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

Abstract 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 CO 2 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 CO 2 injection. Laboratory results included rapid changes in pH (8.6 to 5.7), alkalinity (243 to 1295 mg/L as HCO 3 ), 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 CO 2 injection. These chemical changes, which are in general agreement with those obtained from sampling the ZERT monitoring wells, could provide early detection of CO 2 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.