Radiometric interpretation of geologic data and its effect on groundwater in Abuja, North-Central Nigeria

Abstract

The purpose of this study is to evaluate the quality of groundwater in different locations for water consumption at Dei-Dei, Kubwa, Gosa and Lugbe area of Abuja, North-Central Nigeria. Vertical electric sounding and shuttle radar topography mission was used to determine the depth of groundwater bearing formation and map lineaments structures underlying the area. Boreholes with the geophysical log data were drilled and rock samples in each layer lithologically were collected for ?-ray analysis. The activity concentrations of 238U, 232Th, and 40K from the borehole rock samples were determined using high-purity germanium ?-detector. The activity concentration of radionuclides in Dei-Dei borehole has a mean value of 30.1 ± 2.9 Bq kg?1 for 238U; 67.2 ± 5.2 Bq kg?1 for 232Th, and 832.3 ± 105.0 Bq kg?1 for 40K. Kubwa borehole has a mean value of 34.4 ± 3.2 Bq kg?1 for 238U; 60.5 ± 5.4 Bq kg?1 for 232Th and 573.1± 72.0 Bq kg?1 for 40K. At Gosa borehole, 238U has a mean value of 26.1 ± 2.5 Bq kg?1, 62.8 ± 4.8 Bq kg?1 for 232Th and 573.3 ± 73.0 Bq kg?1 for 40K. At Lugbe borehole 238U has a mean value of 20.0 ± 2.0 Bq kg?1, 46.8 ± 4.9 Bq kg?1 for 232Th and 915.2 ± 116.1 Bq kg?1for 40K . Significantly higher concentration of 238U and 232Th occurred in samples collected from Dei-Dei borehole was attributed to granitic intrusions produced by denudation and tectonism. Inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the concentrations of 238U and toxic elements (i.e. Pb, Cr, Cd, Zn, Ni, As and Mg) in water samples collected from the boreholes and public water supply in the study area. The activity concentration of 238U in groundwater-based drinking was noted higher at Lugbe borehole with a value of 2736 µBq L?1 when compared with other boreholes. In the study area, the inhabitants permanently used water from the boreholes for daily drinking and household requirements. The annual effective dose was estimated to be in the range from 1.46 x 10-5 to 9.03 x 10?5 mSv yr?1 for boreholes with the highest value noted in Lugbe borehole with a value of 9.03 x 10?5 mSv yr?1. The group receives about 5.55 x 10?5 mSv of the annual collective effective dose in the study area due to 238U in drinking water. The highest radiological risks for cancer mortality and morbidity were found to be low, with highest values of 1.03 × 10?7 and 1.57 × 10?7 obtained from Lugbe borehole. The chemical toxicity risk of 238U in drinking water over a life time consumption has a mean value of 4.0 x 10?3 µg kg?1 day?1 with highest value of 6.0 x 10?3 µg kg?1 day?1 obtained from Dei-Dei and Lugbe boreholes. The elemental concentration of Pb was noted to be higher than the recommended permissible limit at Lugbe borehole and Public Nigeria Water Board with values of 0.014 and 0.012 mg L?1 , respectively. Other results obtained were below the recommended acceptable level by World Health Organization and United State Environmental Protection Agency. Results of the measurements could be of importance in radio-epidemiological assessment, diagnosis and prognosis of uranium induced cancer in the population of the inhabitants of Abuja.