Investigation for DNAPL Migration in Porous Media and Contaminant Mass Flux Changes by Water Table Rising

Abstract

In this study, three dimensional laboratory experiments and two dimensional flow cell experiments were conducted to investigate the effect of water table rising on DNAPL migration, dissolved-phase contaminants mass discharge (Md), and residual NAPL saturation (Sr). In 3-D column experiment, the area of residual DNAPL was calculated to produce morphological 3-D pool. The results of 3-D column experiments show that the wider areas of residual DNAPL were observed at depth of water table and top of capillary zone comparing to the areas from other depths. This means TCE migrated laterally at theses depths. Additionally, two-dimensional flow cell tests were performed to observe the migration of DNAPL. It has been observed that water table and capillary zone have apparent effect on DNAPL migration. The spilled TCE on the surface was accumulated on the three different points; at the depths of water table, top of capillary zone, and uncertain point in saturated zone. It was also detected that TCE dissolution behavior was influenced by water table rising. The DNAPL pool in the capillary zone was dissolved with increasing the water table, leading to the increase of mass discharge: the average mass discharge at the effluent port was 1.13 • 10-2 mg/sec before water table raising while the average mass discharge after water table raising was 3.27 • 10-2 mg/sec. Experiments with fine and coarse grained lenses were conducted to investigate the effect of source zone grain size (coarse and fine lenses) on NAPL dissolution behavior. The results showed that the accumulated TCE removal ratio from fine lenses was 18% higher than the ratio from the coarse lenses after approximately 140 pore volume of water passed through the aquifer system. These results imply that the NAPL dissolution behavior was affected by water table rising and the source zone media characteristics. It also shows that the presence of NAPL in unsaturated zone could be a long-term source point for groundwater contamination in a real aquifer system.