Fallout deposition m the Marshall Islands @ HL Beck er a and 1958 tests, however, deposited relatively little fallout 129 the Marshall Islands m the early 1960's was estimated to and, thus, any uncertamty mtroduced by these stmulations has very little 1mpact on our estimates of total fallout deposition be about 56 kBq m™* X 016 = 09 kBqm™ This 1s '7Cs ground deposition due to global fallout fallout '°’Cs deposited m Marshall Islands, we estimated Becausethefallout estimates of most radionuchdes were derived from measured activities of "Cs m soil, careful attention was pard to the separation of the ’Cs activity due to fallout durmg thefirst week orso after the test—when the radioactive cloud was passmg over the similar to the estimate of 07 kBq m~* denved by Whitcomb (2002) on the basis of a literature survey Usmg the estmate of 09 kBq m™~fortotal global the residual global fallout component m 1978 and 1994 when soil activity measurements were made For the purposesofour calculations, we have assumedthatall of the global fallout was deposited m 1961 Marshall Islands for the first time (so-called “local Effective decay rate of °’Cs in the Marshall Islands fallout after the radioactive debris from thattest or other tests had circled the globe once or more (producing “global fallout”) the upper horizons of soil m the Marshall Islandsat a rate higher than due to radioactive decay alone, as suggested fallout’”)}— compared to the ""Cs activity resultmg from The estimated concentrations of °’Cs m soil sam- ples collected m 1978 by Robison et al (1981, 1997) and m 1991-1993 by Simon and Graham (1997) had two sources (1) the local fallout due to tests conducted at Bikim and Enewetak, and (2) the global fallout due to nuclear weapons tests conducted monthsor years earlier im the Pacific as well in other sites of the northern hemisphere Theglobal fallout of 'Cs m the Marshall Islands was estimated im this work based on a comparison of the deposition of “Sr, measured m steel pots at Majuro, Enewetak, and Ponape Micronesia (alternate spellmg Pohnpe1) during 1960-1970 with that collected m NYC Weaccounted for the fact that '’Cs was lost from by Robison et al (2003) Failure to account for this weathermg effect resulted m previous estrmates of the deposition density at the time of fallout, based on contemporary measurementsof '*’Cs mvyentory assummg only stmple radioactive decay, bemg sigmficantly underestimated (by as much as a factor of three to four for somesouthern atolls) Contemporary '*’Cs to “Srratios m soils collected by Robison et al (1981) in 1978, as well as a comparison of '*’Cs msouls ofatolls sampled by both Robison et al (1981) m 1978 and Simon and Graham (1997) m 1991-1993, confirm the Robison et al (2003) findmgthat '*7Cs 1s lost from the soil profile to mventory from global fallout m NYC In NYC, 26 kBq ground water m the Marshall Islands Because the soils of the coral atolls have virtually no clay content, and because the soil is pure CaCO, below the soil horizon where the orgamic matter 1s located (at most about 50 compared to 0.43 kBq m~“at Mauroforthe sameperiod, a ratio of 17% Similar “Sr deposition ratios for the same horizons durmg the same period (Larsen 1983), and the total °’Cs m™~ of “Sr was deposited between 1960 and 1970, period are obtamed for Enewetak (15%), Kosrae Micronesia (11%) and Ponape Micronesia (15%), even though Mayuro has a much different annual ramfall, about 340 cm compared to 200 cm for Enewetak and 500 cm for Ponape Thus, it appears thatall atolls of the Marshall Islands received on average about 16% as much global fallout as did NYC The similarity of the ratios for locations at different lattudes and with different precipitation levels agrees with the findmgs of Simon and Graham (1997) who noted that the decrease m global fallout deposition with decreasing latitude (UNSCEAR 2000) 1s offset in the Marshall Islands by an mcrease in deposition at lower latitudes because of their greater annual ramfall rates The mventory of "Cs from global fallout m NYC [inferred from measurements of “Sr through 1981 re- ported by Larsen (1983)] was about 5 6 kKBg m~? m the early 1960’s at the end of the period of heavy fallout Hence, the global fallout deposition density of Cs m cm), there 1s a contmual downward migration of ¥’Cs due to the low bmdmg capacity of the upper soil Our estimates of effective half-lives, taking imto account the combimation of the environmental loss and radioactive decay, are given m Table 2 forall atolls of the Marshall Islands, ordered by increasmg average amual precipitation The estimated half-hves for environmental loss ranged from about 15-60 y correspondmg to effective half-hves rangmg from 10 to 20 y Our estimated environmental loss rates are somewhat Jess than those estimated for Bikim and Enewetak by Robison et al (2003) However, the calculations of Robison et al (2003) did not account for fractionation At relatively close-in distances to ground zero for a near surface explosion, fractionation can be very high and thus the ratio of ’Cs to Sr m fallout 1s much lower than the ratio expected based on the atom ratio of these nuchdes from nuclearfission (see next section) When corrected for fractionation, the effective half-life at Bikim, reported by Robison et al (2003), would thus probably be closer to 12 y rather than the 8-10 y they reported Although an