Spatial Distribution of Fallout and Lithogenic Radionuclides Controlled by Soil Carbon and Water Erosion in an Agroforestry South-Pyrenean Catchment

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Primordial radionuclides originate in the geosphere, and anthropogenic radionuclides are increasingly entering the geosphere because of human disposal practices. Mining and milling of uranium ore is one of the largest intrusions into consolidated rock with regard to radioactivity. Although uranium solubilized and was transported to specific deposit locations during the genesis of rock formation, it tends to be relatively immobile once deposited. However, groundwater does flow through the consolidated geosphere in fractures in hard igneous and metamorphic rocks, or through a more general percolation within more permeable sedimentary formations. This groundwater movement can solubilize radionuclides from underground deposits, depending on the chemistry of the groundwater and chemical interactions with specific radionuclides. If underground deposits become soluble, radionuclides are mostly retained in the rock because of chemical exchange along the water transport path. Inert gaseous radionuclides (e.g., 222Rn) move more freely, but transport from deep underground to the surface takes years to millions of years, and radioactive decay often allows only small amounts of radionuclides to reach the surface. The nature of the underground deposit is important. For example, surface concentrations of uranium do not reflect a 1.3-billion-yr-old uraninite deposit 450 m below the surface in northern Canada. With very slow transport of radionuclides from deep underground, surface weathering of rock is the greatest source of radionuclides to the biosphere, hydrosphere, and atmosphere.

Radionuclides may be present in the environment either because they are naturally occurring (e.g. radionuclides of primordial origin and their decay products together with other radionuclides of cosmogenic origin) or because they are artificially produced (e.g. those released from the nuclear fuel cycle). However, in addition to generating artificially produced radionuclides, some human activities can result in enhanced concentrations of naturally occurring radionuclides in environmental media. For example, naturally occurring 226Ra and its progeny are concentrated in scales present in pipework and other equipment used by the oil and gas industries. In this chapter, emphasis is placed on biosphere pathways and modelling relevant to artificially produced radionuclides, but many of the techniques discussed and data reviewed are also relevant to naturally occurring radionuclides.

With Regards,
Sara Giselle
Associate Managing Editor
Journal of Medical Physics and Applied Sciences