Radioactivity of soil in Croatia I: naturally occurring decay chains

Figure 2 shows the distribution of the A of ²³²Th (T_1/2=14 billion years ) in Croatian soil, and Figure 3 that of ²³⁸U (T_1/2=4.5 billion years). The two main naturally occurring decay chains – comprising numerous radionuclides – start with ²³²Th and ²³⁸U. Therefore, these results correspond to the radioactivity levels of the whole chains (peculiarities related to the ²³⁸U chain will be discussed later).

Figure 2

Figure 3

The average values (Ā) of A for ²³²Th and ²³⁸U were similar, being 41 Bq/kg and 45 Bq/kg, respectively. The range of A for ²³²Th was 8–85 Bq/kg and for ²³⁸U, 0–140 Bq/ kg (0 means that measured A was below the corresponding DL). Globally, the reported medians (and ranges) are 30 Bq/ kg (11–64 Bq/kg) for ²³²Th and 35 Bq/kg (16–110 Bq/kg) for ²³⁸U. Our results, therefore, do not depart from those obtained elsewhere (including the rest of Europe) (22).

A comparison of results in Figures 2 and 3 with the sketch of Croatian regions (Figure 1) reveals a similar trend for ²³²Th and ²³⁸U. Activity concentrations were about average or lower in most of region I and sizable parts of region III. In region II and parts of subregion IIIa (western Istria), they were mainly above average. At a few spots, there were deviations from this trend, but the above conclusion still holds well.

In order to explain these differences, at least qualitatively, we ought to address factors that generally affect the formation of soil. Of these, the most important are the geological background of bedrock and substratum, climate, living organisms close to the surface, and relief (18). In region II, the bedrock is a mixture of limestone and dolomite, the living organisms typical of subalpine ecosystems, and the relief varies from highland valleys to steep mountains. These characteristics seemingly favour the presence of U and Th in soil. In region III, the bedrock is also limestone and dolomite, but the other mentioned factors are markedly different, which reduces the presence of Th and U, despite the same geological background. In region I, limestone and dolomite are scarce, whereas the climate, relief, and exchange of matter between living organisms and soil differ substantially from those in regions II and III. These conditions seem to be less favourable for the presence of Th and U in the surface layer of soil. An elucidation of the above findings requires a focused study that would take into account the complexity of the behaviour of U and Th in soil (23).

Figure 4 shows the distribution of the A of ²²⁶Ra (T_1/2=1600 years ) in Croatian soil, and Figure 5 that of ²¹⁰Pb (T_1/2=22.4 years). For both radionuclides, the spatial distribution of A followed the trend observed for ²³⁸U, which was not surprising, since all of them belong to the same decay chain. However, there were differences in the magnitudes of A. For ²²⁶Ra, A varied from 14 to 281 Bq/kg, and Ā=57 Bq/kg. For ²¹⁰Pb, the range was 0–288 Bq/kg, and Ā=63 Bq/kg. Globally, the reported median for ²²⁶Ra is 35 Bq/kg (the same as for ²³⁸U) and the range 17–60 Bq/kg (narrower than for ²³⁸U) (22). However, there are considerable variations of these quantities in the data for Europe, and our finding, therefore, does not represent an anomaly. Since ²²⁶Ra and ²¹⁰Pb appear in the chain later than ²³⁸U and have shorter T_1/2, their activities were expected to be lower or equal to that of ²³⁸U (16).

Figure 4

Figure 5

In order to explain the departure from this expectation, we turn to chemical and physical properties that affect the propagation of Ra and Pb through the environment and their accumulation in soil. Let’s start with ²²⁶Ra, which is a product of five decays, starting with that of ²³⁸U. Chemically, Ra is similar to other alkaline earth metals. The highest A of ²²⁶Ra was measured at locations in region II, i.e., limestone and dolomite bedrock abundant in CaCO₃ and MgCO₃. Due to the chemical similarity of Ca, Mg, and Ra, a substitution of Ca or Mg by ²²⁶Ra is more favourable than the same process for any other element of the ²³⁸U chain. Since the formation of soil depends on the parent material, the concentration of Ra may be enhanced in soil that originates from limestone and dolomite. This is, however, not the only mechanism which can lead to ²²⁶Ra content in surface soil being different from that expected solely from the decay of ²³⁸U. Since Ca and Mg are biogenic elements, they are exchanged between the surface layer of soil and vegetation. Given the mentioned chemical properties of Ra, the participation of its isotopes in the exchange exceeds that of other radionuclides in the decay chains (24, 25). This process might at least partly account for the highest activity concentrations of ²²⁶Ra in the samples from the highly forested northwestern part of subregion IIa (Gorski Kotar). Like with U and Th, a focused study based on the known properties of Ra in soil (23) is required for a full clarification of the observed phenomenon.

