Anomalous concentrations of radionuclides in the groundwater of Ede area, southwestern Nigeria: a direct impact of geology

摘要

An assessment of the groundwater in areas underlain by pegmatite in Ede, southwestern Nigeria was carried out to determine the concentration of Potassium-40 (K-40), Uranium-238 (U-238) and Thorium-232 (Th-232) radionuclides. In the earlier work, it was established that zircons in these pegmatites have suffered a high degree of metamictization that has enhanced continuous loss of some radionuclides since the time of emplacement of the pegmatite host rock to the present. The aim of this work is to determine whether or not there is corresponding increase in the concentration of radionuclides in groundwater in the study area. Fifteen groundwater samples were collected from both hand dug wells and boreholes in the area. Ten samples were from Ede town, two samples each from nearby communities of Iddo and Ekuro and one from Iwoye, where the bedrocks were not pegmatites. All the 15 samples were analyzed with the Sodium Iodide scintillator (NaI [Tl]) detector at the Centre for Energy Research and Development, Obafemi Awolowo University, Ile-Ife, Nigeria. The results showed varying concentrations of the radionuclides in the water samples. Activity concentrations of Potassium-40 (K-40) indicated an average value of 17.149Bq/L for samples from Ede, 9.265Bq/L for Iddo, 6.6Bq/L for Ekuro, while Iwoye has a value of 21.21Bq/L. The Uranium-238 (U-238) series had an average value of 13.64Bq/L for Ede, 13.49Bq/L for Ekuro, 11.685Bq/L for Iddo and 12.04Bq/L for the lone sample from Iwoye. Thorium-232 (Th-232) series had an average value of 11.182Bq/L for Ede, while average values of 7.79Bq/L and 9.025Bq/L, respectively, were recorded for Iddo and Ekuro and 12.25Bq/L for Iwoye. The annual effective dose level of K-40 is generally below the World Health Organization (WHO) recommended dosage of 0.1mSv/y except for three locations, while those of U-238 and Th-232 are in excess of the standard values. The high radionuclides in the groundwater in the study area were not due to anthropogenic sources but directly due to geological processes which release the radioisotopes from rocks after weathering, continuous loss from metamict minerals and natural lateral mobility from regions of radionuclides' release to other areas. There is thus the need to carry out systematic studies of the radionuclides concentrations on regional scale in the area which could lead to investigating on both short- and long-term health effects on organic species in these areas, and development of purification systems before utilizing water from the areas for domestic and industrial purposes.