ANALYSES OF NATURAL RADIONUCLIDES IN GRANITIC ROCKS AND ASSESSMENT OF RADIOLOGICAL HEALTHHAZARDS AT SELECTED QUARRY SITES IN SOUTHWESTERN NIGERIA

Abstract

Quarry business is thriving in southwestern Nigeria because of the abundance of granitic rocks and the high demand for these rocks in building and construction industries. Granitic rocks contain Natural Radionuclides (NRs) (40K, 226Ra and 232Th) and when their concentrations exceed certain level, they pose health concerns to the workers and the environment. The possible health hazards include lung and skin diseases. Presently, there is scarcity of information on the activity concentration of the NRs in rocks with which to assess the radiological health hazards on quarry workers in Nigeria. The main problem being the cost and time for gamma spectrometric analysis. This work was designed to assess the radiological health hazards on the quarry workers and the possibility of estimating activity concentration of NRs from mineralogical data. A total of 213 granitic rock samples were collected from 71 quarry sites, due to non-accessibility of some quarries, in Oyo (36), Osun (21), Ekiti (27), Ondo (69) and Ogun (60) states. Each sample was divided into two for petrographic and gamma spectrometric analyses. Thin sections of the rock were prepared from which mineral composition was determined using a petrological microscope. A calibrated gamma spectrometer comprising 7.6 cm x 7.6 cm NaI(Tl) detector was used to measure the activity concentrations of NRs. The radiological hazard parameters: Radium Equivalent (Raeq), Dose Rate (DR), Annual Effective Dose (AED) and Excess Lifetime Cancer Risk (ELCR) to the quarry workers were estimated from the activity concentrations. To study the strength and usability of the relationships between mineral composition andactivity concentration in the rocks, data were analysed using descriptive statistics and regression analysis was carried out at α_0.05. The ranges of activity concentrations of 40K, 226Ra and 232Th were: 79.26-1187.38, 4.45-67.31and 25.72-271.03 Bqkg-1, respectively. The Raeq, DR, AED and ELCR were: 69.30-499.02 Bqkg-1, 31.38-223.55 nGyh-1, 0.05-0.39 mSvy-1 and 0.87-1.36, respectively. About 6 % of Raeq values exceeded the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) recommended limit of 370 Bqkg-1 for use of granitic rocks as building materials. The AED (0.05-0.39 mSvy-1) and ELCR (0.05 %) estimated were below the UNSCEAR recommended values of 3.0 mSv.y-1 and 0.1 %, respectively, indicating that the workers are less prone to radiological health hazards. The minerals in felsic and mafic parts of the granitic rocks were: Quartz 7.68-19.24%; Microcline 4.68-29.28%; Plagioclase 9.65-37.82%; Biotite 18.21-36.95%; Muscovite 0.81-5.14%; Hornblende 0.02-15.20%; Apatite 0.74-4.05% and Opaque 0.80-5.28%. Significant relationships (R) were observed between Microcline and 40K (R=0.92), Biotite and 226Ra (R=0.86); and Biotite and 232Th (R=0.87). The relationships A_K=38.34M_Mi-94.34, A_Ra=3.30M_B-58.50 and A_Th=13.02M_B-208.90 established could be used for the rapid evaluation of NRs of 40K (AK) in Microcline (MMi), 226Ra (ARa) in Biotite (MB) and 232Th (ATh) in Biotite (MB). The radiological hazard parameters of granitic rock samples from selected quarries in southwestern Nigeria were within internationally accepted safety limits for quarry workers.