CO sub(2) leakage impacts on shallow groundwater: Field-scale reactive-transport simulations informed by observations at a natural analog site

It is challenging to predict the degree to which shallow groundwater might be affected by leaks from a CO sub(2) sequestration reservoir, particularly over long time scales and large spatial scales. In this study observations at a CO sub(2) enriched shallow aquifer natural analog were used to develop a predictive model which is then used to simulate leakage scenarios. This natural analog provides the opportunity to make direct field observations of groundwater chemistry in the presence of elevated CO sub(2), to collect aquifer samples and expose them to CO sub(2) under controlled conditions in the laboratory, and to test the ability of multi-phase reactive transport models to reproduce measured geochemical trends at the field-scale. The field observations suggest that brackish water entrained with the upwelling CO sub(2) are a more significant source of trace metals than in situ mobilization of metals due to exposure to CO sub(2). The study focuses on a single trace metal of concern at this site: U. Experimental results indicate that cation exchange/adsorption and dissolution/precipitation of calcite containing trace amounts of U are important reactions controlling U in groundwater at this site, and that the amount of U associated with calcite is fairly well constrained. Simulations incorporating these results into a 3-D multi-phase reactive transport model are able to reproduce the measured ranges and trends between pH, pCO sub(2), Ca, total C, U and Cl super(-) at the field site. Although the true fluxes at the natural analog site are unknown, the cumulative CO sub(2) flux inferred from these simulations are approximately equivalent to 37.8E-3 MT, approximately corresponding to a .001% leak rate for injection at a large (750 MW) power plant. The leakage scenario simulations suggest that if the leak only persists for a short time the volume of aquifer contaminated by CO sub(2)-induced mobilization of U will be relatively small, yet persistent over 100 a.