Mixing and dilution controls on marine CO2 removal using alkalinity enhancement

Marine CO₂ removal (CDR) using enhanced-alkalinity seawater discharge was simulated in the estuarine waters of the Salish Sea, Washington, US. The high-alkalinity seawater would be generated using bipolar membrane electrodialysis technology to remove acid and the alkaline stream returned to the sea. Response of the receiving waters was evaluated using a shoreline resolving hydrodynamic model with biogeochemistry, and carbonate chemistry. Two sites, and two deployment scales, each with enhanced TA of 2997 mmol m⁻³ and a pH of 9 were simulated. The effects on air-sea CO₂ flux and pH in the near-field as well as over the larger estuary wide domain were assessed. The large-scale deployment (addition of 164 Mmoles TA yr⁻¹) in a small embayment (Sequim Bay, 12.5 km²) resulted in removal of 2066 T of CO₂ (45% of total simulated) at rate of 3756 mmol m⁻² yr⁻¹, higher than the 63 mmol m⁻² yr⁻¹ required globally to remove 1.0 GT CO₂ yr⁻¹. It also reduced acidity in the bay, ΔpH ≈ +0.1 pH units, an amount comparable to the historic impacts of anthropogenic acidification in the Salish Sea. The mixing and dilution of added TA with distance from the source results in reduced CDR rates such that comparable amount 2176 T CO₂ yr⁻¹ was removed over >1000 fold larger area of the rest of the model domain. There is the potential for more removal occurring beyond the region modeled. The CDR from reduction of outgassing between October and May accounts for as much as 90% of total CDR simulated. Of the total, only 375 T CO₂ yr⁻¹ (8%) was from the open shelf portion of the model domain. With shallow depths limiting vertical mixing, nearshore estuarine waters may provide a more rapid removal of CO₂ using alkalinity enhancement relative to deeper oceanic sites.