Simulating geochemical reactive transport in physically and chemically heterogeneous aquifers: implications for CO2 risk assessment and uncertainty

Abstract

Simulating hydro-geochemical processes is crucial to understand a wide range of earth processes including carbon capture utilization and storage (CCUS), and risk assessment. However, simulating heterogeneous fully-coupled non-linear reactive transport at scales applicable to CCUS scenarios is computationally expensive. A streamline approach that simplifies a three-dimensional transport problem to a one dimensional transport problem in residence time is used to model non-linear kinetic geochemical reactions and transport over large domains. The efficiency achieved by the streamline approach allows for the simulation of ensembles of non-linear kinetic hydro-geochemical transport realizations on an 8-core Linux machine. CO2 leakage from a hypothetical CCUS operation into groundwater is used to showcase the streamline approach for simulating complex hydro-geochemical transport within an ensemble of 100 heterogeneous flow field realizations. The 2nd chapter of this dissertation uses the streamline approach to demonstrate that physical heterogeneity determines spreading and residence time, which then largely governs ensemble results. The distribution of solute residence times that resulted from physical heterogeneity, governs the amount of time advected solute has to undergo kinetically controlled geochemical reactions, which then determines the ensemble solute concentrations. The 3rd chapter illustrates the application of the streamline approach to estimating human health risk that considers both subsurface uncertainty and population variability. The contributions that hydrological and geochemical processes have on human health risk estimates are assessed. Chapter three shows that both hydrological and geochemical conditions plays a key role in determining concentrations of lead (Pb) and that simulating non-linear kinetics provides more protective risk estimates compared assuming chemical equilibrium. The 4th and final chapter investigates to the role of spatially heterogeneous reactive surface areas. An interesting dynamic between physical and geochemical heterogeneity is discussed. As a reactive solute approaches geochemical equilibrium the spatial distribution of reactivity can act to lower or higher effective ensemble reactions rates. Beyond the point that a reactive solute achieves geochemical equilibrium, physical heterogeneity is the sole factor controlling effective ensemble reactions rates.