Distribution of Natural Radionuclides in the Surface Soil in Some Areas of Agriculture and Grazing Located in West of Riyadh, Saudi Arabia

The distribution of 238U (Uranium-238), 232Th (Thorium-232), 40K (Potassium-40) and 137Cs (Caesium-137) in surface soil at various sites of agriculture and grazing located in west of Riyadh, Saudi Arabia was investigated under natural field conditions. These sites were located at cities of Huraimla, Thadeq and Sajir, Saudi Arabia. The latitude of these sites was in the range of 25.08 °N to 25.30 °N, the longitude was in the range of 44.43 °E to 46.17 °E and the altitude was in the range of 721 m to 803 m. Data were collected by scanning the surface soil by the help of the Mole (proximal sensor for gamma radiation, the soil company, the Netherlands). The Mole was mounted on fabricated iron carriage. Four surfaces soil in each location were scanned. From the data analysis, average activity concentration was determined for 40K (range from 96.81 to 337.91 Bq/kg), 137Cs (range from 8.04 to 17.13 Bq/ kg), 238U (range from 13.02 to 38.70 Bq/kg) and 232Th (range from 10.47 to 33.73 Bq/ kg). The best-fitting relationship between the concentrations of 232Th and 40K versus 238U and also of 40K versus 232Th was found to be linear type with reasonable coefficients of determination (R2). However, R2 of 0.799 for 232Th vs. 238U, 0.6366 for 40K vs. 238U and 0.4295 for 40K vs. 232Th. The 232Th /238U, 40K /238U and 40K /232Th ratios (slopes) extracted were equal to 0.9124, 8.5351 and 6.6979, respectively. The total annual effective dose for the studied locations was in the range of 0.02 to 0.06 mSv/year. The calculated external hazard index value for the soil samples studied was ranged between 0.10 and 0.30 with an average value of 0.20. The results showed a reasonably low radiation outdoor absorb dose (less than unity), which is a good indication for the farmers for agricultural activities and for animal pasture.