Potential risks to freshwater resources as a result of leakage from CO2 geological storage: a batch-reaction experiment

In assessing the feasibility of widespread deployment of CO2 geological storage, it is prudent to first assess potential consequences of an error or accident that could lead to CO2 leakage into groundwater resources above a sequestration interval. Information about the sensitivity of the groundwater system to introduction of CO2 is needed in order to design groundwater monitoring program. A laboratory-batch experiment was conducted to explore the range of CO2 impact on groundwater quality of a spectrum of representative aquifers, in the Gulf Coast region, USA. Results show that CO2 elevated concentrations of many cations within hours or days. Two types of cations were recognized according to their concentration trends. Type I cations-Ca, Mg, Si, K, Sr, Mn, Ba, Co, B, Zn-rapidly increased following initial CO2 flux and reached stable concentrations before the end of the experiment. Type II cations-Fe, Al, Mo, U, V, As, Cr, Cs, Rb, Ni and Cu-increased at the start of CO2 flux, but declined, in most cases, to levels lower than pre-CO2 concentrations. Dissolution of dolomite and calcite caused the largest increase in concentrations for Ca, Mg, Mn, Ba and Sr. Cation release rates decreased linearly as pH increased during mineral buffering. Experiment results suggest that carbonate minerals are the dominant contributor of changes in groundwater quality. Risk assessments of potential degradation of groundwater and monitoring strategies should focus on these fast-reacting minerals. Mobilization risk of Type II cations, however, may be self-mitigated because adsorption occurs when pH rebounds.