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 CO2 migration on shallow groundwater chemistry, the isotope composition of dissolved inorganic carbon (δ13CDIC) and dissolved strontium (87Sr/86Sr) were evaluated as tracers. Dissolved CO2 in groundwater was introduced using a closed-loop dipole-style well field situated in a shallow sand-dominated aquifer. Baseline δ13CDIC values, oxygen and hydrogen isotope ratios, and 87Sr/86Sr 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 CO2. Baseline groundwater δ13CDIC-PDB, oxygen (δ18OSMOW) and hydrogen (δDSMOW) stable isotope values averaged −17, −4.1 and −19.5‰, respectively. Groundwater 87Sr/86Sr baseline values averaged 0.70840 at MW-3 and 0.70818 at MW-2. Arrival of the dissolved CO2 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 δ13CDIC-PDB rose to an average steady-state value of 0.16 ± 0.3‰ during the test; δ18O and δD of water did not change from their baseline values. 87Sr/86Sr 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 δ13CDIC and the 87Sr/86Sr of dissolved strontium served as excellent tracers of plume movement during this experiment.