Contaminated boars bring physical and agricultural hazards

The Swedish environmental consultancy Calluna has alerted Gävle hunters about packs of “extremely” radioactive wild boar moving north of Stockholm (The Telegraph, 2017). Ulf Frykman told the Telegraph that the organization tested 30 samples of boar and found 24 above the safe level of radioactivity; some by over 10 times, reaching 16000 Bq/kg compared to the safe limit of 1500 Bq/kg.

“Wild boar root around in the earth searching for food, and all the caesium stays in the ground,” he explained, referencing the once airborne contamination from the 1986 Chernobyl nuclear reactor release. “It’s the highest we’ve ever measured.” In contrast, deer and elk feed higher in bushes where there’s less radiocaesium, though within months of the disaster some deer in the area were measured to have 50000 Bq/kg.

“They dig deep holes in the fields, and then you destroy your tractor,” said Mia Brodin, board member of a local farmers’ association.”The crops are destroyed and they eat food you put out for your livestock.”

Boar activity increases movement of radioactivity

Introduction of these wild boar into the area include both their set of farming challenges as well as unique radioactivity concerns. Though these levels are not high enough to produce immediate acute radiation damage like victims of nuclear warfare, the long-term exposure to lower levels of environmental radioactivity have been known to increase rates of cancer beyond national averages.

The article’s reported levels of specific radioactivity characterize the mass-density of radionuclide accumulation in organism as a result of their habits and other ecological processes, also known as bioaccumulation. In an effort to expand radiation protection from the conventional anthropogenic view, the International Atomic Energy Agency (IAEA) has been revising their handbook on ecological radionuclide transfer coefficients from food chain intrusions to those of other environmental entities (Howard et al., 2013). These coefficients are the ratio of equilibrium radioactivity concentrations in bulk organism to that of soil (both in Bq/kg) resulting from continuous bioaccumulation and redeposition mechanisms. The 2013 paper reports compiled data for over 12,000 terrestrial measurements, covering 10 nuclides with roughly 30% Cs observations and 22% mammals. These observations lend themselves to coefficients for the rat and deer RAP (reference animal and plants) models used by the International Comission for Radiation Protection (ICRP) for theoretical studies. Such values for terrestrial rangifers (199 observations) range from 0.59 to 21, with a geometric median of 1.4.

These values are important as they relay a long-term contamination effect of migration into a lesser contaminated area. Measurements of this coefficient are initially much higher; but as these entities undergo the same mixing process which enabled these particulates to accumulate, this concentration ratio recovers through a net deposition into area.

Modeling the deposited radioactivity from boar migration

By treating the contamination as a mass-balance equation, we can determine the amount of radioactivity in the soil which will be in equilibrium with the the amount of radioactivity in the boars. This calculation makes two crucial assumptions which would otherwise lead to a lower transfer to the soil: there are only two compartments (the boar and the soil, the air merely mediates), and there are no sinks which would permanently remove radioactivity (such as vegetation or boar’s internal organs). There is substantial evidence that former influences exist, and the latter compartment-models are successfully used in medical research for evaluating drug dose effects. Therefore, this “toy” model should be taken very lightly.

Using the median terrestial rangifers coefficient and the 16000 Bq/kg sample observation, we can expect lesser contaminated soil to reach 25 mBq to 25 Bq per kg of dry weight depending on the area of deposition, calculated here with 100 and 1 m radius respectively (Fig. 1). Taking into account the 30.08 year half-life of the decaying Cs-137, the deposited radioactivity would suffer up to a 28% reduction; this would occur over a period of time comparable to the rate of accumulation which may vary further on frequency of soil perturbing behaviors, the mixing bioturbation forces, but generally on the order of their 10 to 14 year lifespan.

To put this into context, a 2008 investigation into Swedish soil directly in the pathway of the plume from Chernobyl found a mean of 26 Bq per kg within the depth of 10 to 20 cm (Jarvis et al., 2010).

These calculations demonstrate the magnitude of impact depending on the area of effect- they don’t include how much radioactivity departs by other means, like wind resuspension and soil filtration. As boar are migratory and tend to have many soil interaction events, the model would certainly involve larger areas of effect. Arguably, one could characterize the average surface area a boar covers in its lifespan to obtain an appropriate order of magnitude for deposition, but again addition correction is needed to account for the removal by resuspension and organic uptake.

References

Howard BJ et al. The IAEA handbook on radionuclide transfer to wildlife. Journal of Environmental Radioactivity, 121:55-74; 2013. doi:10.1016/j.jenvrad.2012.01.027

Jarvis NJ et al. Modelling the effects of bioturbation on the redistribution of Cs-137 in an undisturbed grassland soil. European Journal of Soil Science, 61:24-34; 2010. doi:10.1111/j.1365-2389.2009.01209.x

Orange R. Radioactive wild boar spark concerns in Sweden 31 years after Chernobyl. The Telegraph; 2017. Available at http://www.telegraph.co.uk/news/2017/10/06/radioactive-wild-boar-spark-fear-sweden/. Accessed 7 Oct 2017.