto consideration all radionuchdes that
contributed substantially to either external or mternal
radiation exposure from each of the most rmportant
weapons tests conducted at Bikim and Enewetak The
present analysis also attempts to correct for all of the
potential sources of error noted above Based on available environmental measurement data, ground deposition

density for "Cs and 62 other radionuchdes, listed m

Simon et al (2010a, Table 4), have been estrmated for

each nuclear test that we believe deposited substantial

levels of fallout at each atoll or reef island All together,
fallout deposition has been estimated for 20 tests, listed
im Simon et al (2010a, Table 1), at 32 atolls or separate

reef islands The complete list of the 34 atolls and
separate reef islands of the Marshall Islands 1s given in

migration (Robison et al 2003) That gradual loss 1s
primarily a result of heavy tropical ramfal]l m the

Simon et al (2010a, Table 2), deposition estimates are

the absence of clay m the soil which might otherwise


Marshall Islands, which was more abundant m the
southern atolls than in the northerm atolls, as well as

retam '’Cs Therefore, the true amounts of "’Cs

deposited at the tmes ofthe tests were greater than
those derived for the 2004 report with largerrelative
correction made for the southern atolls compared to
the northern atolls because of higher armual precipitation there,

2 For the more southern atolls, global fallout “’Cs

comprises a substantial fraction (as much as 50%) of

the total measured Cs mventory In the 2004report
(DCEG 2004), the contribution ofglobalfallout '*’Cs,

which1s relatively constant over the Marshall Islands,

to the total measured '’Cs, was also overestrmated

The extent of the overestimation was such that negative values were derived for some southern atolls for
the local fallout,

3 Because the 2004 calculations were conservatively

based on the maximum '’Cs observed m soils collected at each atoll] m 1994-1996,ratherthan the best

given for all of them, with the exception of Brkim and
Enewetak, where the tests were conducted


Based on evaluations of several types of data, we

have estimated the deposition density (kBq m~*) of 63
imdividual radionuclides by atoll or island as well as by

nucleartest, plus the cumulatve *°*°Pu from all tests

The various types of data reviewed mcluded environ-

mental measurements of '’Cs and other radonuchdes

(both historical and contemporary), lustorical measurements of exposure rate followmg mdrvidual tests derived
from aerial surveys, ground surveys, and continuous
momitors, historical measurement data of beta activity

collected on gummed film during the years of nuclear
testng, and recent results from meteorological analyses
Findings from the analysis of these data were coupled
with mformation on the predicted mixture of radionuchdes from specific nuclear tests as a function of time
after detonation by Hicks (1981, 1984) to predict deposition densities as a function of fallout TOA

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