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Multiphase, reactive transport modeling was used to identify the mechanisms controlling trace metal release under elevated CO 2 conditions from a well-characterized carbonate aquifer. Modeling was conducted for both batch and column experiments. The column experiments resulted in higher trace metal concentrations because the rock to water ratio was higher. A kinetic desorption model fits the overall trends in release for seven trace metals observed in batch and column experiments exposing Edwards Aquifer material to elevated concentrations of CO 2 . Observed and predicted trace metal concentrations are compared to groundwater concentrations from this aquifer to determine the potential for leaking CO 2 to adversely impact drinking water quality. Finally, a three-dimensional multiphase flow and reactive-transport simulation of CO 2 leakage from an abandoned wellbore into a generalized model of the shallow, unconfined portion of the aquifer is used to determine potential impacts on groundwater quality. As a measure of adverse impacts on groundwater quality, both the EPA’s regulatory limits and the maximum trace metal concentration observed in the aquifer were used as threshold values. Results of the field-scale simulations indicate that CO 2 leakage into a carbonate aquifer is likely to cause decreases in pH and increases in TDS beyond observed ranges in the aquifer and beyond regulatory limits. However, trace metal concentrations are not predicted to exceed either the observed maximums or the regulatory limits. [ABSTRACT FROM AUTHOR]

This study focused on CO2 plume expansion subsequent to wellbore CO2 leakage into a shallow unconfined aquifer post-CO2 injection. The target response variables included CO2 plume size, as well as flux to the atmosphere. Many processes contribute to CO2 plume expansion in the aquifer; here we considered process and model parameters including those affecting the abandoned well leak rate, aquifer hydraulic properties, and aquifer geochemistry. In order to identify the significant factors affecting leakage, we adopted an uncertainty quantification framework to quantify input uncertainty, generate exploratory samples effectively, perform scalable numerical simulations, visualize output uncertainty, and evaluate input-output relationships. We combined quasi-Monte Carlo and adaptive sampling approaches to reduce the number of forward calculations while fully exploring the input parameter space and quantifying the output uncertainty. The CO2 migration was simulated with STOMP-CO2 (water-salt-CO2 module). Response surfaces of model outputs were built with respect to input parameters to determine the individual and combined effects. Four most significant parameters were identified to be dominating the CO2 plume expansion process subsequent to wellbore CO2 leakage: distance between the leaky and injection wells, maximum leakage rate, porosity, and hydraulic conductivity. [ABSTRACT FROM AUTHOR]

This paper provides an overview of the existing data and knowledge presented in recent literature about the potential leaking of CO 2 from the deep subsurface storage reservoirs and the effects on groundwater quality. The objectives are to: 1) present data and discuss potential risks associated with the groundwater quality degradation due to CO 2 gas and brine exposure; 2) identify the set of geochemical data required to develop models to assess and predict aquifer responses to CO 2 and brine leakage; and 3) present a summary of the findings and reveal future trends in this important and expanding research area. The discussion is focused around aquifer responses to CO 2 gas and brine exposure and the degree of impact; major hydrogeological and geochemical processes and site-specific properties known to control aquifer quality under CO 2 exposure conditions; contributions from the deep reservoirs (plume characteristics and composition); and the possibility of establishment of a new network of reactions and processes affecting or controlling the overall mobility of major, minor, and trace elements and the fate of the elements released from sediments or transported with brine. This paper also includes a discussion on the development of conceptual and reduced order models (ROMs) to describe and predict aquifer responses and whether or not the release of metals following exposure to CO 2 is harmful, which are an essential tool for CO 2 sequestration related risk assessment. Future research needs in this area are also included at the end of the paper. [ABSTRACT FROM AUTHOR]