Affiliation: Gillings School of Global Public Health, Department of Environmental Sciences and Engineering
Concerted efforts to remediate subsurface systems contaminated with dense nonaqueous phase liquids (DNAPLs) have achieved limited success when measured by comparing solute concentrations to drinking water quality standards. A novel treatment technology using a class of brine barrier remediation technologies (BBRTs) based on surfactant- and gravity-induced mobilization, dense brine containment and collection, and a vapor-phase extraction polishing step is utilized to remediate such systems. A series of investigations were conducted using one- and three-dimensional laboratory experiments, a field-scale experiment, and modeling to investigate the impact of BBRTs, the effectiveness of source-zone remediation, and to investigate the establishment, persistence, and rate of removal of a brine layer in a controlled system. Laboratory and field-scale experiments are performed using the suggested methodology. It is shown that under certain conditions, less than 1% of the original DNAPL mass remained in the laboratory system. DNAPL mobilization and recovery in the field-scale experiment was relatively ineffective due in part to the low saturation levels of the DNAPL. The results show that essentially complete removal of a DNAPL is required to reach typical cleanup standards and that details of the morphology and topology of a DNAPL distribution, in addition to the saturation, play an important role in determining the rate of mass transfer. The behavior of dense, viscous calcium bromide brine solutions used to remediate these experimental systems consists of a density of 1.7 times, and a corresponding viscosity of 6.3 times, that of water is obtained at a calcium bromide mass fraction of 0.53. The results show that a dense brine layer can be established, maintained, and recovered to a significant extent. Regions of unstable density profiles are shown to develop and persist in the field-scale experiment, which we attribute to regions of low hydraulic conductivity. The saturated-unsaturated, variable-density ground-water flow simulation code SUTRA is modified to describe the system of interest, and used to compare simulations to experimental observations and to investigate certain unobserved aspects of these complex systems.