Geochemical framework for CO2 mineralization in coastal aquifers: Lessons from the Coda Terminal project, Iceland

• The Coda Terminal project in Iceland aims to store 3 MtCO 2 /yr in basaltic subsurface. • The geochemical exploration work shows high potential for CO 2 mineralization. • The groundwater salinity stratification is more complex than previously thought. • Both freshwater and seawater are suitable for CO 2 injection. • Legislation on groundwater protection is still being developed for big scale CO 2 mineralization projects. Freshwater scarcity can limit the scalability of subsurface CO 2 mineralization projects which rely on dissolving CO 2 in water before injection into basaltic reservoirs. While ongoing Carbfix projects primarily use freshwater or process water such as geothermal condensate to dissolve CO 2 , in the Coda Terminal project, in Straumsvík, SW Iceland, it is proposed to use seawater due to its availability. The storage reservoir contains a mix of freshwater, brackish, and saline groundwater, with measured conductivities ranging from ∼100 to ∼40,000 μS/cm. Water chemistry of the brackish and saline water indicates extensive water-rock interaction with depletion of Na, K, B, and Mg, and strong enrichment in Ca - essential for calcite precipitation, the dominant carbonate mineral in low-temperature basalt alteration. Dissolution experiments conducted on Straumsvík basalt show similar trace element mobility in both freshwater and seawater, with only Al, Fe, and Mn exceeding WHO drinking water limits, suggesting low risk of groundwater contamination. Reaction path modelling indicates that mineralization efficiency can reach 100% for both water types, and that mixing of CO 2 -charged injection water with reservoir water does not significantly affect water chemical evolution or secondary mineral formation. These results support the feasibility of using seawater for CO 2 storage in basaltic settings and high potential for CO 2 mineralization in Straumsvík. However, complex groundwater stratification and regulatory framework highlight the need for integrated hydrogeological, geochemical, and regulatory planning to ensure safe and effective implementation of large-scale CO 2 mineral storage at coastal sites.