A Study on Natural Radioactivity, Indoor Radon Levels, and Occupational Exposures: Impact of Quarry Mining and Geothermal Explorations in the Kenyan Rift Valley

Abstract

The East African Rift System is an exceptionally unique environment marked by geological phenomena such as volcanism, faulting, seismicity, geysers, and hot springs. The underlying lithological profile and surface manifestations, such as hot and cooled magmatic rocks, fumaroles, and hot springs, present a great potential for exploiting abundant natural resources. The region has, therefore, attracted significant interest in mining, mineral processing, and geothermal resource exploration. Extraction and processing of these resources of great economic importance involve methods that potentially disperse and redistribute radionuclides, thereby enhancing radiation levels in the surrounding environment. This raises valid concerns about the potential radiotoxicity on human health and the environment. This study aimed to assess the radioecological risks and hazards caused by quarry mining and geothermal exploration in the Kenyan Rift Valley. Thirty-five randomly selected quarries within the Menengai Caldera Hills, were measured for the background ionizing radiation at 1m above the ground using a Wilnos radiation survey meter. The measurements were used to map the spatial distribution of radioactivity in the area. Subsoil samples were collected from each of the 35 quarry mines and were analyzed for radioactivity levels of Uranium-238, Thorium-232, and Potassium-40 using a high-purity germanium detector. Indoor radon concentrations were also measured using active AlphaE meters at Paka Geothermal Prospect. Occupational exposure measurements were also conducted for the quarry and geothermal workers using thermoluminescent detectors worn at their working stations for 30 days. The results of the radioactivity levels were used to determine the radiation hazard indices and the risks of cancer induction for the public and workers. The average gamma radiation field equivalent dose rate was 0.195 ± 0.073 (0.073 - 0.385) μSvh-1. The heat map generated from these results highlighted the quarry areas with the highest background radiation above 0.3 μSv/hr. The mean activity concentrations of 238U, 232Th, and 40K were 30.7±6.4 (23.4 – 60.1) Bq/Kg, 47.2±16.4 (24.3 – 95.9) Bq/Kg, and 520±125 (205 – 741) Bq/Kg, respectively. Activities of 232Th and 40K exceeded the UNSCEAR world average of 45 and 420 Bq/Kg, respectively, with an 87% prevalence of 40K in the area. The average total) excess lifetime cancer risks due to NORM exposure in indoor and outdoor environments was 2.38 ± 1.05 × 10-3 (1.89 – 3.60 × 10-3), exceeding the UNSCEAR threshold limit of 1.45 ×10-3 by approximately 64%. The mean indoor radon dose at Paka Geothermal Prospect campsite was 43.4±2.5 (17.6 - 135.6) Bq/m3. The Lung Cancer Cases (LCC) averaged 118 ±7 (113 - 128) cases per million people per year and was below ICRP permissible limits of 170 – 230 lung cancer cases per million people per year. Personnel dosimetry among the workers yielded a mean of 0.701±0.339 (0.302-1.304) and vii 0.646± 0.354 (0.338-1.367) mSv/month for the cumulative skin and body dose of workers in the Paka Geothermal Prospect, while 0.834±0.351 (0.344 - 1.281) mSv/month and 0.920±0.388 (0.179 - 1.294) mSv/month was the cumulative skin and body dose for quarry workers at Menengai Caldera Hills. Both results were compliant with IAEA limits of 1.67 mSv/month. The results indicate that the quarries are safe from immediate radioecological effects; however, there is a high probability of developing cancer over a lifetime of exposure to the indoor and outdoor environments of the Menengai quarry mines. Indoor radon levels do not pose a health risk to the residents and workers at the Paka Geothermal Prospect. The occupational radiation exposure doses in both study areas complied with the regulatory limits. The study, therefore, recommends continuous monitoring and minimization of radiation doses received by the quarry workers, provision for regulations and guidelines on ionizing radiation effects in Environmental Impact Assessments for mining operations, and the establishment of an acceptable national reference level for public exposure to indoor radon in Kenya. These findings underscore the significance of assessing the radio-toxicological influence and hazards associated with anthropogenic activities in volcanic regions