90 Sr and stable element levels in bones of brown bears: long-term trends in bear populations from Croatia and Poland

The aim of this study was to investigate the temporal trends and geographical differences in 90 Sr and stable element (Ba, Ca, Mn, Sr, Pb, Zn) levels in the bones of Croatian and Polish brown bear ( Ursus arctos ) populations. Experimental data suggest that in the decades after nuclear weapon tests and the Chernobyl accident, 90 Sr bone activity concentrations decreased from 352 to 11 Bq kg −1 in the Croatian bear population (period 1982–2015) and from 831 to 27 Bq kg −1 in Polish bears (period 1962–2020). Calculated effective and ecological half-lives were 9 and 13 years for Croatian bears, and 15 and 31 years for Polish bears, respectively. Different temporal trends were noted in levels of Ba, Mn, Pb and Zn between the two countries with majority of bones having lower Pb, Sr and Zn in Croatian than in Polish bears. Estimated values for the soil-to-bear transfer of 90 Sr were the same order of magnitude in the studied populations. Contrary to this, the estimated transfer of stable Sr was an order of magnitude lower for the Croatian bear population compared to Polish bears. The observed differences in soil-to-bear transfer between stable Sr and 90 Sr found for Croatian bears might suggest the need for careful consideration on the use of stable Sr data as an analogue for 90 Sr. To our knowledge, this is the first study that analysed 90 Sr activity in tissue of brown bears. As such, it provides insight into the fate and behaviour of one of the most relevant anthropogenic radionuclides at the top of the food chain.