**4. Estimation and appraisal of radioactive contamination and its effects on the components of terrestrial ecosystems**

Radioecology is a sub-discipline of ecology concerning the presence and effects of radioactivity on Earth's ecosystems. Some of the risks of ionizing radiation were known in the early twentieth century. Nevertheless, the discipline *de facto* started developing in the period following World War II and the bombings of Hiroshima and Nagasaki [26]. The advent of the Atomic Age not only gave the impetus to study radiation effects on ecosystems, but also gave them powerful tools in the form of radioactive isotopes, which could be used as tracers [26, 27]. Initially, studies were carried out by the US Atomic Energy Commission (AEC) at several sites crucial to the Manhattan Project, principally Oak Ridge, Tennessee, and Hanford, Washington; many of these studies dealt with the cycling with biogenic carbon, phosphorus and oxygen through ecosystems and were conducted with radioactive tracers (14C, 32P, and others) [27]. In parallel, studies were conducted in the former USSR in the closed town of Ozyorsk (Chelyabinsk-40). Some studies were conducted in secret; most of them dealt with dispersal and deposition of bomb radionuclides and with the bioaccumulation of radioactivity in crop plants and farm animals [28–30].

Without a doubt, the most significant contamination event in the context of terrestrial ecosystems is Chernobyl. It is estimated that, at the time of the accident, around 10% of the total core radioactivity was released, including 100% of all noble gases and around 30% of volatile atoms including 30% of the core radiocesium (134Cs and 137Cs), 55% of the core 131I, and ~ 45% of the core 132Te. Less volatile radionuclide species such as radiostrontium (89Sr and 90Sr) were also released in smaller amounts (~5% of core inventory), as well as <3.5% of the core transuranic nuclides (neptunium, plutonium, curium) [31, 32]. The core inventories and releases are summarized in **Table 3**.

The most significant release of radioactivity from the damaged reactor was in the form of noble gasses (85Kr and 133Xe). Nevertheless, fast atmospheric dispersal and the lack of chemical reactivity of noble gasses mean that radioactive krypton and xenon resulted only in trace global contamination. In contrast, the volatile iodine-131, released in significant quantities during the reactor fire, was the predominant problem in contaminated areas during 1986. It is estimated that up to 4000 additional thyroid cancers among people can be attributed to this nuclide [4]. In the long term, the most significant contribution of radiation dose to the biota is attributed to radiocesium (134Cs, 137Cs), particularly 137Cs, due to its long half-life (30.17 years), its propensity to accumulate in plant and fungal matter and animal nerve and muscle tissue. The contribution of 90Sr to the background dose is also significant, but much lower and often indistinguishable from pre-Chernobyl global fallout from atmospheric nuclear testing [34].

Radioecological research after 1986 in Europe involved multinational teams working in the Chernobyl exclusion zone (ChEZ) and the most contaminated areas of Belarus and Russia (Gomel and Bryansk regions), as well as many studies on a

*Radionuclide Contamination as a Risk Factor in Terrestrial Ecosystems: Occurrence, Biological… DOI: http://dx.doi.org/10.5772/intechopen.104468*


*†transuranic nuclides*

#### **Table 3.**

*Core inventories and releases of the most important contaminants originating from the Chernobyl accident. Data obtained from [31–33].*

national level focusing on areas with known contamination. Among the projects conducted in the ChEZ, several exemplary studies of the bioaccumulation of different radionuclides in wildlife stand out [17, 19, 34, 35]. Researchers have demonstrated that the appropriate sentinel species for radioecological studies comprise small rodents including representatives of family Cricetidae like *Myodes glareolus* Schreber, 1870, *Microtus arvalis* Pallas, 1778, *Microtus oeconomus* Pallas, 1776 as well as European murid species: the yellow-necked wood mouse *Apodemus flavicollis* Melchior, 1834 and the wood mouse *Apodemus sylvaticus* Linnaeus, 1758.

