Reactive transport modeling as a tool for the integrated interpretation of laboratory and field studies in environmental geochemistry

Wanner, Christoph (2021). Reactive transport modeling as a tool for the integrated interpretation of laboratory and field studies in environmental geochemistry (Unpublished). (Habilitation, Philosophisch-naturwissenschaftliche Fakultät)

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The interaction between moving water and stationary rock within the Earth's crust and on its surface is typically controlled by a series of coupled processes occurring in porous media at various spatio-temporal scales. An example is chemical weathering, which takes place within a thin layer at the Earth’s surface called the Critical Zone and eventually leads to the formation of soils on the time-scale of hundreds to thousands of years. Chemical weathering is kinetically limited and its rate depends on the transport of CO2 and O2 through the Critical Zone. It is also strongly affected by physical weathering processes controlling the grain size distribution within the Critical Zone and thus the available mineral surface area where chemical reactions can take place. In addition, variations in biological activity that produce CO2 may change chemical weathering rates. The complex feedbacks and interrelationships within such coupled systems cannot be understood by traditional geochemical approaches that combine field observations, laboratory analyses and theoretical treatments of the individual uncoupled processes. Therefore, the quantitative interpretation of field studies requires a numerical simulation tool that is able to capture the coupled behavior of subsurface systems and to upscale experimental findings. The main requirements for such a tool are to (i) simulate the relevant geochemical and biogeochemical processes in a mechanistic way and (ii) simultaneously couple these processes to flow and transport rates. Over the past 30 years, the field of reactive transport modeling (RTM) has mastered these requirements and thus has become an essential method for the entire Earth Sciences.
This Habilitationsschrift describes the contributions by the Author to advancing the field of reactive transport modeling, thereby demonstrating how RTM enables an integrated interpretation of laboratory and/or field studies. In addition, this Habilitationsschrift emphasizes how RTM contributes to solving environmental challenges, such as those identified by the United Nations’ Sustainable Development Goals 6 (Clean Water and Sanitation), 7 (Affordable and Clean Energy), and 13 (Climate Action).
Chapter 1 introduces groundwater contamination, geothermal energy, and silicate weathering as important topics in environmental geochemistry and describes the role of coupled processes in the corresponding systems. Moreover, the Chapter introduces the general concept of reactive transport modeling and illustrates its ability to numerically capture the coupling between non-isothermal geochemical and biogeochemical processes, as well as fluid flow and transport rates.
Chapter 2 provides a detailed summary of the key accomplishments by the Author to advancing RTM. These include (i) the integration of stable isotopes in RTM simulations that address challenges relevant to environmental geochemistry, and (ii) the use of reactive transport models as an exploration tool for geothermal systems. These contributions resulted, among others, in the publication of nine peer-reviewed publications, which are presented in Chapters 3–5 and form the core of this Habilitationsschrift.
Chapter 3 presents three selected RTM applications where Cr or U isotopes are integrated in reactive transport model simulations to understand Cr- and U-contaminated groundwaters. In the first two studies, small-scale laboratory experiments are numerically simulated using a novel approach for obtaining a high spatial resolution of the simulated systems. The model results and their comparison to measured Cr and U isotope ratios provide fundamental insights into the processes controlling the magnitude of Cr and U isotope fractionation occurring in porous media. Obtaining a predictive understanding of such isotopic fractionation is crucial, because it opens the way to quantify the most important processes limiting the mobility of Cr and U in the subsurface. The third application presents a benchmarking exercise, which allowed testing and improvement of the approaches for numerically simulating stable Cr isotope fractionation in geochemical processes. Altogether, the RTM applications presented in Chapter 3 contribute to a more informed assessment and management of groundwater bodies contaminated by Cr and U.
Chapter 4 presents three applications where RTM is used as an exploration tool for geothermal systems. The first one involves 2D simulations carried out for the Dixie Valley geothermal system located in the western USA. The model output is then applied via a geochemical method called solute geothermometry, which is used to estimate the maximum temperature of deep geothermal systems. Eventually, the combined approach demonstrates which particular solute geothermometry method works best for various scenarios of rock–water interactions and thus contributes to an improved estimation of deep reservoir temperatures. Obtaining reliable temperatures is important because the reservoir temperature is a major limit on the energy that can be exploited from the deep subsurface. In the other two applications, RTM simulations are performed to quantitatively assess the geothermal potential of two geothermal systems in the Swiss Alps. All simulations are carried out in 3D and are constrained and calibrated by multiple field observations, such as chemical and isotopic compositions of thermal and cold springs, as well as temperature measurements along tunnels. The model results suggest that mountain belts such as the Swiss Alps are more promising targets for geothermal power production than previously thought, and they identify the favorable geological settings for such systems, which leads to useful implications for exploration.
Chapter 5 presents a sequence of three RTM applications, which for the first time integrate Li isotopes in the simulations. The motivation for numerically simulating the fate of Li isotopes is that their ratios serve as proxies to track and quantify the rates of silicate weathering, which in turn constitutes a major natural sink for atmospheric CO2. The first study describes a new numerical approach to integrate Li isotopes in RTM simulations. In the first and second applications, the developed approach is used to simulate the fate of Li and its isotopes in granitic and basaltic rainwater catchments, respectively. Subsequently, the model results are compared to Li data from major worldwide rivers and from a series of small streams draining the Columbia River Basalt area in the western USA. This model-based, integrated interpretation of measured Li isotope ratios permits identification of the processes governing Li isotope ratios in groundwater, riverwater and even seawater. Moreover, the results imply that seawater Li isotope ratios may be closely related to the amount of CO2 globally consumed by continental silicate weathering. In the third application, the numerical approach to simulate the fate of Li isotopes is expanded to account for the limited amount of Li that precipitates in secondary minerals. The updated approach is used to unravel a complex set of Li data collected from groundwater samples discharging into the Gotthard railway base tunnel in the Swiss Alps. This study reveals that the behavior of Li isotopes in environmental samples is more complicated than hitherto realized. Overall, the applications presented in Chapter 5 contribute to assessing the use of Li isotopes as a proxy for silicate weathering and may help to better quantify this important natural sink for CO2.
Chapter 6 summarizes the main implications of this Habilitationsschrift regarding the use of RTM in environmental geochemistry and how it contributes to meeting the listed Sustainable Development Goals. In particular, this final chapter concludes that the inclusion of stable isotopes into RTM simulations provides fundamental new insights into the processes controlling stable isotope ratios in environmental samples. This underscores how RTM serves as a powerful tool for the integrated interpretation of multiple datasets obtained from field and laboratory studies. Moreover, the Chapter discusses how RTM simulations are limited by the availability of thermodynamic and kinetic data, and by the need for detailed geochemical, biogeochemical, hydrological, and geophysical site-characterizations in order to calibrate such simulations. Finally, the Chapter contains a brief outlook emphasizing the numerous opportunities for new code development and for laboratory as well as field studies to constrain and calibrate future RTM applications. These activities will enable even more powerful RTM simulations to meet the environmental challenges of the future.

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