Development, Numerical Implementation, and Application of Methods to Simulate Solute Transport Processes in Porous Media Systems
Type of DegreePhD Dissertation
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Solute transport in porous media has been the attention of a plethora of studies over the last several decades. This research identifies gaps in existing knowledge within the realm of solute transport in porous media systems and has made several new contributions. The scientific contributions of this dissertation effort can be broadly classified into three categories: (1) development of numerical methods for solving solute transport equations; (2) implementing numerical modeling capabilities and solution techniques in solute transport simulators; and (3) application of numerical modeling simulators to understand transport processes in various real-world problems. The numerical modeling simulators involved in this research primarily comprise of simulators that are commonly known as MODFLOW-family of codes, owing the name to the underlying flow simulator called MODFLOW. MT3D is the associated solute transport simulator, SEAWAT is the variable-density flow and transport simulator, and RT3D is the reactive transport simulator, and all these codes use MODFLOW as the flow simulator. MODFLOW-USG and MODFLOW 6 are the latest versions of MODFLOW that, among other enhancements, offer unstructured grid capability. As part of this research, a rigorous formulation for the dissolved and adsorbed mass storage for the finite-difference approximation of solute transport equation was developed. The new formulation for the convective form of equation conserves mass, an important consideration for solving solute transport equations. The reformulation was implemented in MT3D. Other improvements to MT3D include: incorporation of solute transport capabilities in the lake and stream flow; implementation of a contaminant treatment system to simulate above-ground pump and treat systems; multi-species kinetic reaction capabilities; and handling of transport within dry model cells. The new version of MT3D was called MT3D-USGS, released by U.S. Geological Survey (USGS) as a public domain code. During the development of MT3D-USGS code we discussed several transport issues with the MODFLOW-USG development team, including an inherent lack of capability in the governing equation of solute transport to handle any flow balance errors originating from an iterative flow solution, resulting in anomalous concentrations in the solute transport solution. To address this issue, we jointly developed a practical, new solution technique that stabilizes the concentration values. Finally, a new numerical code, DRT3D, was also developed by coupling reaction simulation capabilities of RT3D and density-dependent code SEAWAT. Numerical models were developed to study several applied physical problems. First, the SEAWAT code was used to study the saltwater intrusion processes occurring within low-lying island aquifers. The study showed that lake formation on small islands can substantially deplete subsurface water resources. In a second study, the temporal trends of freshwater depletion and replenishment on small islands during dry and wet conditions were examined. The findings from this research suggest that freshwater is replenished at a faster rate relative to the depletion during dry conditions. Finally, the unstructured grid functionality offered by MODFLOW-USG was utilized to design coaxial circular grid cells to a variety of axisymmetric problems using the MODFLOW-USG modeling framework. The axisymmetric model design can also be implemented with the unstructured grid functionality offered by MODFLOW 6. The research included here is a collection of studies that have contributed to the development of numerical methods, source code improvement, and application of the models for solving practical problems. The results of these research efforts were published in a peer-reviewed USGS report and four peer-reviewed journal publications.