Tracing CO₂ leakage and migration using the hydrogeochemical tracers during a controlled CO₂ release field test

A critical environmental issue in carbon capture and storage (CCS) is potential CO₂ leakage, which accompanies geochemical reactions with aquifer materials. To investigate the hydrogeochemical effects of CO₂ leakage on shallow groundwater at the early stage of CO₂ leakage and to evaluate a hydrochemical or isotopic tracer for CO₂ migration, a controlled CO₂ release experiment was performed in a siliciclastic aquifer at the Environmental Impact Evaluation Test (EIT) site, South Korea. After the baseline survey of hydrochemical and carbon isotope (δ¹³C_DIC) compositions, CO₂-infused water was injected at a rate of 5.5 m³/day for 26 days at ∼22 m below ground level in the ∼40 m thick heterogeneous aquifer whose pressure gradient was increased to approximately 10 times the natural gradient to make a flow path along monitoring wells. The arrival of CO₂ plume was determined at each monitoring well by the decrease in pH and the increase in the partial pressure of CO₂ (P_CO2) and EC. δ¹³C_DIC decreased at the arrival of CO₂ plume and showed high correlations with log P_CO2 since the δ¹³C_DIC of injected CO₂ (−24.7‰) was distinct from that of ambient groundwater (−16.7‰) and little carbon sources existed in the aquifer. The spatial and temporal evolution of hydrochemical and isotopic compositions observed using a monitoring well network indicated that the CO₂ plume migrated along a preferential pathway overwhelming induced pressure gradient due to water table mounding at injection and that the plume sank to some degree probably due to its large density. Concentrations of hydrochemical elements displayed three types of behavior: (1) pulse-like with rapid increases at the arrival of CO₂ plume and decreases despite the continuous injection of CO₂ similar to EC (HCO₃, Ca, Mg, Na, K, Sr, and Ba), (2) pH dependent with relatively slow increases and decreases in concentrations (SiO₂ and Mn), and (3) rapid increases but slow decreases (Li). The hydrochemical variations indicated the dissolution of a limited amount of reactive minerals such as calcite, followed by cation exchange at the early stage of CO₂ leakage in siliciclastic aquifers. Based on the study result, Li was an effective hydrogeochemical tracer to monitor the migration of CO₂ in siliciclastic aquifers as well as pH, EC, and δ¹³C_DIC.