MEASUREMENT OF SOIL-GAS RADON CONCENTRATION AND GEOGENIC RADON POTENTIAL MODELLING FOR SOUTHWEST NIGERIA

Abstract

Radon-222 is a radioactive gas in the natural decay series of Uranium-238. It easily emanates from the soil to constitute radiological hazard and is the leading cause of lung cancer apart from smoking. High indoor radon buildup could occur in buildings sited over high radon-bearing bedrocks. Radon hazard, expressed as Geogenic Radon Potential (GRP), is due to a combination of soil-gas radon concentration ( ) and soil-air permeability ( ), both of which depend on bedrocks. Data on these two quantities over different bedrock formations and soil types in Southwest (SW) of Nigeria are very scarce resulting in limited knowledge on radon hazard and lack of requisite radon control guidelines. This study was designed to measure , determine GRP and model the distribution of GRP over different bedrocks of SW Nigeria. Measurements of were made using a calibrated real-time semiconductor radon monitor at a depth of 0.80 – 1.00 m in 150 randomly selected locations across 20 bedrocks in the six states of SW Nigeria. Saturated hydraulic conductivities of undisturbed soil samples taken from these locations were measured with a constanthead permeameter in order to determine . The GRP for each location was calculated from and and categorised using Neznal classification for radon hazard ratings. A Levenberg-Marquardt feed-forward-back-propagation artificial neural network was employed to develop a predictive model for . Data was randomly split in 70:15:15 for training, testing and validation, respectively, for six different architectures and the best was chosen following standard procedure. Goodness-of-Prediction (G), Average Validation Error , Mean Bias Error and Root Mean Square Error were used to determine performance and validation of the model. The and GRP maps were generated on existing geological map for SW region. The measured ranged . The ranged , while ranged . The GRP ranged . Sedimentary formation had highest of , while granitic bedrocks had highest and GRP of and , respectively. Radon hazard classification showed that , and of the sites were of low, medium and high radon hazard rating, respectively. Out of the 13 sites with high radon hazard rating, granitic and metamorphic bedrocks presented more sites (84.6%). The best performing architecture was 2 x 8 x 1. Performance indices of the model, yielded G of 73.5%, of 0.073, of 0.42 and of 4.62 kBqm-3. Validation indices yielded G of 86 , of , of and of 1 , indicating good model performance. Values of measured and GRP were used to generate maps which showed spatial distribution of low, medium and high radon hazard ratings. The values of measured soil-gas radon concentration and determined geogenic radon potential were highest in granitic bedrocks. The performance indices of the developed neural network model showed good reliability in predicting geogenic radon potential for southwest Nigeria.