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Mobility of radioisotopes in marine surface sediments

December 31, 1993

Transport of a radioisotope in a sediment-water system can be retarded by sorption of the isotope to solid; which is controlled by the affinity of the radioisotope for the sediment particles. In order to study trace metal and ra- dionuclide mobility on the sea floor, the following measurements were carried out: (1)effective diffusion rates in sediments in the laboratory and on the sea floor, (2) laboratory distribution ratio (Kd) values, which measure the affinity of an element for the solid phase, and (3) field Kd values. Effective dffusion rates and Kd values for the radioisotopes 134Cs125Sb, 203Hg, 133Ba, 7Be, 65Z, 54M, 60Co, 59Fe, and 113Sn were measured for si= marine sediments: carbonate ooze, siliceous clay, red clay, metalliferous sediment, hemipelagic sediment, and terrigenous clay. The terrigenous clays analyzed are similar in composition and texture to sediments from sites of present and potential radioactive waste contamination in the arctic.

Based on the effective diffusion coefficients (Ds) determined in the laboratory, the isotope mobility fell into five groups. Diffusive transport rates fall into similar mobility groups for all sediment types tested both in the laboratory and on the sea floor. Interaction of the radionuclide with solids was the primary mechanism for reducing its mobility in the sediment-water system. As it is difficult to measure in situ diffusion rates directly, this suggests that Kd values based on measured sediment properties can be used to help determine in situ diffusion rates for radioactive waste near the sediment-water interface for other locations, such as the Arctic. The relative contributions of diffusion, bioturbation, irrigation, and sediment transport to radionuclide transport can then be assessed and used to predict the interactions and fate of the radioactive waste in the sedimentary environment.