Alkalinity enhancement with sodium hydroxide in coastal ocean waters.

Carbon Dioxide Removal (CDR) is increasingly recognised as essential for achieving net zero emissions to limit the impacts of climate change. Ocean Alkalinity Enhancement (OAE) presents a potentially scalable marine CDR (mCDR) technique. Here we report on the first OAE field trial in Australia, conducted at a coastal site in Tasmania using continuous addition of aqueous sodium hydroxide (NaOH). The resulting plume of modified seawater was effectively tracked, and changes in surface carbonate chemistry were quantified using a containerised laboratory. At the point of NaOH release, partial pressure of CO[Formula: see text] (pCO[Formula: see text]) decreased by up to 370 μatm with alkalinity increasing by approximately 545 μmol kg[Formula: see text]. Maximum downstream decreases in pCO[Formula: see text] ranged from 22 to 77 μatm, corresponding to signal strengths of < 1 - 5%. This small-scale field trial confirmed that the dispersion of a plume of modified seawater occurs rapidly and within meters of the site of addition, and that with appropriate tools, these changes can be measured directly in a coastal ocean location. These results suggest that the deployment of shore-based OAE, in combination with local coastal infrastructure and regional models, have potential as an mCDR approach.