Coastal alkalinity addition within safe environmental thresholds: numerical experiments in Halifax Harbour and surrounding areas (Canada)

Surface ocean alkalinity enhancement (OAE), through the release of alkaline materials (feedstock), is an emerging abiotic marine technology for marine carbon dioxide removal that could increase the storage of anthropogenic carbon in the ocean. Alkaline feedstock may, in theory, be released at any location in the surface ocean, but the use of pre-existing coastal infrastructures (e.g., sewage outfalls, cooling pipes) is cost efficient and lowers the emissions associated with the transport of feedstock. Release at these locations is regulated and needs to occur within safe environmental thresholds. It is therefore essential to understand how point source feedstock release alters the carbonate system to 1) maximize dosing while 2) ensuring the resulting perturbations remain within the safe zone of carbonate system parameters. Influencing factors may be the dosing level, the type of feedstock, pipe design and proximity of neighboring pipes, the background state of the carbonate system, and local circulation. Given the spatial distribution of some of these factors, their importance may vary regionally. Here, we use a coupled physical-biogeochemical model that is specifically designed for coastal OAE research to investigate where and how dosing can breach environmental thresholds in the Halifax Harbour and surrounding coastal areas. Simulations with various dosing rates and release sites are carried out and their results analyzed with respect to environmental thresholds (pH>9, precipitation risk). Benthic exposure to feedstock (particulate phase) is also considered.