Experimental and Numerical Analysis of Variable-density Flow and Transport Scenarios

Abstract

Saltwater contamination has received a lot of attention in the past few years with natural disasters such as tsunamis and hurricanes causing widespread groundwater pollution. Groundwater modeling of saline contaminant plumes is complex due to the underlying density effects. The research presented here aims to improve the understanding of variable-density processes under both laboratory and field conditions.

New benchmarking datasets were developed for saltwater intrusion (saltwedge) scenarios under controlled laboratory conditions. Three types of datasets- steady-state, transient and flux- were presented for the purpose of verifying the numerical codes. Results from SEAWAT, a widely used variable-density model, were tested against the acquired datasets and shown to match the experimental results. Other variable-density models (SUTRA, SWI) were also tested against the proposed datasets and produced close matches.

Image analysis procedure was used to measure the spatial concentration profiles in the experimental tanks during variable-density experiments. A detailed error analysis method was developed to estimate accuracy in the application of image analysis procedure. Also, the proposed error estimation method was shown to be better than the often used methods available in literature. The image analysis procedure was improved based on the results from the analysis and a more accurate concentration dataset was obtained using the improved procedure.

Variable-density experiments were conducted to obtain datasets with and without instabilities. The datasets were then modeled using SEAWAT with different numerical parameters. It was shown that MOC (method of characteristics), TVD (total variation diminishing) and regular finite difference methods are not suitable to be used for modeling variable-density scenarios when instabilities can develop as they lead to wrong predictions. Finite difference in conjunction with heterogeneity was found to be the best method.

Variable-density flow and transport theory was applied to study two different phenomenon- a) migration of zero-valent iron nanoparticles (ZVI); and b) fate and transport of saltwater trapped in open wells after a saltwater contamination event such as a tsunami or a hurricane.