Caesium-137 in the muscles of game animals in 2015-2022 – levels and time trend

The research material consisted of muscle tissue of game animals: the wild boar (Sus scrofa), the roe deer (Capreolus capreolus) and the red deer (Cervus elaphus). Samples for testing were collected by the veterinary inspectorate from processing plants in Poland. Figure 1 shows the number of samples tested in each year. A total of 858 game animal muscle samples were tested: 508 from wild boar, 145 from roe deer and 205 from red deer.

Fig. 1.

An accredited test method (AB 95; Polish Centre for Accreditation, Warsaw, Poland) using gamma-ray spectrometry with a solid-state detector (high-purity germanium HPGe detector) and a scintillation detector (NaI (TI) crystal) placed in a lead shield was used to determine the radioactive concentration of caesium-137 (25). The laboratory at the Department of Radiobiology of the National Veterinary Research Institute has participated in tests of its proficiency in this method several times and has passed on each occasion. Crushed, ground and homogenised muscle samples were transferred to Marinelli measuring containers (450 cm³). The geometry of the multi-nuclide MBSS 2 calibration source (The Czech Metrology Institute, Jihlava, Czech Republic), which was used to calibrate the detectors, was preserved. The measurement time was 72,000 s (20 h). The collected gamma-ray spectra were analysed using Genie 2000 software (Mirion Technologies, Atlanta, GA, USA) (Fig. 2).

Fig. 2.

A summary of the analyses performed was made for the matrix data, and then the mean concentration, standard deviation and median were calculated for the obtained data using Microsoft Excel (Redmond, WA, USA). The minimum and maximum measured concentrations and the number of samples above and below the minimum detectable activity (MDA) were summarised in the way described by Done and Ioan (6). Slope coefficients were determined for the calculated mean and median values for the years 2015–2022 using the linear regression method in Microsoft Excel. The fit of the function to the data was verified using the value of the coefficient of determination R² with the same software. It was judged against the generally accepted criterion for a satisfactory or very good fit of 0.6 < R2 < 1. Concentrations over time were additionally checked using the nonparametric Mann–Kendall (M-K) test published by Salmi et al. (28). The normality of distribution for each of the three studied groups was evaluated with the Kolmogorov–Smirnov (K-S) statistical test in Statistica v. 13.1 (Dell, now TIBCO, Palo Alto, CA, USA).

A concentration factor is commonly used to quantify the transfer of radionuclides into meat (23). The transfer of radionuclides from the environment to the bodies of wild animals has been described in the literature several times (1, 15). It was assumed to be a dependence on the concentration ratio (CR) and is described by the following formula: CR= Activity concentration in whole organism (Bq/kg fresh weight ) Activity concentration in soil (Bq/kg dry weight )$${\rm{CR}} = {{{\rm{\;Activity concentration in whole organism\;}}({\rm{Bq}}/{\rm{kg\;fresh weight\;}})} \over {{\rm{\;Activity concentration in soil\;}}({\rm{Bq}}/{\rm{kg\;dry weight\;}})}}$$

To calculate the CR, making the same assumption as Beresford et al. (3), Kapala et al. (20) and Yankovich et al. (32), the concentration of muscle activity was assumed to be the concentration of activity of the whole organism. Data from the Central Laboratory for Radiological Protection (CLOR) published in annual monitoring reports on the concentration of Cs-137 in soil were used to calculate the CR in the individual years 2016, 2019 and 2020 (5, 17).

Estimating the dose of Cs-137 absorbed by an adult through consumed game is extremely difficult because data is lacking on the amount of game consumed in Poland. Therefore, 1 kg was assumed for the calculations as the annual intake for an adult. The literature contains information obtained from hunters from north-eastern Poland. Kapala et al. (20) stated that the annual consumption of game by an adult is 20–50 kg. The effective dose E_ff consumed in contaminated meat by the reference adult resulting from radionuclide uptake was calculated using the following formula: Eff=AM×k×m$${{\rm{E}}_{{\rm{ff}}}} = {\rm{AM}} \times {\rm{k}} \times {\rm{m}}$$ where E_ff is the effective dose (Sv), AM is the arithmetic mean Cs-137 concentration in muscles in Bq/kg, k is the adult dose conversion factor of 1.3 X 10⁻⁸ Sv Bq⁻¹ used for Cs-137 (16) and m is the annual intake by adults (kg).

The research conducted over the stated 2015–2022 timeframe provided valuable information and enabled the creation of a large database. Every year in the studied period, a certain number of determinations were made in wild boar, roe deer and red deer muscle samples (Fig. 1). The measurements of these samples facilitated the determination of average concentrations of caesium-137 as well as the determination of its minimum and maximum concentration in each tested matrix (Table 1).

