Metal Release from Sandstones under Experimentally and Numerically Simulated CO₂ Leakage Conditions

Leakage of CO₂ from a deep storage formation into an overlying potable aquifer may adversely impact water quality and human health. Understanding CO₂-water-rock interactions is therefore an important step toward the safe implementation of geologic carbon sequestration. This study targeted the geochemical response of siliclastic rock, specifically three sandstones of the Mesaverde Group in northwestern Colorado. To test the hypothesis that carbonate minerals, even when present in very low levels, would be the primary source of metals released into a CO₂-impacted aquifer, two batch experiments were conducted. Samples were reacted for 27 days with water and CO₂ at partial pressures of 0.01 and 1 bar, representing natural background levels and levels expected in an aquifer impacted by a small leakage, respectively. Concentrations of major (e.g., Ca, Mg) and trace (e.g., As, Ba, Cd, Fe, Mn, Pb, Sr, U) elements increased rapidly after CO₂ was introduced into the system, but did not exceed primary Maximum Contaminant Levels set by the U.S. Environmental Protection Agency. Results of sequential extraction suggest that carbonate minerals, although volumetrically insignificant in the sandstone samples, are the dominant source of mobile metals. This interpretation is supported by a simple geochemical model, which could simulate observed changes in fluid composition through CO₂-induced calcite and dolomite dissolution

Kinetic Metal Release from Competing Processes in Aquifers

Understanding groundwater time scales wherein kinetic metal-desorption and mineral-dissolution are important mechanisms is essential for realistic modeling of metal release. In this study, release rate constants were compiled and the Damköhler number was applied to calculate residence times where kinetic formulations are relevant. Desorption rate constants were compiled for arsenic, barium, cadmium, copper, lead, mercury, nickel, and zinc, and span 6 orders of magnitude, while mineral-dissolution rate constants compiled for calcite, kaolinite, smectite, anorthite, albite, K-feldspar, muscovite, quartz, goethite, and galena ranged over 13 orders of magnitude. This Damköhler analysis demonstrated that metal-desorption kinetics are potentially influential at residence times up to about two years, depending on the metal and groundwater conditions. Kinetic mineral-dissolution should be considered for nearly all residence times relevant to groundwater modeling, provided the rate, solubility, and availability of the mineral generates a non-negligible concentration. Geochemical models of competitive desorption and dissolution for an illustrative metal demonstrate total metal concentrations may be sensitive to dissolution rate variations despite the predominance of release from desorption. Ultimately, this analysis provides constraints on relevant processes for incorporation into transport models