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Radiocesium dynamics in irrigation ponds in Okuma, Japan




Byrnes, Ian, author
Johnson, Thomas, advisor
Brandl, Alexander, committee member
Borch, Thomas, committee member

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The objective of this research is to examine the variability in the vertical migration of 137Cs in ponds located in the 10 km deposition zone surrounding the Fukushima Dai-ichi nuclear accident site in Fukushima Prefecture, Japan. Four ponds, utilized for the irrigation of local rice production, were selected for sampling. Three types of samples were collected from each pond. Sediment cores were extracted to a minimum depth of 30 cm. Grab samples of the top sediment layer were taken at each core location. Finally, water samples were obtained to determine hydrochemical properties and radiocesium concentration suspended in the water. In the laboratory, the sediment cores were cut into sections and dried. Once the water was removed, the sediments sections were counted on a HPGe detector. Counting data results were used to construct a profile of the 137Cs activity concentration by depth. Water samples, used to determine the solid-liquid distribution coefficient Kd, were filtered through a 0.45 micrometer filter and analyzed to determine the hydrochemical composition of the pond water as well as counted for 137Cs activity concentrations. The grab samples were dried and packaged for HPGe counting and sequential extractions. Sequential extractions were used to determine the ratio of radiocesium that is bound to the sediments versus available to transfer through the environment. Two extractions were completed on the grab samples; an ammonium acetate extraction and an extraction with hydrogen peroxide solution (35%). The results show that 137Cs is very mobile in the top sediment layers. The highest concentrations of 137Cs in the cores is not limited to the upper sediment layers suggesting some combination of a higher mobilization of sediments in the ponds and post extraction mixing within the core resulting from the extraction technique. Similarly, the Kd values found are similar to those found in rivers in the affected areas of Fukushima. Collectively, the results suggest the ponds are open water bodies with the capacity to exchange 137Cs with local agriculture.


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