Collaborative Research: Unraveling Transport in Porous Media through the Integration of Isotopic Tracers, Geophysical Data, and Numerical Modeling

Various estimates suggest that economic liabilities associated with contaminated groundwater sites in the U.S. run into the hundreds of billions of dollars. Each of these sites poses a potential threat to the environment and human health; nearly 90% of the conventional court-mandated strategies fail to remediate polluted sites adequately. In many cases, these failures can be attributed to remediation strategies that rely on classical conceptual models of contaminant transport, which neither represent subsurface heterogeneity sufficiently nor capture preferential flow and low permeability solute traps. As a result, the application of models to practical problems retains significant uncertainty. While many different models for transport exist, understanding which is 'most correct' for a given problem is fundamental to the prediction of contaminant transport and reaction and site clean-up. This project will utilize methods from the fields of contaminant transport, geophysics, elemental and isotope geochemistry, and mixing-driven reactions to address this need. The research will ultimately enable significantly improved remediation design strategies and improved assessment of human health risk from contaminated groundwater sites. As part of this research, new high school chemistry labs will be developed following the Colorado high school curriculum focusing on acid-base chemistry, pH buffers, and metal geochemistry in collaboration with a Denver public high school. These labs will be disseminated beyond local development via the National Earth Science Teachers Association's 'The Earth Scientist' Journal.

The reliability of contaminant transport predictions depends on the appropriateness of the numerical/conceptual model to site geology. The primary questions are: (1) which physically measureable parameters control solute mass transfer and geochemical reactions, (2) which parameters can be predicted a priori, and (3) which mathematical formulations accurately describe site specific transport under varying flow rates and heterogeneity. The main limitation of many existing transport models is the poor connection between fitting parameters governing solute transport in models and the physical system. The objective of the work is to explore controls on the poor predictive ability of existing physical transport models, especially those that include chemical reaction. Diffusion-induced elemental fractionation, diffusion-induced lithium isotopic fractionation, and geophysical characterization will be integrated to explore the controls on solute transport and reaction. While conservative tracer concentrations have been used to constrain advective-dispersive transport parameters in the past, isotopic tracers fractionate during diffusion and can therefore provide information on immobile pore space in cases where diffusion dominates ion transport. The power of this project will be to add significant constraints to the controls of known parameters in solute transport modeling, determine their controls on transport processes, and quantify the distribution of transport length scales active in a porous medium and the subsequent impact on reaction kinetics. The experimental data produced will be used to validate existing theories commonly used by the hydrology community (i.e., 'local' versus 'non-local' transport), and explore new theories as motivated by emerging data, if needed, to develop new insight into transport and reaction behavior.