Baseline, mapping and dose estimation of natural radioactivity in soils of the Brazilian state of Alagoas

Soils consist of the result of several formation factors, such as the material of origin, climate, relief, living organisms etc. The natural radioactive elements K-40, Ra-226 and Ra-228 are present in the mineral material as well as in the organic matters, which make up the soil and account for approximately 86% of the dose of radioactivity in humans. While several countries have conducted radiometric surveys in their territories, in Brazil, studies on radioactivity mapping in soil are scarce and mostly carried out in areas of high natural radioactivity, primarily in the Southeast region. In order to provide useful information for geological and epidemiological studies, and knowing the distribution of such elements in soils in states of other regions, it is of utmost importance, since some of them present areas with radioactive anomalies. Such a measure also contributes to building a national database to be used, among other purposes, to evaluate changes in radionuclide concentrations in soils due to human activities. This study aimed at knowing the occurrence of radionuclides K-40, Ra-226 and Ra-228 in soils in the State of Alagoas, located in the Northeast region of Brazil. A total of 41 samples were collected in the first 25 cm of soil, in order to represent the entire state territory. Samples of the aforementioned radionuclides were processed and analyzed by gamma spectrometry. Activity concentration ranged from (6 +/- 1) Bq kg(-1) to (2160 +/- 81) Bq kg(-1) for K-40 (average of 630 +/- 25 Bq kg(-1)), from (5.0 +/- 0.4) Bq kg(-1) to (63 +/- 2) Bq kg(-1) for Ra-226 (average of 28 +/- 1 Bq kg(-1)) and from (9.5 +/- 0.5) Bq kg(-1) to (157 +/- 5) Bq kg(-1) for Ra-228 (average of 54 +/- 2 Bq kg(-1)). While the average concentration for Ra-226 agrees with the worldwide median of 35 Bq kg(-1), the average values found for Ra-228 and K-40 were above the worldwide median (30 Bq kg(-1) and 400 Bq kg(-1), respectively). The maximum concentration of K-40 found in the backlands in the state of Alagoas (2160 Bq kg(-1)) is compatible with its highest concentrations worldwide. In contrast, the coastal region and the interior of the state showed a significant difference between them regarding the concentrations of the radionuclides analyzed, probably caused by climatic and geological differences. Radiological maps were developed to show the spatial distribution of the studied radionuclide concentrations.