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 reasons 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 125 estimateof the average mventory at undisturbed sites, some, but not all, of the underestimation due to losses from the expected deposition was compensated for, and 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 MATERIALS AND METHODS 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 Overview 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