We took samples of uncultivated soil from the surface layer (0–10 cm) at 138 sites from all over Croatia and measured their radionuclide activity concentrations with high-resolution gamma-ray spectrometry. This second part of our report brings the results on ⁴⁰ K and ¹³⁷ Cs to complement those on the ²³² Th and ²³⁸ U decay chains addressed in the first part. Together they give the most complete picture of radioactivity of Croatian soil so far. Activity concentrations of ⁴⁰ K were the highest in the Pannonian region, and there was an opposite trend for ¹³⁷ Cs. We found that the concentrations of ¹³⁷ Cs tended to increase with altitude, annual precipitation, and vegetation density. The concentration ratio of ¹³⁷ Cs and K in soil, which indicates the potential for ¹³⁷ Cs entering food chains via uptake by plants, was the lowest in agriculturally important areas in the east of the Pannonian region. In addition, we used the obtained results on activity concentrations to calculate the related absorbed dose rate as a measure of external exposure to ionising radiation from soil. The sum of the absorbed dose rates for naturally occurring radionuclides and ¹³⁷ Cs showed that external exposure was generally the highest in the Dinaric region and Istrian Peninsula.

The assessment of environmental radioactivity much relies on radionuclide content in soil. This stems from the significant contribution of soil to both external and internal exposure to ionising radiation via direct emission of gamma radiation and soil-to-plant radionuclide transfer, respectively. This motivated us to carry out a systematic research on the radioactivity of soil in Croatia to obtain relevant data that can be used as a basis for understanding the related effects of geomorphological, biogeographical, and climatological properties of the environment. We collected samples of the surface layer of uncultivated soil (0–10 cm) at 138 sites from all over the country and measured them for radionuclide activity concentrations by means of high-resolution gamma-ray spectrometry. This resulted in radioactivity maps containing data on activity concentrations of representative radionuclides in the environment. In this paper, which is the first in our two-part presentation, we focus on the naturally occurring ²³² Th and ²³⁸ U decay chains and their correlations with the diversity of Croatian regions. For both of the chains, activity concentrations were the highest in the Dinaric region, the lowest in the Pannonian region, and intermediate in the Adriatic region. Relatively high concentrations of ²²⁶ Ra in the soil of the Dinaric region implied a possibility of an enhanced emanation of its progeny ²²² Rn into the air. Activity concentrations of ²¹⁰ Pb were additionally elevated in areas with dense vegetation, most probably due to an atmospheric deposition of airborne ²¹⁰ Pb onto the surface of plants and their eventual decomposition on the ground.