Changes in the chemistry of groundwater reacted with CO2: Comparison of results from laboratory experiments and the ZERT field site, Bozeman, Montana, USA

Abstract The Zero Emission Research and Technology (ZERT) field experiment at Bozeman, Montana, USA, was a multi-disciplinary field pilot designed to evaluate near-surface CO 2 transport and monitoring technologies applicable to its potential leakage from deep storage reservoirs. As part of this program, sediment cores from two wells at the site were reacted with a solution having the approximate composition of groundwater from the site. A total of 50 water samples were collected from 7 containers, one with its sediments saturated with Na, placed for 15 days in a glove box with 1 atm of food-grade CO 2 . The objective of this laboratory study is to investigate the changes in the concentrations of major, minor and trace inorganic compounds, and to compare these changes with those observed in groundwater at the site following CO 2 injection. The aim of the Na saturated experiment was to investigate the role of sorption/ion exchange on the observed chemical changes. Laboratory results showed rapid changes in pH (8.6–5.7), alkalinity (243–1295 mg/L as HCO 3 ), and electrical conductance (539–1822 μS/cm) following CO 2 exposure. Results showed an increase in concentrations of Ca (28–297 mg/L), Mg (18–63 mg/L) and Mn (2–837 μg/L) following CO 2 injection. Results for the sodium saturated sample showed a significant role for sorption/ion exchange indicated by lower increases in the concentrations of Ca (32–189 mg/L), Mg (2–14 mg/L), Fe (7–28 μg/L) and Mn (3–465 μg/L) following CO 2 exposure. These easily monitored chemical species could provide early detection of CO 2 -leakage into shallow groundwater. These experimental results, with the exception of Fe, are also in general agreement with those obtained from sampling the ZERT monitoring wells. The main geochemical processes responsible for the observed increases in the concentrations of solutes include: Dissolution of calcite and dolomite; dissolution/reduction of Mn-oxyhydroxides; and desorption-ion exchange from clays and Fe-oxyhydroxieds due to lowered pH values upon CO 2 exposure. The trace metal concentrations in samples from both the laboratory experiments and ZERT wells are all significantly below the maximum contaminant levels (MCLs).