Cubatão River is located in Santos Basin, São Paulo State, Brazil. This region is characterized by the occurrence of estuaries
and mangrove. Due to its location, near the coastal line, it is also an important industrial area, where phosphate fertilizer
plants, petrol refineries, and chemical and steel industries are present. Such human activities contribute to the enhancement
of elemental composition in sediments and, in some cases, also increase the radionuclide concentrations, the so called Technologically
Enhanced Natural Occurring Radioactive Materials (TENORM). The contamination of land and sediments by TENORM is of major concern.
The activity concentration of U and Th series radionuclides was determined in five sediment samples from Cubatão River. The
activity concentration ratio was also determined. Equilibrium was observed for the ratio 234U/238U. The activity ratios of Th/238U, 228Ra/226Ra and 210Pb/226Ra were higher than the unity. In the first two cases, the observed values are due to the higher activity of Th in the sediment
and in the last case are probably due to the atmospheric deposition of 210Pb.