PNNL

Abstract

Large scale deployment of CO2 geological sequestration requires the assessment of the risks. One of the potential risks is the impact of CO2 leakage on shallow groundwater overlying the sequestration site.The understanding of the key chemical processes and parameters are critical for building numerical models for risk assessment. Model interpretation of laboratory and field tests is an effective way to enhance such understanding. Column experiments in which CO2 charged synthetic groundwater flowed through a column packed with material from High Plains aquifer was conducted and concentration of several constituents in the effluent water was analyzed. In this paper, reactive transport model was developed to interpret the observed concentration changes, attempting to shed light on the chemical reactions and key parameters that control the concentration changes of these constituents. The reactive transport model catches the concentration changes of pH, Ca, Mg, Ba, Sr, Cs, As and Pb fairly well. Calcite dissolution and Ca-driven cation exchange reactions are the major drivers for the concentration changes of Ca, Ba, Sr, and Cs. The pH-driven adsorption/desorption reactions lead to a concentration increase of As and Pb. The volume fraction and reactive surface area of calcite, CEC and sorption capacity are key parameters in determining the magnitude of concentration increase. Model results also show that the dissolution of calcite with Ba impurity could be an alternative explanation of the increase in Ba concentration.