The K-S statistical test showed the lack of normal distribution for each of the three studied groups. For wild boar muscles, the K-S test parameters were d = 0.42594 and P-value < 0.01, for roe deer muscles they were d = 0.292960 and P-value < 0.01, and for deer muscles they were d = 0.27901 and P-value < 0.01. Figure 3 shows the average concentrations of caesium-137 in samples of wild boar, roe deer and red deer muscle and time trends covering the period from 2015 to 2022.

Fig. 3.

The regression analysis performed for wild boar muscles indicated a slow decreasing trend for the medians (slope coefficient −0.441 Bq/kg), which was also confirmed by the M-K test, which indicated a negative trend. However, regression analysis for average values apparently indicated an increase in concentrations over time (slope coefficient 1.405 Bq/kg), and the M-K test did not confirm statistically significant changes and indicated no trend over time. These discrepancies may have resulted from the occurrence of individual high concentrations in individual samples, which would have significantly affected the arithmetic mean. This also caused the coefficient of determination for the mean to be very low (3%) and indicated unsatisfactory fit. The coefficient of determination for the median was higher and amounted to (48%), but it was also not satisfactory in fit.

In roe deer muscles, the mean and median showed a downward trend (respectively −1.234 and −0.441 Bq/kg), while the M-K test conducted in both cases did not confirm statistically significant changes. The coefficient of determination for the means indicated satisfactory agreement (75%), while for the median it was not satisfactory and amounted to 48%.

The regression analysis performed for deer muscles indicated a slow decreasing trend for the medians (−1.234 Bq/kg) and mean values (−6.131 Bq/kg). The M-K test for mean values showed a negative trend, while the regression coefficient was unsatisfactory (43%). For the median, the M-K test did not confirm any significant statistical changes, but the regression coefficient was satisfactory and amounted to 75%.

Figure 4 shows two maps of Poland. The map on the left (Fig. 4A) is reproduced from the 2011 edition of the Radiation Atlas of Poland (17). It presents the distribution of Cs-137 concentration in the surface layer (0–10 cm) of soil in Poland and shows that Poland is contaminated with this radionuclide in an uneven manner. The illustration on the right (Fig. 4B) shows the highest recorded concentrations of caesium-137 in the muscles of game animals in 2015–2022. Even though almost 40 years have passed since the Chernobyl accident, there are similarities between the areas contaminated after the disaster and the areas where the highest concentrations of caesium-137 were recorded in the muscles of game animals in the years 2015–2022. Extensive literature data confirm the levels of Cs-137 concentrations in soil occurring in and characteristic of specific regions of the country (20, 24).

Fig. 4.

Caesium-137 activity was determined in 508 wild boar muscle samples. In 321 samples (63.2%), the results exceeded the MDA value, and in 9 (1.8%) the measured concentrations exceeded the applicable limit (600 Bq/kg). The highest measured wild boar muscle concentration was 4,195 ± 372.0 Bq/kg. The average concentration in this matrix was 42.84 Bq/kg, the median was 2.370 Bq/kg and the calculated standard deviation showed a value of 251.4 Bq/kg.

In the 145 roe deer muscle samples tested, 96 samples (66.2%) showed caesium-137 activity above the MDA value. However, in no case was the foodstuff limit of 600 Bq/kg exceeded. The highest concentration recorded in roe deer muscles was 111.5 ± 12.50 Bq/kg, and the average measured concentration was 9.909 Bq/kg, while the median was 3.680 Bq/kg. The standard deviation calculated for roe deer muscle samples was 16.11 Bq/kg.

The last group of game animals examined were European red deer. A total of 205 muscle samples were tested. Measurement was possible in 127 samples (62%) of their caesium-137 concentrations, which, as in the case of the concentrations in roe deer muscles, did not exceed the MDA and averaged 5.265 Bq/kg during the period examined. The median was 2.570 Bq/kg. The highest concentration recorded was 86.67 ± 3.470 Bq/kg. The calculated standard deviation value for the tested red deer muscle samples was 9.952 Bq/kg.

Table 2 presents a summary of the analyses performed and the data obtained for the indicated matrices, i.e. average concentration, minimum measured concentration, maximum measured concentration, standard deviation, median and the number of samples above and below the MDA.

The collected data on the concentration of Cs-137 in the soil for the years 2016, 2019 and 2020 sufficed for the determination of the CR for each tested matrix (Table 3). For the tested samples of wild boar muscles, the determined ratio was always above 1.000 Bq/kg, with the highest value (4.365 Bq/kg) recorded in 2016. However, for the tested muscle samples of both roe and red deer, the determined ratio was considerably lower at 0.500 Bq/kg, except in 2016, when it was 1.068 Bq/kg in roe deer tissue. With the collected data on the average concentration of Cs-137 in the years 2015–2022 in the muscles of wild boar, roe deer and red deer, it was possible to determine the effective equivalent doses. The estimated doses of Cs-137 absorbed by an adult through consumed game ranged from 0.209 to 1.040 μSv. For the roe deer and red deer muscle samples tested, the estimated dose did not exceed 0.213 μSv except in one case: in red deer tissue in 2015 (Table 3).