Technology Status Report: In Situ Flushing - CLU-IN

In Situ Flushing Project Summaries
GWRTAC Case Study Database
referred to as nonaqueous phase liquids. Denser-than-water nonaqueous phase liquids (DNAPLs)
frequently enter the subsurface as a separate phase and migrate downward due to gravity. After
the bulk of the DNAPL migrates through the aquifer, discrete "blobs" or ganglia of DNAPL are
entrapped, held in the pore spaces by capillary forces. Under low flow conditions typical in aquifers,
these blobs of residual DNAPL are immobile and dissolve very slowly, thereby creating long-term
sources of contamination (3).
Surfactants can be used to dramatically reduce remediation times via increased solubilization
through micellar partitioning, increased mobilization through reductions in interfacial tension, or a
combination of both. Mobilization has a greater potential to increase remediation efficiency over
solubilization alone, but, if uncontrolled, the mobilized DNAPL may sink deeper into the aquifer
where it is more difficult to remediate and may contaminate previously pristine portions of the
aquifer. Particular mobilization regimes are more easily controlled and thus preferred during
surfactant flushing. Bank formation, the creation of a continuous phase of DNAPL at the surfactant
front, can lead to increased efficiency in DNAPL removal and may be used to control migration of
mobilized DNAPL (4, 5).
The purpose of this investigation was to examine the mobilization of residual DNAPL in wellcharacterized,
two-dimensional homogeneous porous media during surfactant flushing to 1)
determine the critical conditions that lead to different mobilization regimes, 2) quantify the factors
leading to DNAPL bank formation, and 3) examine the use of a dimensionless grouping for
predicting DNAPL bank formation as a function of viscous and buoyancy forces. For all
experiments, a 1:1 mixture of two food-grade, anionic surfactants, sodium diamyl sulfosuccinate
(Aerosol AY) and sodium dioctyl sulfosuccinate (Aerosol OT) were selected for the surfactant
flushing solution. Tetrachloroethylene, a representative groundwater pollutant, was selected as the
DNAPL. Experiments were conducted in two well-characterized cells packed with glass beads.
Explanations of the mobilization regimes are given using a dimensionless grouping of system
parameters defined as the bank number. The bank number was defined as the total force acting on
a DNAPL ganglion in the flow direction divided by the force acting on the same ganglion in a
direction perpendicular to the flow. A bank number of 1 indicates a balance in viscous and
buoyancy forces. Different bank numbers were achieved by varying the angle of the cell (i.e., the
angle of flow) and the surfactant injection rate.
For each experiment, an aqueous solution of 1% Aerosol AY/OT was injected into the cell. Upon
contact with the surfactant solution, the vast majority of the PCE was immediately mobilized as a
separate phase for all experiments.
In five experiments, PCE was immediately mobilized downward upon contact with the surfactant
solution, forming a PCE pool at the bottom of the cell. This mobilization regime was termed
"downward mobilization." As the experiments progressed, the pool grew, moved through the cell,
and finally exited the cell. Bank numbers for these experiments ranged from 0.41 to 1.34. Nearly
complete downward mobilization in these experiments was expected due to the dominating effect
of the gravitational force on the mobilized DNAPL ganglia. Figure 1 is a representative set of
images taken at three separate times during a surfactant experiment where downward mobilization
was observed, resulting in the formation of a DNAPL pool.
Figure 1. Downward mobilization with DNAPL pool formation (Refer to internet source to view)
Ground-Water Remediation Technologies Analysis Center
Operated by Concurrent Technologies Corporation
Appendix - Page 157 of 164
Copyright GWRTAC 1998
Revision 1
Tuesday, November 17, 1998