Exposure to ionising radiation in the environment stems from both naturally occurring and anthropogenic radionuclides. Since life on Earth has evolved in the absence of anthropogenic radionuclides, their presence in any quantity is threatening. Nuclear fission is the main source of anthropogenic radionuclides, of which 137Cs is considered particularly harmful to living organisms due to its long half-life
Under non-accidental conditions, most of external exposure to ionising radiation originates from soil. In spite of a vast amount of related studies, radioactive properties of soil continue to attract attention and are investigated with regard to agronomy (2, 3), human activities (4, 5, 6, 7, 8, 9), and the ecology of intact nature (10, 11). In the first part of the report (12) on the radioactivity of uncultivated soil in Croatia, we presented the activity concentrations (
Figure 1 shows a widely used division of Croatia with respect to geomorphological, biogeographical, and climatological characteristics. A more detailed description of the main regions and subregions has been given in the first part of this report (12), and here we repeat the main points only.
Main geomorphological, biogeographical, and climatological regions of Croatia. Region I belongs to the Pannonian Plain, with subregion Ia comprising hilly areas and subregion Ib being a flat lowland. Region II belongs to the Dinaric Alps. In subregion IIa, the climate is cold continental and vegetation is subalpine, whereas in subregion IIb, karst prevails, and the climate is Mediterranean. Region III is Mediterranean in both climate and vegetation. In subregion IIIa the influence of regions I and II is stronger than in subregion IIIb. Sizeable areas of dense forests are indicated by letter F
Region I is part of the Pannonian Plain. Its western subregion (Ia) comprises hilly and flat terrains, whereas subregion Ib in the east is a flat lowland. The vegetation, soil types, and climate are typical of Central Europe, and the annual precipitation decreases from the west (~1200 mm) to the east (~500 mm) of the region. Region II is part of the Dinaric Alps, underlain by limestone and dolomite bedrock. In subregion IIa the vegetation is subalpine and the climate is cold continental, with annual precipitation mostly exceeding 1500 mm and being as high as 3500 mm in densely forested mountains. Subregion IIb is transitional and influenced by the vicinity of the Mediterranean. Annual precipitation is 1000–1500 mm. Region III comprises the eastern Adriatic coast, accompanying islands, and parts of the hinterland. The bedrock is again limestone and dolomite, whereas the climate and vegetation are Mediterranean. The annual precipitation ranges from ~700 mm in the southeast to ~1500 mm in the highest northeast of the Istrian Peninsula. Subregion IIIa is affected by the vicinity of subregion IIa, whereas subregion IIIb is barely influenced by the continental climate.
Regions I and II contain sizeable areas of dense forests, and these are indicated by letter F in Figure 1.
Our experimental procedure has been explained in more detail in the first part of the report (12). Briefly, we sampled the surface layer of soil (0–10 cm) at 138 locations throughout Croatia in 2015 and 2016 following a recommended procedure for sampling and preparations for measurements (14). Radionuclide activity concentrations were determined by means of high-resolution gamma-ray spectrometry using a setup based on a high-purity germanium coaxial detector. We also applied corrections for self-attenuation (15) and coincidence summing effects (16). Detection limits were typically 2 Bq/kg for 40K and 0.3 Bq/kg for 137Cs.
Figure 2 shows the distribution of the measured
Distribution of the
40K accounts for 0.012 % of the total K found in nature, and its distribution is actually the same as that of K. Since K is a biogenic element, its concentration in soil depends not only on underlying geology but also on the exchange of matter between soil and living organisms. While the
137Cs, in contrast, represents a serious radioecological threat. The origin of most of the present-day 137Cs in Croatia is radionuclide release during the Chernobyl disaster in 1986. Much smaller contributions come from nuclear weapons tests and the Fukushima accident (9, 19). The distribution of the
Distribution of the
Generally, the
In order to understand results in Figure 2, one has to address several factors which might have affected the deposition of (initially airborne) 137Cs onto the ground and its fixation in soil. These factors are expected to be primarily related to atmospheric phenomena and geomorphological properties, including physical and chemical properties of soil (2). 137Cs is still abundant in the upper atmosphere and is deposited on the ground mainly via precipitation. When we compare results in Figure 2 with altitude and annual precipitation (20) at sampling locations, we notice that higher altitudes and more precipitation have
Due to similar electronic structures K and Cs have similar chemical properties, and at insufficient concentrations of K in soil, plants tend to take up chemically similar elements, including 137Cs (21). This is, in fact, why fertilisers rich in K have been used to reduce 137Cs uptake in heavily polluted areas (22). For the same reason the ratio
where
Ratio of the concentrations of 137Cs and K in Croatian soil
Internal exposure to ionising radiation due to soil radioactivity depends on numerous factors and has to be estimated on the basis of specific conditions. On the other hand, there is a well-established method of assessing external exposure from measured activity concentrations of radionuclides in soil and their gamma emissions. For naturally occurring radionuclides, the absorbed dose rate
where
Absorbed dose rate due to naturally occurring radionuclides in Croatian soil
In the case of 137Cs, there is no universal equation for calculating the related absorbed dose rate
expressed in nGy/h. By taking into account measured
Absorbed dose rate due to 137Cs in Croatian soil
Figure 7 shows the distribution of the total absorbed dose rate (
Total absorbed dose rate due to radionuclides in Croatian soil
In this second part of our report on uncultivated soil radioactivity in Croatia we focus on 137Cs, 40K, and absorbed dose rate. Since 137Cs and K have similar chemical properties, they compete in uptake by plants, which implies that their relative concentrations in soil are of interest for the propagation of 137Cs through food chains. Measured activity concentrations of 40K were the highest in the Pannonian part of Croatia, and 137Cs showed the opposite trend. Their ratio was the smallest in the Pannonian region, especially in its eastern part, where the majority of Croatian intense agricultural production has been taking place. Activity concentrations of 137Cs tended to be above average at high altitudes, in areas with dense vegetation and annual precipitation above average. By combining these results with those for the 232Th and 238U decay chains presented in the first part, we determined that the highest dose rates were in the Dinaric region and on the Istrian Peninsula. Since this study relied on long-lived radionuclides, our findings are likely to remain current for a prolonged period of time and may also serve for reference in case of accidental radioactivity bursts in the environment of Croatia.