Numerical dye tracer experiments in Bedford Basin in support of Ocean Alkalinity Enhancement research

Ocean Alkalinity Enhancement (OAE) is considered as a potential technique to mitigate ocean acidification and remove carbon dioxide (CO₂) from the atmosphere. In this study, a suite of numerical tracer experiments was conducted using a high-resolution nested model to support ongoing OAE field trials in Halifax Harbor and Bedford Basin. We first estimated the residence time, which provides an overall description of the circulation, for different seasons over the past 20 years (2003-2022). Results show a clear seasonal pattern in residence time which is longest in July and shortest in January. Particles with different dissolution rates and sinking velocities were then added continuously through the cooling outfall of a local power plant for three months to simulate the dissolution, dispersion, and movement of different alkaline mineral feedstocks. To account for inter-annual variability, the years with the longest and shortest residence time in each season were selected to perform these simulations. Furthermore, tracer simulations will be compared with ongoing Rhodamine WT field trials. Results obtained thus far show that the surface alkalinity signal due to OAE is most likely to be detected near the cooling outfall but depends on the tidal stage and the local circulation and weather conditions. Detectability is highest in July because the residence time is longest. In addition, the detectability increases with faster dissolution rate and slower sinking velocity.