Physical and Numerical Modeling of Buoyant Groundwater Plumes

Abstract

In coastal states, the injection of treated, relatively fresh, wastewater into deep saline aquifers offers a disposal alternative to ocean outfalls and discharge directly into local waterways. The density contrast causes 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 important to be able to determine the fate and transport rates of the plume. In this study, both physical and numerical modeling were undertaken to investigate and understand buoyant plume behavior and transport.

Physical models using a 2D tank filled with glass beads were carried out under different ambient density scenarios. The experiments consisted of injection of a freshwater pulse-source bubble in an initially static system with no ambient flow.

Using the finite-difference numerical code SEAWAT v.4 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 spatial resolution, results from three different grid sizes were compared to determine a reasonable compromise between computing times and numerical accuracy. Furthermore, a comparison of advection solvers was undertaken to identify the best solver to use for this specific problem. 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. The finite element based numerical code SUTRA_MS was also used for intercode comparison with SEAWAT v.4.

Dimensionless analysis of the flow and transport governing equations was undertaken to determine important physical problem parameters and a characteristic plume travel time. From the derived dimensionless numbers, it was hypothesized that density, hydraulic conductivity and dispersivity should all play important roles in this problem. A parameter sensitivity analysis on these parameters was performed. The parameter sensitivity investigation involved a quantitative comparison based on moment analysis. It was determined that the problem was most sensitive to density contrast and hydraulic conductivity in regards to vertical velocity rates. Dispersivity changes played an important role in affecting fingering.