EAR-PF: Evaluating the relationship between physical heterogeneity and stable isotope fractionation factors during subsurface reactive transport

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Dr. Jennifer Druhan has been awarded an NSF Earth Science Postdoctoral Fellowship to carry out a combined experimental and modeling study of the influences of physical heterogeneity on the measurement and analysis of stable isotope fractionations associated with subsurface reactive transport. While stable isotopes are commonly used to analyze a wide variety of complex hydrogeochemical systems, models of fractionation routinely require simplifying assumptions such as homogeneous, well-mixed reactivity, leading to misrepresentation of system processes. The experimental aspect of this study will involve a suite of meso-scale, flow-through column reactors packed with homogeneous and heterogeneous permeability distributions. The columns will be designed specifically to allow precise characterization of flow-path heterogeneity using nuclear medial imaging techniques. The modeling aspect will involve both inverse parameter estimation methods to obtain heterogeneous permeability distributions from nuclear medical imaging data, and forward geochemical reactive transport simulations of fractionation through these permeability distributions to characterize the relationship between heterogeneous solute transport and observed fractionation. The goal of the work is to demonstrate the difference between effective fractionation factors measured in homogeneous and heterogeneous systems, to show the effects of hierarchical heterogeneity on these measurements, and to demonstrate the temporal influence of evolving permeability fields on isotopic ratios. Results of this work will have direct application to analysis of isotopic datasets in near-surface environments in studies ranging from drinking water resources to contaminant remediation to reclaimed water storage.The NSF supported research will be carried out at Stanford University in the Department of Geological and Environmental Sciences. Through the Stanford SURGE program aspects of the study will be leveraged to support mentorship of undergraduates in the earth sciences. The study will further serve as a seminal example in the development of a short course in reactive transport modeling of stable isotope fractionation.