Changes in the Chemistry of Groundwater Reacted with CO2: Comparison of Laboratory Results with the ZERT Field Pilot

As part of the ZERT program, sediments from two wells at the ZERT site, located in Bozeman, Montana, USA were reacted with a solution having the composition of local groundwater. A total of 50 water samples were collected from 7 containers placed for 15 days in a glove box with one atmosphere of CO₂ to investigate detailed changes in the concentrations of major, minor and trace inorganic compounds, and to compare these with changes observed in groundwater at the ZERT site following CO₂ injection. Laboratory results included rapid changes in pH (8.6 to 5.7), alkalinity (243 to 1295 mg/L as HCO₃), electrical conductance (539 to 1822 μS/cm), Ca (28 to 297 mg/L), Mg (18 to 63 mg/L), Fe (5 to 43 μg/L) and Mn (2 to 837 μg/L) following CO₂ injection. These chemical changes, which are in general agreement with those obtained from sampling the ZERT monitoring wells, could provide early detection of CO₂ leakage into shallow groundwater. Dissolution of calcite, some dolomite and minor Mn-oxides, and desorption/ion exchange are likely the main geochemical processes responsible for the observed changes.