²²⁶Ra decays into ²²²Rn, a noble gas with T_1/2 =3.8 days. Radon is the only gaseous radionuclide appearing in the naturally occurring decay chains. Of its isotopes, only ²²²Rn is of interest for environmental radioactivity, because the other ones (²¹⁹Rn and ²²⁰Rn) have T_1/2 under one minute. The gaseous nature of ²²²Rn and its relatively long T _1/2 lead to its appearance in the air close to the ground. This is a consequence of its diffusion through soil and release into the air above (26, 27). Hence, the ²³⁸U chain differs from the ²³²Th and ²³⁵U chains in the appearance of ²²²Ra, which can easily migrate out from its matrix (in our case, from soil). By consequence, decay products of ²²²Rn are subject to airborne decay and atmospheric deposition onto different surfaces (28). Of these, ²¹⁰Pb has the longest T_1/2 and it therefore deserves a special consideration.

The distribution of the A of ²¹⁰Pb followed the trend observed for ²³⁸U and ²²⁶Ra, but its Ā and maximum A were higher than those of the mentioned radionuclides from the same decay chain. In particular, by comparing Figures 1, 4, and 5, one can note that the A of ²¹⁰Pb tended to be higher than that of ²²⁶Ra in forested areas. Radionuclides in the part of the ²³⁸U chain between ²²²Rn and ²¹⁰Pb have short T_1/2 (less than 30 minutes), and this results in the appearance of airborne ²¹⁰Pb (16). Since the migrations of gaseous ²²²Rn and aerosols (containing Pb) are influenced by atmospheric processes, concentrations of ²¹⁰Pb in soil might depart from those of ²²⁶Ra. Airborne ²¹⁰Pb not only falls onto the ground but also accumulates on various surfaces and ends up in soil through the decomposition of plants (e.g., leaves) (28, 29, 30, 31, 32). The latter process might be extended over a number of years, and it is not surprising that the concentration of ²¹⁰Pb in soil could be higher in areas with rich vegetation (29, 31). These effects provide a plausible explanation of the fact that the A of ²¹⁰Pb in our samples exceeded that of ²²⁶Ra mainly in forested areas or elsewhere where the vegetation was dense enough (16).

The ²³²Th and ²³⁸U decay chains in soil contribute significantly to the external exposure of humans and biota to ionising radiation, as many of the members of the chains are gamma emitters (33). When their activity concentrations are small to moderate – which is our case – this contribution usually does not pose a threat. However, the presence of airborne ²²²Rn is a different matter. High-energy charged particles – especially gamma particles – are more dangerous in internal exposure than gamma photons, which makes the inhalation of ²²²Rn (an gamma emitter) a serious health hazard (34). Actually, there have been substantiated arguments which relate the inhalation of ²²²Rn to the risk of lung cancer (35). In turn, radioecological and radiotoxicological concerns with regard to the naturally occurring decay chains are in most cases related to the concentration of ²²²Rn in air, especially in dwellings.

Areas of potentially higher risk from radon are usually termed “radon prone areas”. Their identification, however, is a complex issue, as it must take into account numerous factors that affect the appearance of ²²²Rn in air and its accumulation in dwellings. There are different approaches to this issue, and some of them include potential formation of ²²²Rn as a starting point (36). Our data allow us to only determine areas in Croatia where the formation of ²²²Rn in the topmost layer of soil is comparatively enhanced because ²²⁶Ra concentrations are above average. From our results, it follows that these areas in Croatia are mainly in region II and, to a lesser extent, region III. Preliminary results of direct measurements of indoor radon in different parts of Croatia within the Action Plan for Radon 2019–2024 suggest that elevated radon concentrations have indeed been found mainly in these regions (37).