Investigation of the Impact of CO₂ on Water-Rock Interactions Using Sequential Extractions

Abstract

INVESTIGATION OF THE IMPACT OF CO2 ON WATER-ROCK INTERACTIONS USING SEQUENTIAL EXTRACTIONS by Sarah R. Beers Release of CO2 into groundwater has an impact on groundwater geochemistry, including a decrease in pH, and increases in alkalinity and cation concentrations. From an experiment in Bozeman, Montana, it was concluded that the cations were released by interactions between groundwater, CO2, and aquifer sediment. To evaluate these interactions, ions from the aquifer sediment were sequentially extracted to determine which cations and trace metals were associated with different phases in the sediment. These phases included ion exchange sites, carbonate, oxide, and sulfide minerals, and organic material. The data were then compared with the groundwater chemistry data to determine the likely reactions occurring in the aquifer with the addition of CO2. Geochemical groundwater modeling revealed that the groundwater was undersaturated with respect to calcite both before and during the addition of CO2. Thus, calcite dissolution was contributing to the groundwater chemistry. In addition to calcite dissolution, a decrease in pH in the aquifer affected ion exchange and manganese oxide dissolution. Comparisons between the sequential extraction results and groundwater chemistry showed that uranium and barium were closely related to carbonate dissolution, while magnesium and strontium were primarily contributed by ion exchange. Also, manganese oxides contributed a majority of the cobalt in the system. Ion exchange, carbonate dissolution, and manganese oxide dissolution are responsible for a majority of the groundwater geochemical changes as a result of CO2 injection.