Modelling Radon and Thoron Exhalation and Measurement of Total Natural Radiation Exposure in Mrima Hill, Kenya
Chege, Margaret Wairimu
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Mrima hill in Kwale County is one of the regions in Kenya with the highest levels of background radiation. Rich deposits of mineral ores such as niobium and manganese are also known to exist. Small-scale farming is the mainstay of the economy with cassava as the main cash as well as food crop; raw soil constitutes the primary building material while hand-dug wells serve as the source of water for the residents. The main objective of this research was to determine the exposure associated with radioactivity in indoor air, crops and well water in the region. This involved the determination of 222Rn and 220Rn concentration in dwellings and measurement of 226Ra, 232Th, and 40K concentrations in building materials, soil, cassava and well water. A model on radon and thoron exhalation rates was developed and validated through exhalation measurements. In addition, physico-chemical parameters of water that included heavy metals, pH and conductivity were investigated. CR-39 SSNTD were used for simultaneous measurements of 222Rn and 220Rn; accumulation chamber coupled with RAD7 monitor for exhalation measurements; HPGe detectors for radioactivity measurements in crops and water samples; and the AAS for heavy metal determination in water samples. Conductivity and pH were measured on site using standard digital meters. A total annual effective dose of 24.1 mSv/y was obtained with indoor air, crops and water, and building materials contributing 56 %, 36 % and 8 % of the dose respectively. All radon concentration values were below 200 Bq m-3 while 65 % of thoron measurements were above 300 Bq m-3. A mean thoron concentration of 652.8±397.0 Bq m-3 was obtained against that of radon of 35.2±13.9 Bq m-3. Building materials registered average radon and thoron exhalation rates of 0.0043 Bq m-2 s-1 and 19.6 Bq m-2 s-1 respectively and average concentrations of 226Ra and 232Th of 134 Bq kg-1 and 431 Bq kg-1 respectively. Modelled and measured isotopes exhalation values showed good agreement which meant that exhalation rate was dependent on the content of mother radionuclide in the building material. A correlation coefficient of near unity was observed between 226Ra and 232Th content in building materials and in soil. Over 70 % of cassava tubers and leaves had detectable amounts 226Ra with average concentrations of 60±5 Bq kg-1 and 141±11 Bq kg-1 respectively. 232Th, with an average concentration of 35.3±61.5 Bq kg-1 was detected in 28 % of the tubers; it was not detected in the leaves. 37 % and 7 % of water samples detected for 226Ra and 232Th with average concentrations of 4.3±0.3 Bq kg-1 and 2.0±0.1 Bq kg-1 respectively. 40K was present in all samples in averages of 842±539 Bq kg-1, 1708±552 Bq kg-1 and 91.4 Bq kg-1 in cassava tubers, leaves and water respectively. In terms of heavy metals, over 90% of the water samples were enriched with at least one metal with Mn and Cd as the main contaminants. The average electrical conductivity was 862±949 μSv/cm; 17 % of the samples had pH values lower than 6.5. From the results obtained, the main source of exposure in Mrima hill region is indoor air with thoron as the main source of the radiation dose. Exposure through ingestion is mainly due to 226Ra and 40K. Groundwater in the region is generally of poor quality mainly due to heavy metal enrichment.