Comparative study of Cs-137 activity concentration between attic dust and urban soil from Salgotarjan city, Hungary

The ¹³⁷Cs (t_1/2 =30 years) is a principal radioisotope that was artificially introduced into the environment through the atmospheric bomb tests took place from the middle of the 1940s to the 1980s and from the major nuclear accidents (i.e., Chernobyl, 1986 and Fukushima, 2011). From the atmosphere, ¹³⁷Cs easily adsorbs to particles and it returns to lithosphere (pedosphere) by wet and dry deposition as a radioactive fallout component. Due to the Chernobyl nuclear accident, the released contaminated air mass, containing Cs-137, largely propagated, deposited, and distributed across several European countries in the ambient environment (Balonov et al., 1996). These particles also reached houses (e.g. through open windows, cracks, and vents) in an urban environment and deposited inside resulting in the exposition of the habitants to ¹³⁷Cs, especially in areas that are not accessible for a regular cleaning like attics. Following the nuclear accidents, primary attention was drawn to agricultural areas and less attention was paid to urban environments. Accordingly, the goal of this study is to compare the ¹³⁷Cs activity in attic dust as undisturbed samples, and urban soils as disturbed environmental materials to determine the ¹³⁷Cs distribution in urban environment. 

Attic dust (AD) samples were collected from 14 houses, which were built between 1900 and 1990 14 urban soil (US) samples were collected nearby the houses at a depth of 0-15 cm in Salgótarján, a former industrial city. To obtain a representative local undisturbed soil sample, a forest soil sample was collected from the upwind direction (NW) of the city. To check the ¹³⁷Cs content of the local industrial waste material, we also collected fly-ash slag sample from a waste dump.   AD and US samples were analyzed by a well-type HPGe and with an n-type coaxial HPGe detector in a low background iron chamber, respectively.

Cs-137 activity in the studied AD ranges from 5.51±0.9 to 165.9±3.6 Bq kg^-1, with a mean value of 75.4±2.5 Bq kg^-1 (decay corrected in 2016). In contrast, US samples show ¹³⁷Cs activity ranging between 2.3±0.4 and 13.6±0.6 Bq kg^-1.  The brown forest soil sample has elevated ¹³⁷Cs activity concentration (18.5±0.6 Bq kg^-1), compared to the urban soils. The fly-ash slags activity is below the detection limit (0.7±0.5 Bq kg^-1).

The average ¹³⁷Cs activity in AD is ~15 times higher than that of US. This result clearly indicates that attic area provides a protected (hardly or unchanged) environment, therefore physical condition of the dust remains constant in time, and there is a small chance for chemical reaction. Forest soil proves that US were highly disturbed by anthropogenic activity. This is supported by fly-ash slag activity results.  Whereas, ¹³⁷Cs activity concentration of the AD samples shows significantly higher than that of the studied soils in Hungary. This confirms again US cannot show the historical atmospheric ¹³⁷Cs pollution such as attic dust. A statistically significant relationship (p=0.003, r²=0.05) were found between the AD and US samples. Therefore, it can be considered that attic dust remained undisturbed for decades and preserve past record of components of atmospheric pollution.