Spatial trends in the distribution of natural radioisotopes in the bottom sediments of Santos Basin (Brazil)

Abstract

Activities related to the marine exploration of oil and gas reservoirs tend to cause a concentration of natural radionuclides in related materials. As such, knowledge regarding the distribution of radionuclides in sedimentary basins with current oil operations is essential for modeling radiological hazards and possible risks of site contamination. This study investigated the distribution of 226Ra and 228Ra, radioisotopes from the 232U and 232Th decay chains in bottom sediment of the Santos basin. Sediment samples were collected from sites in a sampling grid based on depth contours and were analyzed through high-resolution gamma spectrometry. A distribution model of the spatial variation of these isotopes, the first of its kind for the Basin, supported the interpretation of similar tendencies in their distribution. From studying the spatial trends of the mean levels of the isotopes with descriptive statistics and variance analysis, latitudinal and bathymetric differences in the content of radionuclides emerged. These differences are probably derived from the distinct sources of these elements, as 226 Ra originates in the deep ocean from the decay of parent230Th and from the patterns of sedimentation driven by open ocean circulation, while 228Ra is supplied by terrigenous materials transported by the northward-flowing Brazilian Coastal Current.