Viscosity-driven Enhanced Hydrological Connectivity

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Remediation or cleanup of contaminated groundwater resources is a well-known societal problem for which more effective solutions that are based on a better understanding of how water mixes with chemicals in aquifers is required. Many chemicals that are either responsible for contamination or used as a remedial agent for decontamination have viscosities and densities different from water. These differences lead to complex flow patterns, especially in aquifers with heterogeneous hydraulic properties, as one fluid is pushed by the other during natural groundwater flow or pump-and-treat operations for groundwater cleanup. Viscous fingering is an example of such a complex flow pattern that develops when a less viscous chemical or water pushes a more viscous water or chemical through the aquifer. The fingers that form spontaneously at the boundary between the two fluids control how fast and how far the contamination or decontamination spreads. Understanding and predicting the growth of viscous fingers and the associated spreading and mixing within contaminated/remediated regions is the goal of this project. Lab experiments and numerical simulations of viscous fingering will be conducted for a range of fluid viscosity contrasts and aquifer heterogeneities representative of U.S. aquifers to generate data for testing hypotheses on spreading and mixing of contaminant/remedial fluids. The proposed research is particularly relevant to megacities suffering from fuel leakage from underground storage tanks at gas stations because of the viscosity contrast between gasoline additives and water. Educational activities that involve undergraduate and graduate students in learning the mechanisms of groundwater contamination and remediation through hands-on numerical and physical modeling tools are also planned in this project. This project will improve our current understanding of how the physical properties of a solute plume interact with permeability heterogeneity to affect plume spreading and mixing during hydrodynamically unstable transport through strongly heterogeneous rocks. This understanding is required to predict and control contaminant remediation and removal processes because of the variability in the physical properties of different types of organic/inorganic contaminants. Based on high-resolution numerical simulations, theory of hydrodynamic dispersion and instability, and novel experiments of solute transport in consolidated rocks, the interplay between permeability heterogeneity and viscosity contrast and its control on mixing and spreading of a solute plume will be quantified. By evaluating new metrics of plume connectivity that are defined in terms of the breakthrough curve and the topology of the control plane concentration map, which are field observables, the contribution of the viscosity contrast to the solute arrival time and the mixing zone length will be quantified. Understanding how viscosity contrasts, in the presence of heterogeneity, impacts solute connectivity and transport is highly relevant for aquifer remediation, risk analysis, underground waste disposal, and the chemical industry. Undergraduate and graduate students will be trained in conducting Hele-Shaw cell experiments, building physical aquifer models of contamination and remediation, and organizing a viscous fingering art exhibition.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.