During the 200 s, researchers reported very high internal doses in Cricetidae, particularly the bank vole (*M. glareolus*) due to high dietary intake of 137Cs [17, 34]. This has been confirmed by subsequent monitoring studies in the ChEZ [19, 35, 36], as well as in Alpine ecosystems in Bulgaria [37, 38]. Recent monitoring data suggest that *M. glareolus* is potentially the best rodent zoo monitor for residual contamination in Europe. A selection of results from two groups of monitoring programs, mentioned above is presented in **Table 4**.


#### **Table 4.**

*A summary of the findings of five radioecological studies using small mammals as zoo monitors.*

The summarized works show evidence for the high value of *M. glareolus* as a monitoring species for residual radioactivity from the Chernobyl accident due to its propensity to accumulate radiocesium. While accounting for the differences in values obtained by the various research groups, and the different time frames, another aspect of Chernobyl contamination becomes apparent: There are significantly higher depositions and animal body burdens of radiostrontium (90Sr) within the Chernobyl exclusion zone, as opposed to very low amounts of 90Sr present at greater distances from the accident site; this can be explained by the much lower volatility of strontium compared to cesium. This is one of the main reasons why 90Sr is still a significant contaminant within the ChEZ, but in most of Europe the largest part of the Chernobylassociated dose burden to the biota comes from 137Cs.

During recent monitoring studies, conducted in Bulgaria in the period 1996– 2020, small mammals such as rodents and insectivores were selected mainly due to their positions in the food chain like primary consumers, rapid maturity, large population number, and rapid biological reaction to the environmental changes [38]. The possible biological response of the organism was studied at different levels of organization of living matter, and evaluated the population number and structure, food spectrum, total beta-activity in target tissues, and organs of the investigated animals, standard hematological methods—to determine hemoglobin contents, hematocrit, and morphological characterization of erythrocytes, as well as cytogenetic methods. The food spectrum was analyzed as a basis for further investigations on the transfer of beta-emitters through the rodent populations and the whole ecosystem.

*Radionuclide Contamination as a Risk Factor in Terrestrial Ecosystems: Occurrence, Biological… DOI: http://dx.doi.org/10.5772/intechopen.104468*


#### **Table 5.**

*Whole-body total β-beta activities at two localities (Rila Mountain, Bulgaria), 2019–2020 [38].*

The total body burden of β-emitters of a species depends on the trophic chain position, food, life mode, physicochemical composition of the atmospheric precipitation, total suspended dust content in atmospheric air, and other factors. The total β-activities in Bq/kg of some small mammal species were investigated at two different altitudes in Rila Mountain, Bulgaria. The results, obtained in 2019–2020, are presented on **Table 5**.

All values were below 480 Bq/kg and were considered as referent.

Significant differences between mice and voles were obtained only due to the difference in their food specialization. Mice are omnivorous, while voles are mainly herbivorous species. Green vegetable parts accumulate radiocesium more actively than seeds and the quantity of the consumed low-caloric green food by animals is higher.

The comparison of the results obtained with the data 20 years ago makes it obvious that the values of total β-activity decreased by about 10 times in the period 1995–2019. Data obtained in the bodies of different monitor species of small mammals from Rila Mountain during 1995 varied from about 3500 Bq/kg in the yellow-necked wood mouse to 5000 Bq/kg in the snow vole. The total level of beta-activity in bank vole and yellow-necked wood mouse from Beli Iskar region during 1995 was between 2000 and 3000 Bq/kg [37].

High doses of radiation can influence the normal function of the blood and disturb the hematopoiesis. These were possible basophilic granulations that appear in enhanced, but also disturbed erythropoiesis, basophilic DNA fragments observed in a blood smear, frequently as a result of decreased spleen function, anemia, and overloaded bone marrow. However, the given results do not suggest such changes, and they have not been established.

A correlation between total beta-activity loading and chromosome aberration frequency in bone marrow cells was established. The percentage of chromosome aberrations in mice was about 1.6% and breaks were 0.2% and in herbivorous voles respectively 7.0 and 2.5%. The percentage of aberrant bone marrow cells of mice from the investigated regions is visibly lower than in vole species. This fact correlates with the recorded total body burden of β-emitters in herbivorous species in comparison with the omnivorous murids.
