Understanding the evolution of oil sands tailings deposits using tritium-helium dating and noble gases

Oil sands tailings are deposited in tailings ponds to allow fines to settle over time. Knowledge of the age of tailings allows for an improved determination of the settling rates of tailings as well as an understanding of geochemical processes within the tailings ponds. In this study we tested the use of the tritium (3H) – helium (3He) age dating method to date pore water of oil sands tailings deposits. This method exploits the decay of naturally occurring 3H and its radioactive daughter product 3He to calculate the ages of tailings. Samples were collected from four oil sands tailings ponds. Many of the samples have noble gas concentrations that were below the air equilibrated water concentrations and some samples were supersaturated in methane (CH4). The concentrations are interpreted to be lower than expected as a result of stripping by the production of CH4. When samples supersaturated in CH4 are excluded, the calculated ages of Pond A samples ranged from 1.5 ± 0.1 to 32.0 ± 0.9 years, Pond B ranged from 5.3 ± 0.3 to 23.0 ± 0.8 years, Pond C ranged from 0 ± 0 to 6.5 ± 0.3 years, and Pond D ranged from 16.6 ± 0.6 to 27.3 ± 0.8. The tailings samples obtained from Pond A, Pond C and Pond D show a general trend of increasing tailings age with depth. •First use of tritium-helium dating to date oil sands tailings.•Samples commonly undersaturated in noble gases due to stripping associated with CH4 production.•Samples generally within expected age range based on ages of tailings ponds.