On-site dissolved-gas analysis and electric-resistivity tomography as new tools to trace $$\hbox {CO}_{2}$$ mineral sequestration in aquifers

Mineral sequestration of [Formula: see text] in basalt aquifers is a promising pathway for permanent carbon storage. Within the DemoUpStorage R&D project, [Formula: see text] was dissolved in saline groundwater and co-injected with He as an inert tracer into a deep basalt aquifer at a pilot injection site near Helguvik, Iceland. We tested a combination of two novel geochemical and geophysical tools to monitor the subsurface [Formula: see text] dynamics. Real-time, on-site monitoring of dissolved gases was conducted over one year using a gas-equilibrium membrane-inlet mass spectrometer (GE-MIMS). Electrical resistivity tomography (ERT) was used to track changes in aquifer resistivity before and after injection. GE-MIMS measurements revealed a clear increase in He concentrations downstream of the injection point, confirming fluid transport. However, no corresponding [Formula: see text] increase was observed, indicating substantial [Formula: see text] retardation relative to the fluid transport. No anomalies in He or [Formula: see text] were detected in the overlying freshwater aquifer, indicating minimal upward migration. ERT data showed localized resistivity decreases suggesting basalt dissolution near the injection borehole. Together, GE-MIMS and ERT provided complementary, resource-efficient insights into [Formula: see text] transport, reaction, and storage processes in the basalt aquifer. These tools enhance monitoring and assessment capabilities, supporting the development of safe and effective geological [Formula: see text] storage.