Radioactivity levels in the environment depend on geological aspects, mainly on the composition of rocks and soil, where natural radionuclides are found in varying concentrations (Raykov 1978; Négrel et al. 2018). The Rhodopes are rich in uranium and other ore deposits. Exposure to various human activities can increase the proportion of natural radioisotopes in the effective dose. This can pose a potential risk to humans and living organisms, as 75% of the radiation received by mankind is due to natural sources of radiation. (Ghiassi-Nejad et al. 2001).

A comparison is made between the radioisotopes content in the soils from the studied areas and the average values of natural radionuclides in undisturbed soils from different regions of the world, Bulgaria, the Western Rhodopes, and the Sofia field. (Yordanova et al. 2005, 2015; Lazarova et al. 2019).

The publications of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 2000) report average values of the content of natural radionuclides in Bq/kg in undisturbed soils from different regions of the world. The data is presented in Table 2.

For the study region, the specific activity of the natural radionuclides ²³⁸U, ²²⁶Ra, ²³²Th, ⁴⁰K, ²¹⁰Pb in Bq/kg was in the following range: ²³⁸U – 9÷62; ²²⁶Ra – 11÷92; ²³²Th –16÷82; ⁴⁰K – 260÷1432 and ²¹⁰Pb – 26÷64 (Figs 5–7).

Content of natural radionuclides in the studied soils was within the background amounts and was comparable to global averages (Figs 5–7). The differences between the Eastern and Western Rhodopes could be explained by the predominance of sedimentary rocks in the Eastern Rhodopes. Sedimentary rocks, especially of biogenic origin, have a very low content of radioactive elements.

Potassium is an important element. It is found in all living organisms. The importance of the radioactive isotope ⁴⁰K is mainly due to its long half-life (1.28 × 109 years) and its ubiquity.

World average specific activity of ⁴⁰K (activity per unit mass of soil) is 370 Bq/kg, ranging from 100 to 700 Bq kg (Mcaulay and Moran 1988).

Content of ⁴⁰K in Bulgarian soils varies significantly – from 40 to 800 Bq/kg. For the studied area it ranges from 30 to over 1400 Bq/kg (Fig. 8). This wide range is characteristic of cinnamon forest soils, predominant in the studied region, due to the great variety of soil-forming rocks in these soils. In the soil samples from the Ada Tepe gold mine and the Madjarovo mine, the measured values were two times higher than the world average. This can be explained by the presence of sedimentary rocks rich in organic remains.

A shift in the radioactive equilibrium between ²³⁸U and ²²⁶Ra and higher content of radium was detected in some of the soil samples. It may be due to the lower mobility of radium in soils, the low humus content and the greater solubility of uranium salts allowing U migration along the soil profile.

No significant differences were found in the radiation status of arable and non-arable soils. The results of the studied radionuclides correspond to the average or slightly above average values, typical for the geographical latitude of Bulgaria and were within the values cited in the literature as normal for the respective regions.

Radioactive contamination introduces new elements into the ecosystem. As a result of nuclear tests in the 1960s and the Chernobyl accident, ¹³⁷Cs entered the environment. For the study area, the specific activity of the technogenic radionuclide is in the range (1–87) Bq/kg.

After the Chernobyl nuclear accident, the southern part of Bulgaria was most affected. Four to five times higher activity concentrations were measured in the soils of Southern Bulgaria than in those of the Northern part (Yordanova et al. 2007). The Rhodopes have received significant amounts of radioactive ¹³⁷Cs. Through the rains, it entered the soil, bound to the surface soil layer and was redistributed into the ecosystem, where it remained for a long time due its long half-life.

From the data on the specific activity and dynamics of ¹³⁷Cs it can be seen that the soil pollution was non homogeneous as a result of air transport. It varied between 3 and 1700 Bq/kg, even within small areas (tens of square meters) (Yordanova et al. 2007). This was also confirmed by the present studies where its content was ranging from <1 to 87 Bq/kg (Fig. 9). The total technogenic γ-activity in the soils of Bulgaria has increased between 10 and 300 times after the Chernobyl accident. For ¹³⁷Cs this excess was 3–10 times, in some cases reaching up to 50 times above the characteristic background values. BFSA (2012).

Until 1986, the average value of ¹³⁷Cs specific activity in Northern Bulgaria was 10 Bq/kg and in Southern Bulgaria – 26 Bq/kg. (Naydenov and Staneva 1987a, b). After 1990, cesium-137 could be detected in all soil samples. (Yordanova et al. 2014). In 1996, due to the high inhomogeneity, the average values of the isotope varied between 160 and 280 Bq/kg for Southern Bulgaria and between 40 and 60 Bq/kg – for Northern Bulgaria. (Yordanova et al. 2007). According to the data of the Executive Agency for Environment (2016), the highest values were registered in the Western Rhodopes (Chetroka region – 261/kg and the town of Laki – 189 Bq/kg;). 35 years after the Chernobyl accident, the present study recorded specific activity of cesium-137 from <1 to 87 Bq/kg. This decrease of ¹³⁷Cs activity in the surface soil layer was mainly due to its radioactive decay.

The Eastern Rhodopes are rich in water. Sites such as the Ada Tepe gold mine, the Madzharovo mines, the Kardzhali Lead-Zinc Complex, are located in close proximity to settlements or to large water sources. A study on the radiological impact of uranium mining on surface waters and sediments closure shows that the migration of U_nat, ²²⁶Ra, ²¹⁰Pb and ²³²Th through surface water is one of the major pathways for contamination spread. (Ivanova et al. 2015). Most of the sites are near surface water bodies and are potentially dangerous for groundwater bodies. This poses an environmental risk in case of possible pollution, not only for humans but also for all living organisms, as the Eastern Rhodopes have the greatest species variety in Bulgaria and is one of the richest territories in aspect of biodiversity in Europe (Ministry of Environment and Water 2013). Many Balkan endemics and highly endangered species inhabit the region. The studied areas are close to regions of the European ecological network Nature 2000 and are of growing interest for tourism. In this regard a further research and assessment of the radiological status of ground and surface water in the region could be recommended.

The radium equivalent index (Ra_eq) and the external hazard index (H_ex) were used to assess the results obtained with respect to potential radiation hazard to the population. The data for H_ex do not exceed the permissible upper limit 1. Thus, with an external hazard index below 1 and low Radium equivalent activity, in relation to natural radionuclides in the soil, the study area of ​​the Eastern Rhodopes is within normal background amounts and does not pose a radiation hazard for the population and biota in the area.