Investigation of Occurrence and Distribution of Radionuclides and Radiation Levels in “Machine Cut Stone” Quarries in Juja Sub County, Kiambu County

Abstract

Radiation occurs naturally in the environment, with its levels largely influenced by the presence of naturally occurring radionuclides. Open-cast mining operations can expose these radionuclides, previously confined deep within the Earth. Of particular concern is the presence of these radionuclides in quarry products used in construction materials. This study aimed to determine the occurrence and activity concentrations of naturally occurring radioactive materials (NORM), specifically Thorium-232 (232Th), Uranium-238 (238U), and Potassium-40 (40K), in machine-cut stone quarries located in Kiambu County, Kenya. A sampling frame was developed from a comprehensive list of quarries in Kiambu County, from which 15 active quarries in Juja Sub- County were randomly selected. Quarry dust samples were collected from both the top and bottom sections of each quarry. Data collection continued until saturation was achieved. The samples were homogenized by crushing them into powder and sieving to achieve consistency. Gamma spectrometry, using SpectraLineGP software, was employed to determine the activity concentrations of the radionuclides, with results expressed in Becquerels per kilogram (Bq/kg). The duration of quarry activity ranged from 6 to 24 months, with most having been operational for 24 months. The elevation of the quarries ranged from 1,466 to 1,531 meters above sea level, with a mean elevation of 1,494.88 ± 13.88 m. The activity concentrations of radionuclides followed the order: 40K > 232Th > 238U. The mean activity concentrations ranged from 71.1 to 152.2 Bq/kg for 232Th, 40.5 to 107.4 Bq/kg for 238U, and 763.8 to 1,661.2 Bq/kg for 40K. The quarry dust was also found to contain metallic elements such as silicon, iron, aluminum and potassium, with trace amounts of calcium, titanium and manganese. The findings indicate that the environmental radiation levels in machine-cut stone quarries in Juja are within the range of natural background radiation and are thus considered safe for use in construction. The study recommends regular monitoring of radiation levels in stone quarries by relevant Ministries, Departments, Agencies and Counties (MDAC) in the built environment. Additionally, the implementation of guidelines for material selection based on radiological assessments and awareness programs on radiation safety can help ensure public safety in construction practices.