Regional variation in radionuclide concentrations and radiation dose in caribou (Rangifer tarandus) in the Canadian...

Gamma-emitting radionuclides and 210Po, an alpha-emitting product of the 238U decay chain, were analysed in tissues from 16 caribou (Rangifer tarandus) collections in the Canadian North between 1992 and 1994. The study was conducted to determine the regional variability of anthropogenic and natural radionuclides in Canadian caribou, and to estimate the background radiation dose to caribou tissues. 137Cs, 40K, 210Pb and 210Po were consistently found in most herds. Mean muscle 137Cs varied from below detection limits on Banks Island to 231 Bq kg-1 wet weight (ww) in the Beverly herd in the central Arctic. Comparison of 1992-94 levels with published values from the 1960s and 1980s showed that 137Cs in caribou is declining with an effective half-life (Teff) of 9.9 years. The highest mean bone and liver 210Pb activities were observed on south Baffin Island, which included one bone sample with 3800 Bq kg-1 ww of 210Pb and 3070 Bq kg-1 ww of 210Po. The distribution of 137Cs and 210Pb between herds was attributed to higher atmospheric deposition rates between 60 and 65 degrees N, and changes in plant community structure and lichen species composition. The highest mean absorbed dose (30 mGy year-1) was observed in 1993 in the liver of caribou from Lake Harbour. This dose translated into a weighted absorbed dose of 300 mGy year-1, assuming a radiation weighting factor (wR) of 10 to account for the increased biological efficiency of alpha particles for deterministic effects. The Lake Harbour site also had the highest individual weighted absorbed dose in bone (810 mGy year-1) and liver (530 mGy year-1) in the study. There was no significant relationship between bone 210Pb activity and age for individual or pooled herds, indicating that the estimated doses are probably present throughout the lifetime of the caribou. Because some herds number several hundred thousand individuals, collective doses may also be very high. As yet, there have been no reports of observable effects due to these relatively high exposures and many of the herds continue to thrive and increase in size.