Back to list of UTIG abstract submissions, Fall Agu 2003

* Shi, M.
EM: mshi@mail.utexas.edu
AF: The University of Texas at Austin, Geological Sciences Department, Austin, TX 78712 United States
Sharp, J
jmsharp@mail.utexas.edu

Absract
Study of free convection in heterogeneous geologic units poses challenges to hydrogeologists in solute transport modeling and in predicting the effective Rayleigh Number, which is the ratio of bouyancy driving forces to the viscous resistance to flow and the diffusive dissipation of solutes. Downward convection of dense fluid through heterogeneous low-permeability strata follows preferential pathways of groundwater flow and contaminant transport. In this study, the concepts of boundary layer and critical Rayleigh number are used to delineate free convection pathways in a suite of permeability field realizations. A boundary layer is formed when the upward evaporative flux stabilizes the downward groundwater movement in a saline lake environment. Onset of free convection occurs when the effective Rayleigh Number (NRa) exceeds certain value, the critical Rayleigh Number, The threshold value of permeability that permits free convection in a system where denser fluid overlies a less-denser fluid is approximately 10-14 m2 for a boundary layer of 0.315 m and a NRa of 8.5 to 10. Invasion percolation theory (an invasion process that proceeds along a path of least resistance) is used in the searching algorithm that identifies the paths for free convection in 60 different heterogeneous permeability fields. By comparing with modeling results of variable density flow through these different k-simulations, the delineation of pathways provides insight into predicting the occurrence of free convection, groundwater flow, and solute distribution in heterogeneous low-permeability systems. Individual long and wide pathways promote free convection, which indicates that local heterogeneity controls solute transport. Vertically continuous zones of high permeability form pathways that support the initiation and growth of freely-convecting solute plumes.