Effect of Geology on Soil Radioactivity and Risks to Humans Based on Data from Several Towns in Nigeria

Abstract

Gamma ray spectrometric technique has been used to determine activity concentrations of some primordial radionuclides in the naturally occurring uranium, actinium and thorium series and of non-series ⁴⁰K in soil samples collected from five major towns in Oyo State, Southwestern Nigeria. Activity concentration determination was done using a high purity germanium (HPGe) detector. It ranged from 85.83 ± 4.04 Bq kg⁻¹ to 1,850.60 ± 36.83 Bq kg⁻¹ for ⁴⁰K; 5.23 ± 0.34 Bq kg⁻¹ to 198.31 ± 4.10 Bq kg⁻¹ for ²⁰⁸Tl (indicator for ²³²Th) and 7.82 ± 0.42 Bq kg⁻¹ to 64.65 ± 2.00 Bq kg⁻¹ for ²¹⁴Bi (indicator for ²³⁸U). The outdoor absorbed dose rate in air due to each of the primordial radionuclides was calculated from the mean values of the activity concentration in each sample. Values ranged from 3.60 ± 0.17 nGy h⁻¹ to 77.73 ± 1.55 nGy h⁻¹ for ⁴⁰K, 3.48 ± 0.23 nGy h⁻¹ to 132.07 ± 2.73 nGy h⁻¹ for ²⁰⁸Tl and 3.35 ± 0.21 nGy h⁻¹ to 27.73 ± 0.86 nGy h⁻¹ for ²¹⁴Bi. The absorbed dose rate in air was the highest for rocky areas, while the lowest value came from non-rocky, non-industrialized areas. The estimated annual outdoor effective dose equivalents for the two rocky areas were 385.76 μSv y⁻¹ and 221.28 μSv y⁻¹. The estimated annual outdoor effective dose equivalents for the two industrialized areas were 64.05 μSv y⁻¹ and 47.65 μSv y⁻¹. The value for non-rocky, non-industrialized areas was 19.19 μSv y⁻¹. The average value for the study area was 147.59 μSv y⁻¹. The estimated collective effective dose equivalent for the area was 9.77 × 10² man-Sv y⁻¹. The expected number of people in the study area at risk of incurring cancer per year as a result of radioactivity in the soil was estimated to be 11; this value represents approximately 7% of the estimated 160 people for 18 Nigerian cities, approximately 69% of 16 people estimated for oil producing areas of Nigeria and less than the United States Environmental Protection Agency regulatory limit of 1 cancer case to 10,000 people. Also, this result shows that local variability of natural radionuclides in soil depends on local geology of the area. The incidence number was 2.4 people per million.