Tracing CO2 leakage into groundwater using carbon and strontium isotopes during a controlled CO2 release field test

During a carbon sequestration field study to simulate the impact of CO 2 migration on shallow groundwater chemistry, the isotope composition of dissolved inorganic carbon ( δ 13 C DIC ) and dissolved strontium ( 87 Sr/ 86 Sr) were evaluated as tracers. Dissolved CO 2 in groundwater was introduced using a closed-loop dipole-style well field situated in a shallow sand-dominated aquifer. Baseline δ 13 C DIC values, oxygen and hydrogen isotope ratios, and 87 Sr/ 86 Sr values of groundwater were established in four monitoring wells (MW-1 to 4) and one up-gradient background well (BG-1) prior to the introduction of dissolved CO 2 . Baseline groundwater δ 13 C DIC-PDB , oxygen ( δ 18 O SMOW ) and hydrogen ( δ D SMOW ) stable isotope values averaged −17, −4.1 and −19.5‰, respectively. Groundwater 87 Sr/ 86 Sr baseline values averaged 0.70840 at MW-3 and 0.70818 at MW-2. Arrival of the dissolved CO 2 plume at the monitoring wells is modeled using a 1-D analytical equation, which yields breakthrough curves with flow velocities that are consistent with prior numerical modeling estimates. The δ 13 C DIC-PDB rose to an average steady-state value of 0.16 ± 0.3‰ during the test; δ 18 O and δ D of water did not change from their baseline values. 87 Sr/ 86 Sr dropped sharply by 0.00022 at MW-3 and 0.00005 at MW-2 in the first two weeks after plume arrival at the wells, and then slowly increased toward baseline values, correlating with the behavior of dissolved Na, K, Ca, Sr and Si. Carbonate dissolution and desorption from organic matter and Fe-bearing phases at the low-pH plume front is the likely mechanism producing this behavior. The δ 13 C DIC and the 87 Sr/ 86 Sr of dissolved strontium served as excellent tracers of plume movement during this experiment.