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Abstract
Introduction
At present days, inhabitants of Polissya zone in Ukraine receive the main radiation doses due to the use of milk, some types of agricultural plants, and minor forest products. Inhabitants of the region use these products throughout a year since they are traditionally harvested, dried and preserved in big quantities. At the same time, it is known that the minor products of the forest contain a significant amount of radioactive elements now, the content of which in certain areas exceeds the acceptable levels. These circumstances require constant radiation control of the minor forest products, the materials of which, in addition to the practical value, have a certain scientific importance since they provide information on the redistribution of radionuclides in forest ecosystems.
It should be noted that a lot of attention was devoted to the analysis of radiation control materials of forestry products after the Chernobyl accident. The researchers concluded that there are significant problems with the use of most minor products of the forest in areas with rather small levels of radioactive contamination of the soil. Recommendations of scientists have focused on the need to strengthen radiation control and prohibit the use of edible mushrooms, wild berries, and medicinal herbs. In the mid-1990s in the forests of Polissya zone in Ukraine, long-term, large-scale works were carried out on the study of levels of radioactive contamination of wild berry plants around populated areas. Researchers noted that there is a large amplitude of the value of the specific activity of 137Cs in the products and a significant excess of the acceptable levels in most of the tested samples. Similar results were obtained by aggregating data on 90Sr content in wild berries.
The most detailed analysis of radiation control materials for forestry products in Zhytomyr region was made in 2000. The researchers reviewed the frequency distribution of samples of forestry products by levels of specific activity of 137Cs in certain ranges and noted that in most types of forest products, the frequency distribution is approaching normal, closer to the region of lower values of 137Cs content. It has been shown that in berries the proportion of samples, the content of 137Cs in which exceeded the acceptable levels, was 55.6 % of the total number of samples. However, 30 years have passed since the Chernobyl accident. In the forest ecosystems, the radiation situation has changed radically due to the decay of radionuclides, their consolidation in the soil and the redistribution of the constituents. This, to a certain extent, should affect the levels of radioactive contamination of minor forest products.
The purpose of our research was to establish modern levels of the content of the main dose-forming radionuclides (137Cs and 90Sr) in the minor products of the forest in the Zhytomyr region.
Materials and Methods
The objects of the research were samples of forest minor products (fresh blueberries – 25 samples, birch juice – 18 samples and honey – 5 samples) in the Zhytomyr region; specific activity of 137Cs and 90Sr was measured in the laboratory of radiology of Polissky branch of URIFFM.
For radiation analysis, samples of 1 liter were used, which were chopped and homogenized. Measurements of the specific activity of 137Cs and 90Sr in the samples were performed on a spectrometer СЕБ-01-150 „АКП-С” № 22506. The relative error of measurement in the samples, depending on their activity, fluctuated within 10–25 %.
The results of radiological control of the contents of 137Cs and 90Sr in each type of product were united into a single data set and after their processing, the corresponding histograms were constructed.
Results
The most significant fluctuations in the values of the specific activity of 137Cs and 90Sr were observed in samples of wild berries. The frequency distribution of 137Cs content in fresh blueberries for the research period moved to the region of minimum values. It was found that in 82.6% of samples the 137Cs content was lower than the acceptable level (500 Bq kg-1). If we compare this data with the previously obtained data, we can see that the proportion of samples with 137Cs content below 500 Bq kg-1 increased by 23.23 %.
Similar results were obtained from the analysis of data containing another radionuclide. Frequency distribution of 90Sr content in fresh blueberries for the study period also moved to the region of minimum values, with 95.65 % of the samples having a content of 90Sr lower than the acceptable level (50 Bq kg-1).
The results of the study of radioactive contamination of birch juice were quite positive. In 100 % of the samples of birch juice, 137Cs content was lower than the acceptable level (20 Bq kg-1). Similar results were obtained for the specific activity of 90Sr: for the research period, its content in 100% of samples of birch juice was twice lower than the acceptable level (20 Bq kg-1).
One of the few products of radiation control was honey. The obtained data indicate that the honey samples are dominated with a maximum content of 137Cs: the proportion of samples with a specific activity above 200 Bq kg-1 (that exceeds the acceptable levels) is 66.67 % of the total number of analyzed samples. It should be noted that the honey obtained in the northern areas of the Zhytomyr region contains a considerable amount of 137Cs, which is probably due to the fact that the main bee plants in this region are the plants that accumulate intensively the mentioned radionuclide (blueberries (Vaccinium myrtillus L.), cranberries (Vaccinium vitis-idaea L.), buckthorns (Vaccinium uliginosum L.), heather (Calluna vulgaris (L.) Hill.), and ramina (Rhamnus frangula L.)). The content of 90Sr in the honey samples was 100% lower than the acceptable level
(50 Bq kg-1) in the 100 % of the samples.
Conclusions
In 2017, for analyzed types of minor forest products, a decrease in the frequency of samples was observed, the content of 137Cs in which exceeded the valid acceptable levels: the content of 137Cs in fresh berries corresponded to the currently acceptable levels in 82.6 % of samples, 90Sr in fresh berries corresponded to the current acceptable levels in 95.65 % of samples. The content of 137Cs and 90Sr in birch juice samples corresponded to the current acceptable levels in 100 % of samples. In 66.67 % of honey samples, the specific activity of 137Cs exceeded acceptable levels. The content of 90Sr in 100 % of samples of honey was half the acceptable levels.
The presence of a large number of samples, the content of radionuclides in which exceeds the acceptable levels, requires continuing the radiation monitoring of minor products of the forest in Zhytomyr region.
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