The Northern Marshall Islands Radiological Survey

Abstract

Fallout from atmospheric nuclear tests, especially from those conducted at the Pacific Proving Grounds between 1946 and 1958, contaminated areas of the Northern Marshall Islands. A radiological survey at some Northern Marshall Islands was conducted from September through November 1978 to evaluate the extent of residual radioactive contamination. The atolls included in the Northern Marshall Islands Radiological Survey (NMIRS) were Likiep, Ailuk, Utirik, Wotho, Ujelang, Taka, Rongelap, Rongerik, Bikar, Ailinginae, and Mejit and Jemo Islands. The original test sites, Bikini and Enewetak Atolls, were also visited on the survey. An aerial survey was conducted to determine the external gamma exposure rate. Terrestrial (soil, food crops, animals, and native vegetation), cistern and well water samples, and marine (sediment, seawater, fish and clams) samples were collected to evaluate radionuclide concentrations in the atoll environment. Samples were processed and analyzed for 137Cs, 90Sr, 239+240Pu and 241Am. The dose from the ingestion pathway was calculated using the radionuclide concentration data and a diet model for local food, marine, and water consumption. The ingestion pathway contributes 70% to 90% of the estimated dose. Approximately 95% of the dose is from 137Cs. 90Sr is the second most significant radionuclide via ingestion. External gamma exposure from 137Cs accounts for about 10% to 30% of the dose. 239+240Pu and 241Am are the major contributors to dose via the inhalation pathway; however, inhalation accounts for only about 1% of the total estimated dose, based on surface soil levels and resuspension studies. All doses are computed for concentrations decay corrected to 1996. The maximum annual effective dose from manmade radionuclides at these atolls ranges from .02 mSv y(-1) to 2.1 mSv y(-1). The background dose in the Marshall Islands is estimated to be 2.4 mSv y(-1). The combined dose from both background and bomb related radionuclides ranges from slightly over 2.4 mSv y(-1) to 4.5 mSv y(-1). The 50-y integral dose ranges from 0.5 to 65 mSv.

Radionuclides in sediments and seawater at Rongelap Atoll

“…Gradients decrease in a southerly direction to regions near the South Pass where 241 Am is below detection in the surface sediments. The different isopleths roughly coincide with the estimated fallout path of differently contaminated debris from the 1954 Bravo event (Robison et al, 1997). There is approximately a factor of 10 difference in concentration associated with surface sediments from the north compared to samples from the southern part of the atoll.…”

“…During the past 20 years, radiological surveys have focused on documenting levels of specific radionuclides in local foods, drinking water, and soils from islands of the atoll (Noshkin et al, 1981a;Robison et al, 1994;Robison and Conrado, 1996a,b;Robison et al, 1997), and in fish and invertebrates from the lagoon (Robison et al, 1981;Noshkin et al, 1981b). These surveys were undertaken to generate data to assess any potential radiological dose to present day and future inhabitants from external exposure, inhalation, and ingestion pathways.…”

“…During the 1978 Northern Marshall Island Survey (Robison et al, 1997), a few surface sediment and seawater samples were obtained for analysis from shallow depths on the lagoon reef to provide preliminary information on types and concentrations of radionuclides associated with the seawater and sediments (Noshkin et al, 1987). A more detailed sampling program was undertaken at Rongelap in 1981 to better define the distribution of persistent gamma-emitting radionuclides in the surface sediments.…”

Accelerator Mass Spectrometry Measurements of Plutonium in Sediment and Seawater from the Marshall Islands

“…This was also estimated for the gamma dose rates on the northern and southern islands one day after the Bravo test . Regarding soils and vegetation samples, concentrations on the southern islands of Rongelap are about a factor of five lower than those on the northern islands of the atoll (Robison et al, 1997).…”

The Marshall Islands Data Management Program

“…In 1978 a radiological survey was conducted at the atolls of Taka, Bikar, Rongerik, Ailinginae, Likiep, Ailuk, Wotho and Ujelang and the two islands of Mejit and Jemo where preliminary dose assessments were done. (Robison et al, 1982a andNoshkin 1981).…”