Determination of natural (Ra-226, Pb-210, Ra-228, K-40) and artificial (Cs-137) radionuclides in surface sediments of the Cananéia-Iguape Estuarine-Lagunar Complex

The Cananéia-Iguape Estuarine-Lagunar Complex, located in the south of the State of São Paulo, Brazil, is a protected area recognized by UNESCO as part of the Biosphere Reserve, due to its importance as a natural ecosystem. This region underwent important changes due to the opening of the Valo Grande Channel, connecting the Ribeira de Iguape river to the Complex, causing contaminants to enter the ecosystem. In order to deepen the studies of this ecosystem, a Research Project entitled 'Environmental study of coastal systems exposed to important anthropogenic actions in the southeast region by means of biogeochemical tools' was approved. This project aimed to use biogeochemical tools in the study of coastal systems stressed by anthropic influences such as land use for urbanization purposes, agriculture, industrial activities, renovation/installation of port structures and coastal shelf exploitation, keeping in mind, the recommendations given for the preservation and management necessary in the coastal areas. Within the scope of this project, the present work aimed to determine the concentration of the natural radionuclides Ra-226, Pb-210, Ra-228, K-40 and artificial Cs-137 in 34 samples of surface sediments collected in the Cananéia-Iguape Estuarine-Lagunar Complex. The samples were measured by gamma spectrometry using a Germanium Hyperpure - HPGe detector. The mean concentration values obtained in the surface sediments for Ra-226 were 14.2 ± 8.7 Bq kg-^1; for Pb-210 was 31.1 ± 30.4 Bq kg^-1; for Ra-228 of 19.2 ± 14.9 Bq kg^-1 and for the K-40 318 ± 119 Bq kg^-1. The results indicated that there is no anthropogenic contamination and that these values can be considered as the base line of the region. However, the results obtained for the relative standard deviation presented high values, indicating a large dispersion of the results around the mean value. This large dispersion is mainly due to the physical and chemical characteristics of the ecosystems where the sediments were sampled. The Principal Component Analysis obtained from the Pearson correlation matrix showed that in most of the sampling points, the radionuclides are predominantly present in the clay + silt portion and aggregated to the organic matter. Within this set of sampling points, it was possible to visualize three outliers, the sampling point MY3 that presented a more positive correlation with the Ra-226 and Ra-228 concentrations and the Pedra do Tombo and R6 sampling points that presented the highest correlation with the clay + silt and organic matter fraction. The other set of sampling points presented a positive correlation with sand, which indicates a hydrodynamic pattern strongly influenced by the currents generated by the tidal oscillations and by the salinity of the water in these locations. The highest concentration of radionuclides found in the Pedra do Tombo sampling point is due to the fact that this location presents high levels of pelitic sediments, alternation of energy flow, with changes in the direction and intensity of the water flow, allowing the deposition of fine sediments. Moreover, is this location the channel presents a narrowing, which hinders the flow of the fine fraction that ends up accumulating, facilitating the deposition of radionuclides. The results for Cs-137 ranged from <0.5 to 3.2 Bq kg^-1. From the results obtained for the concentration of Cs-137 in the 34 samples analyzed, 93% are below the minimum detectable concentration. The statistical analysis of the results obtained for the Cs-137, using the software ProUCL, allowed to evaluate the average concentration of the sampled data set. The value obtained for the third quartile (0.8 Bq kg^-1) can be used as the recommended value most representative of the background radiation of the region for Cs. (author)