Vertical distribution and radiological risk assessment of 137Cs and natural radionuclides in soil samples

The aims of this study were to investigate the vertical distributions of natural radionuclides ²³²Th, ²²⁶Ra and ⁴⁰K as well as anthropogenic radionuclide ¹³⁷Cs in soil samples and to analyze the correlation among the radioactivity of these radionuclides and the physiochemical characteristics of soil samples namely pH, grain size, carbonate content and organic matter. Risk assessment of the radiological hazard has also been estimated. Forty-four soil samples were collected from eleven locations in Qatar at four depth levels from 0 to 16 cm. The average concentrations of ²³²Th, ²²⁶Ra, ⁴⁰K and ¹³⁷Cs in the soil depth of 16 cm were 10, 17, 201 and 4 Bq/kg, respectively, which were within the reported world mean. The external absorbed gamma dose rate, the annual effective dose, the mean radium equivalent activity, the external hazard index and the lifetime cancer risk were 22 nGy/h, 0.027 mSv/y, 47 Bq/kg, 0.125 and 0.096 × 10⁻³, respectively. These values were far below the minimum recommended international values. The level of radioactivity concentrations in the soil was affected by the physiochemical characteristics of the soil. The positive correlation with highest R² value was found among the radioactivity concentrations of ²³²Th and ⁴⁰K and the soil clay content. Total organic carbon was also positively correlated for ²²⁶Ra and ¹³⁷Cs activity concentrations, whereas, carbonate content was negatively correlated with the radioactivity concentrations of ²³²Th and ⁴⁰K. As far as soil moisture content is concerned, the positive correlation with highest R² value was obtained for ²²⁶Ra activity concentrations.