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Density variability occurs in many subsurface-contaminant transport problems. Our major contributions in this area include developing and validating criteria for gravitational instabilities, experimentally revealing different unstable behavior of variable density contaminants, and developing numerical approaches for dealing with the unstable contaminant transport processes. My Ph.D. thesis documenting these contributions was selected as the best Ph.D. thesis of the year by Soil Science Society of America.
Subsurface heterogeneity and its effect on subsurface contaminant transport have been key research topics for many years. Our major contributions in this area include for the first time reporting the multifractal behavior of permeability distributions, demonstrating the Levy-fractal behavior of permeability distributions in fractured rock, revealing the scale-dependency of Levy index when using Levy fractals for characterizing subsurface heterogeneity, and also developing a number of fractal-based approaches for characterizing and modeling subsurface heterogeneity. These contributions have been very well received in the subsurface hydrology community.
Matrix diffusion has been a classic research topic in fracture hydrology because of its importance for retarding solute transport. We for the first time reported the potential scale-dependency of the effective matrix diffusion coefficient, which has significant implications for understanding and modeling contaminant transport processes in fractured rocks.
Coupled hydrological and mechanical processes are important for many practical applications including nuclear waste disposal and CO2 geological sequestration. My contribution in this area is the development of a general relationship between stress and elastic strain for porous and fractured rock, based on a hypothesis that a natural rock consists of “hard” and “soft” parts and different parts follow different varieties of Hooke’s law. This development allows for unifying a large number of empirical relationships between stress and mechanical properties within a rather simple theoretical framework. Modeling large-scale coupled hydrological and mechanical processes is currently a major research activity in this area.