Cs-137 in game

The radiocesium content that accumulates in the muscles of wild animals is determined by the amount of this nucleid in the food the animal ingests as well as by the kinetics of the nucleid in the animals’ bodies. This Cs-137 input is controlled by the following factors:

Amount of food eaten per time unit
Amount of activity in the forage plants Both variables are in turn influenced by such factors as climate, site conditions etc.

Roe deer and red deer almost exclusively feed on above ground vegetation and every now and then some mushrooms. Their food spectrum depends on the seasonally available vegetation which leads to corresponding fluctuations in their contamination patterns.

The food choice of wild boar is in comparison much more complicated and is determined by many factors so that a uniform seasonal variation in contamination levels cannot be given. This, however, does not at all affect the long term trend – the stagnation of the Cs-137 concentration at a relatively high level.

Cs-137 in wild boar
Figure 1 shows the temporal course of Cs-137 activity in wild boar from the investigation area Bodenmais and an adjacent hunting district in Zwiesel from 1987 – 2003. The soil contamination is similarly high in both areas.

The Cs-137 concentration in the wild boar stagnated at a high level during the investigation period. Thus the average Cs-137 activity of wild boar during the first half year of 2001 was 8,990 Bq/kg fresh meat (n = 7) which was higher than for the whole year 1987 with 7,240 Bq/kg (n = 14). The slope of the regression lines shows that the contamination of the animals increased. Due to the discontinuous distribution of the measured data during the investigation time period and due to the low numbers of samples obtained over the last years, these results should only be regarded as indicating a trend.

Fig. 1: Temporal course of Cs-137 in wild boar from Bodenmais and Zwiesel,
1987 to 2003 (n = 213)

Samples of wild boar were also examined from the investigation area of Göttingen in 2000. The Cs-137 values ranged from 1.5 to 3.1 Bq/kg (n = 4) with an average value of 2.2 Bq/kg. These activities could be some of the lowest Cs-137 values among wild boar in Germany, in contrast to which the results for Bodenmais are among the highest. The contamination of wild boar thus varies within 4 orders of magnitude.

Cs-137 in red deer
Figure 2 presents the course over time of Cs-137 activity in red deer from the investigation area of Bodenmais and an adjacent hunting district in Zwiesel, from where the wild boar samples were also taken.

The state forestry office Bodenmais is a district without red deer i.e. red deer are not tolerated here. When red deer migrate into the district, they must be shot. Hence, the numbers of samples of this species for investigation here are correspondingly low.

Fig. 2: Course over time of Cs-137 activity in red deer from Bodenmais and Zwiesel
with regression line, 1987 to 2000 (n = 172)

The Cs-137 activity of red deer distinctly decreased during the investigation period. The samples from 1987 contained on average 1,510 Bq/kg (n = 32), those taken since 1995 always had less than 1,000 Bq/kg. An effective half-life of 5.7 years can be calculated from the regression line, the reduction in contamination is highly signficant (P<0.0001).

Cs-137 in roe deer
In the muscles of roe deer the Cs-137 activity has a pronounced seasonal variation every year, with low values in the spring and clearly higher values in the autumn. In Fig. 3 the distribution of Cs-137 activity in roe deer from Bodenmais between the years 1987 to 2001 is presented in terms of the monthly median values. The differences between the minimum contamination in the spring and themaximum contamination in the fall are considerable, equivalent to one order of magnitude.

Since the biological half life of Cs-137 in deer is relatively low (ca. 10 days), the concentration of this nucleid in the animals also decreases rapidly after the first snow. In general more than 1 m of snow falls during the winter in Bodenmais. At this time deer mainly feed at feeding stations in which little contaminated food is present. In addition they browse on the buds of bushes and saplings which rise above the snow, or on other forest plants which they scrape out of the snow. The buds of plants are usually less contaminated than the leaves of most forest plants during the growing season. Consequently the muscles of the roe deer only contain low amounts of Cs-137 during the winter. In spring the roe deer begin feeding on forest plants with relatively high concentrations of Cs-137 and so start the opposite cycle of increasing Cs-137 contamination in their muscles.

Fig. 3 Course over time of Cs-137 activity in roe deer meat from Bodenmais.
Monthly medians from 1987 to 2003 (n = 1,557)

From 1987 to 2001 the Cs-137 activity in roe deer decreased. The highest median values were measured in October,1987, 7.040 Bq/kg. In 1993 the highest monthly median in October was 3,810 Bq/kg, and in 1999 in September it was 1,220 Bq/kg.

In the year 2000 a distinct increase in concentrationn during the month of November, 3.370 Bq/kg was observed, the highest monthly median since 1993.

An effective half-life of Cs-137 of 6 years was determined for roe deer (n = 1,460) for the total investigation period from 1987 to 2001 (P< 0.0001). This value results from the combination of 2 phases with greatly differing dynamics: in the first phase from 1987 to 1995 a rlelatively rapid reduction in activity was observed with a half life of 4.6 years (n = 1.091, P< 0.0001); while a distinct slowdown in the reduction of activity occurred, with a half life of 89 years (n = 338, P<0.8454) from 1996 to 2000. Although this result is not significant and is particularly influenced by the high contamination levels of the deer in 2000, a trend toward longer effective half lives is evident since 1996.

Four samples were analyzed from the investigation area Göttingen in October 2000, the month in which Cs-137 activity in roe deer is normally most pronounced. All samples contained less than 0.5 Bq/kg. The Cs-137 contamination in deer as well as in wild boar varies over a span of 4 magnitudes within the Federal Republic of Germany.

This research was conducted with funds of the Federal Ministery for Environment. Nature Protection, and Reactor Safety.
This report reflects the views and opinions of the contractor and need not necessarily correspond to those of the sponsor.

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