Tracking local radiocarbon releases from nuclear power plants in southern Ontario (Canada) using annually-dated tree-ring records

Understanding the radiocarbon (¹⁴C) content of atmospheric carbon dioxide (CO₂) underlies many fields of research. This study shows how one can track the influence of nuclear power plants at the local and regional scale in Canada’s largest urban area. This area is subject to significant ¹⁴CO₂ depletion due to CO₂ emissions from fossil fuel burning. Tree cores collected across southern Ontario in 2018 show that tree-rings dated annually record the same decadal trends as atmospheric measurements at a background site. Tree cores taken closer to ¹⁴CO₂ or fossil fuel sources reflect those local influences. Data of ¹⁴C from a site 20 kilometer downwind from a nuclear power plant is highly correlated (R²=0.76) with annually reported emissions from the plant for 2009-2018. Extending the analysis back to the 1990s shows that the emissions of ¹⁴CO₂ were 4-8 times higher than those at present. At that time, ¹⁴CO₂ emissions were sufficiently strong to affect a remote background monitoring site. Concerning the urban fossil fuel CO₂ emission signature, ¹⁴CO₂ in the cellulose of a tree from downtown Toronto has an extremely depleted Δ¹⁴C signature. The local signal from traffic emissions overshadows any ¹⁴CO₂ from nuclear emissions in the region. This study suggests that, with more cores to reflect the emissions of nuclear ¹⁴CO₂ before 1990 and more suitable urban sampling locations (i.e., representative of a neighborhood rather than one road), this approach has potential to better track the long-term impact of urbanization and nuclear power plants in Canada, and potentially elsewhere around the globe.