HR: 15:10h
AN: H13K-06    [Abstracts]
TI: Physical and Numerical Modeling of Buoyant Groundwater Plumes
AU: * Brakefield, L K
EM: brakelk@auburn.edu
AF: Department of Civil Engineering, Auburn University, 238 Harbert Engineering Center, Auburn University, AL 36849, United States
AU: Abarca, E
EM: ema0004@auburn.edu
AF: Department of Civil Engineering, Auburn University, 238 Harbert Engineering Center, Auburn University, AL 36849, United States
AU: Langevin, C D
EM: langevin@usgs.gov
AF: U. S. Geological Survey, Florida Integrated Science Center, 3110 SW 9th Avenue, Ft. Lauderdale, FL 33315, United States
AU: Clement, T P
EM: clemept@auburn.edu
AF: Department of Civil Engineering, Auburn University, 238 Harbert Engineering Center, Auburn University, AL 36849, United States
AB: In coastal states, the injection of treated wastewater into deep saline aquifers offers a disposal alternative to ocean outfalls and discharge directly into local waterways. The density of treated wastewater is similar to that of freshwater but is often much lower than the ambient density of deep aquifers. This significant density contrast can cause upward buoyant movement of the wastewater plume during and after injection. Since some wastewater treatment plants inject more than 100 MGD of this treated wastewater, it is of the utmost importance to be able to not only determine the fate and transport rates of the plume, but to be able to best determine locations for monitoring wells for early detection of possible problems. In this study, both physical and numerical modeling were undertaken to investigate and understand buoyant plume behavior and transport. Physical models using a 2D cross-sectional Plexiglas tank filled with glass beads were carried out under different ambient density scenarios. The experiments consisted of injection of a freshwater pulse-source bubble into a fully saline tank. The injection occurred in an initially static system with no ambient flow. In the scenarios, the freshwater plume migrated vertically upward until reaching the top of the tank. Fingers developed because of the heterogeneity of the density dependent flow field. The vertical velocities and transport patterns of these plumes were compared to one another to investigate variances due to different ambient water densities. Using the finite-difference numerical code SEAWAT to simulate variable density flow, the experiments were numerically modeled and compared with the physical model results. Due to the sensitivity of this problem to numerical resolution, results from three different grids were compared to determine a reasonable compromise between computer runtimes and numerical accuracy. Furthermore, a comparison of advection solvers was undertaken to identify the best solver to use for this specific problem. This involved a comparison between finite- difference, total variation diminishing and mixed Eulerian-Langrangian methods. From these scenarios, the Method of Characteristics (MOC) advection solver with the fine resolution grid (0.1 cm x 0.1 cm x 2.7 cm cells) resulted in a simulation that was in good agreement with the physical experiments. This model was determined to be the base-case problem for further sensitivity analysis. To further verify both the physical and numerical model, SUTRA_MS was also used for comparison. Dimensionless analysis of the flow and transport governing equations was undertaken to determine important physical problem parameters. From these derived dimensionless numbers, it was hypothesized that density, hydraulic conductivity and dispersivity should all play important roles in this problem. A parameter sensitivity analysis was performed using the numerical model base-case. The parameters investigated were hydraulic conductivity, ambient groundwater density, longitudinal dispersivity and injection volume. It was determined that the problem was most sensitive to ambient density, hydraulic conductivity and dispersivity changes as hypothesized, with all three affecting both vertical mass transfer rates, plume fingering and mixing between the fresh and saline waters. The sensitivity to injection volume was not seen to be an important parameter, except for the obvious effect of change in size of the plume.
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1835 Hydrogeophysics
DE: 1847 Modeling
DE: 1869 Stochastic hydrology
SC: Hydrology [H]
MN: 2007 Fall Meeting