Elizabeth H. Keating

Probabilistic evaluation of shallow groundwater resources at a hypothetical carbon sequestration site

Carbon sequestration in geologic reservoirs is an important approach for mitigating greenhouse gases emissions to the atmosphere. This study first develops an integrated Monte Carlo method for simulating CO2 and brine leakage from carbon sequestration and subsequent geochemical interactions in shallow aquifers. Then, we estimate probability distributions of five risk proxies related to the likelihood and volume of changes in pH, total dissolved solids, and trace concentrations of lead, arsenic, and cadmium for two possible consequence thresholds. The results indicate that shallow groundwater resources may degrade locally around leakage points by reduced pH and increased total dissolved solids (TDS). The volumes of pH and TDS plumes are most sensitive to aquifer porosity, permeability, and CO2 and brine leakage rates. The estimated plume size of pH change is the largest, while that of cadmium is the smallest among the risk proxies. Plume volume distributions of arsenic and lead are similar to those of TDS. The scientific results from this study provide substantial insight for understanding risks of deep fluids leaking into shallow aquifers, determining the area of review, and designing monitoring networks at carbon sequestration sites.

CO2/Brine transport into shallow aquifers along fault zones.

Unintended release of CO(2) from carbon sequestration reservoirs poses a well-recognized risk to groundwater quality. Research has largely focused on in situ CO(2)-induced pH depression and subsequent trace metal mobilization. In this paper we focus on a second mechanism: upward intrusion of displaced brine or brackish-water into a shallow aquifer as a result of CO(2) injection. Studies of two natural analog sites provide insights into physical and chemical mechanisms controlling both brackish water and CO(2) intrusion into shallow aquifers along fault zones. At the Chimayó, New Mexico site, shallow groundwater near the fault is enriched in CO(2) and, in some places, salinity is significantly elevated. In contrast, at the Springerville, Arizona site CO(2) is leaking upward through brine aquifers but does not appear to be increasing salinity in the shallow aquifer. Using multiphase transport simulations we show conditions under which significant CO(2) can be transported through deep brine aquifers into shallow layers. Only a subset of these conditions favor entrainment of salinity into the shallow aquifer: high aspect-ratio leakage pathways and viscous coupling between the fluid phases. Recognition of the conditions under which salinity is favored to be cotransported with CO(2) into shallow aquifers will be important in environmental risk assessments.

Reactive transport modeling of redox geochemistry: Approaches to chemical disequilibrium and reaction rate estimation at a site in northern Wisconsin

The purpose of this study is to investigate the hydrology and redox geochemistry of shallow groundwater discharging to a stream in northern Wisconsin. In this organic-rich aquifer, we observe both oxygen reducing zones and iron reducing zones whose boundaries are roughly constant over time. To investigate the apparent steady state between solute fluxes and redox reaction rates, we develop a reactive transport model of carbon oxidation. We use a “quasi-kinetic,” “partial-equilibrium” approach to modeling redox reactions, a hybrid approach between traditional equilibrium approaches and fully kinetic approaches that require large computer resources. Our model suggests that observed trends in redox sensitive elements can only be explained by oxidation rates that are both dependent on the predominant electron acceptor and are spatially variable. Our coupled models provide field-based estimates of redox kinetics, which are otherwise difficult to obtain in hydrologically complex systems.

Using reactive solutes to constrain groundwater flow models at a site in northern Wisconsin

We propose a method for constraining a groundwater flow model both by head observations and concentrations of nonconservative solutes such as calcium, using reaction-path modeling. When calibrating flow models in small watershed in northern Wisconsin using head data alone, we encountered problems of nonuniqueness. However, by coupling the flow models with a plagioclase dissolution model, we were able to greatly reduce the number of plausible flow models. First, by using flow modeling and reaction path modeling in parallel, we tested the consistency of residence times predicted by the flow models with solute concentrations predicted by the geochemical models. Mineral dissolution rate parameters were assumed to be spatially uniform; without this condition the geochemistry data would not provide additional constraints to the flow modeling process. For a more comprehensive test of our models, we used reactive-transport modeling to predict the spatial distribution of ions at each site. The models qualitatively reproduced the observed data and our calibrated silicate dissolution rates closely matched those reported in a field study of nearby site. There were also discrepancies between predictions and observations. We attribute these to transient effects and sediment heterogeneities that were not included in the models. While the resulting models are not unique, our approach demonstrates the ability of fairly simple models to explain much of the observed variability in a complex system.

A high-performance workflow system for subsurface simulation

The U.S. Department of Energy (DOE) recently invested in developing a numerical modeling toolset called ASCEM (Advanced Simulation Capability for Environmental Management) to support modeling analyses at legacy waste sites. This investment includes the development of an open-source user environment called Akuna that manages subsurface simulation workflows. Core toolsets accessible through the Akuna user interface include model setup, grid generation, sensitivity analysis, model calibration, and uncertainty quantification. Additional toolsets are used to manage simulation data and visualize results. This new workflow technology is demonstrated by streamlining model setup, calibration, and uncertainty analysis using high performance computation for the BC Cribs Site, a legacy waste area at the Hanford Site in Washington State. For technetium-99 transport, the uncertainty assessment for potential remedial actions (e.g., surface infiltration covers) demonstrates that using multiple realizations of the geologic conceptual model results in greater variation in concentration predictions than when a single model is used. Akuna provides integrated toolset needed for subsurface modeling workflow.Akuna streamlines process of executing multiple simulations in HPC environment.Akuna provides visualization tools for spatial and temporal data.Example application demonstrates risk with remediation impacting infiltration rates.

Convolution-based particle tracking method for transient flow

A convolution-based particle tracking (CBPT) method was recently developed for calculating solute concentrations (Robinson et al., Comput Geosci 14(4): 779–792, 2010). This method is highly efficient but limited to steady-state flow conditions. Here, we present an extension of this method to transient flow conditions. This extension requires a single-particle tracking process model run, with a pulse of particles introduced at a sequence of times for each source location. The number and interval of particle releases depends upon the transients in the flow. Numerical convolution of particle paths obtained at each release time and location with a time-varying source term is performed to yield the shape of the plume. Many factors controlling transport such as variation in source terms, radioactive decay, and in some cases linear processes such as sorption and diffusion into dead-end pores can be simulated in the convolution step for Monte Carlo-based analysis of transport uncertainty. We demonstrate the efficiency of the transient CBPT method, by showing that it requires fewer particles than traditional random walk particle tracking methods to achieve the same levels of accuracy, especially as the source term increases in duration or is uncertain. Since flow calculations under transient conditions are often very expensive, this is a computationally efficient yet accurate method.

Advanced Simulation Capability for Environmental Management (ASCEM)

The United States Department Energy (DOE) Office of Environmental Management (EM) determined that uniform application of advanced modeling in the subsurface could help reduce the cost and risks associated with its environmental cleanup mission. In response to this determination, the EM Office of Technology Innovation and Development (OTID), Groundwater and Soil Remediation (GW&S) began the program Advanced Simulation Capability for Environmental Management (ASCEM). ASCEM is a state-of-the-art scientific tool and approach for integrating data and scientific understanding to enable prediction of contaminant fate and transport in natural and engineered systems. This initiative supports the reduction of uncertainties and risks associated with EM’s environmental cleanup and closure programs through better understanding and quantifying the subsurface flow and contaminant transport behavior in complex geological systems. This involves the long-term performance of engineered components, including cementitious materials in nuclear waste disposal facilities that may be sources for future contamination of the subsurface. This paper describes the ASCEM tools and approach and the ASCEM programmatic accomplishments completed in 2010 including recent advances and technology transfer.Copyright