SedimentOguta Lake Excess lifetime cancer risk a b s t r a c tThe distributions of naturally occurring radionuclides 226 Ra, 232 Th and 40 K in sediments of Oguta Lake, Nigeria were determined using gamma ray spectrometry in order to assess the radiological health hazards and excess lifetime cancer risks associated with the use of the sediments. The mean activity concentrations of 226 Ra, 232 Th and 40 K were found to be 47.89 ± 18.67 Bq kg À1 , 55.37 ± 32.74 Bq kg À1 and 1023 ± 474 Bq kg À1 , respectively. The results of the radiological indices and dose rates obtained in this study were all higher than their worldwide mean values but lower than their maximum recommended limits indicating that the use of the sediments as building materials do not constitute any excessive radiological hazards. The area is known to be subjected to environmental degradations due to oil exploration. Therefore, the results of this study could serve as an important radiometric baseline data upon which future epidemiological studies and environmental monitoring initiatives could be based.

Radioactivity measurements have been carried out using gamma-ray spectroscopy to determine the radionuclide concentrations in soil samples in the premises of the Warri Refining and Petrochemical Company located in the Niger Delta region of Nigeria and also in communities around it. The radionuclide contents of the crude oil and petroleum additives were also similarly determined. Results indicate that on the premises used by the company, the ranges of activity concentrations of (40)K, (226)Ra and (228)Th in the soil samples were 261.3-932.3 Bq kg(-1),<4.2-23.0 Bq kg(-1) and<5.1-10.2 Bq kg(-1), respectively. For the 13 villages studied, activity concentrations ranged from <17.2-766.3 Bq kg(-1),<4.2-104.7 Bq kg(-1) and<5.1-13.7 Bq kg(-1), respectively, for (40)K, (226)Ra and (228)Th. The lowest radionuclide concentrations were found in the crude oil relative to the petroleum additives. The highest outdoor effective dose rate obtained in the study was 47.5 microSv y(-1) for the villages and 35.2 microSv y(-1) within the company premises. These values are less than the world average outdoor value of 70 microSv y(-1) given by UNSCEAR. Overall, it can be concluded that the long duration of refining activities does not seem to have affected radionuclide concentrations in the environment. Measurements also showed that the observed radiation dose rates did not vary significantly from the previously determined Warri City outdoor gamma radiation dose rates, for which the probability of occurrence of any health effects of radiation was judged low. The measurements, therefore, are interpreted to represent baseline radiometric data for future reference and research in the area.

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This work was carried out in collaboration between all authors. Author H U EMELUEcollected the samples from the communities, packaged them and took them to the laboratory. He also designed the study, performed the statistical analysis, wrote the protocol, and the first draft of the manuscript. 'Author B C EKE proofread the manuscript and managed the literature search while N N JIBIRI supervised the research work, provided answers to the technical questions and updated the references. All the authors have read and approved the final manuscript.

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