Investigation of Radon Exhalation Rates from Geological Formations in Selected States in the Middle Belt Region

The Middle Belt region of Nigeria, encompassing Benue, Kogi, Niger, Nasarawa, and Plateau states, is underlain by diverse geological formations including granites, gneisses, and sedimentary rocks with varying uranium and radium content. Radon exhalation from soil and rock surfaces represents the primary source of indoor radon in buildings constructed directly on these geological substrates. This study determines radon exhalation rates from geological formations at 40 sites across Benue, Nasarawa, and Plateau states using the closed chamber accumulation method with RAD7 electronic radon detectors. Soil samples are concurrently collected for Ra-226 activity concentration measurement by gamma spectrometry to establish the geogenic control on exhalation. Emanation coefficients are computed from the ratio of measured exhalation to the theoretically derived exhalation based on Ra-226 concentration and soil physical properties. Results indicate surface radon exhalation rates of 18 to 312 mBq per square metre per second across the sampled geological units, with highest values from Precambrian granite outcrops in Plateau State. A significant positive correlation (r = 0.78) between Ra-226 activity concentration and radon exhalation rate confirms the geogenic control. Spatial mapping of exhalation rates provides a first-order geogenic radon potential map for the Middle Belt region. Keywords: radon exhalation, geogenic radon, Middle Belt, gamma spectrometry, geological formations