Technology Status Report: In Situ Flushing - CLU-IN

In Situ Flushing Project Summaries
GWRTAC Case Study Database
In two experiments, distinct DNAPL banks were formed. For these experiments, the bank numbers
were 0.96 and 1.05. Upon contact with the surfactant solution, PCE was immediately mobilized
downward, forming a pool at the bottom of the cell. The surfactant solution flowed preferentially
along the top of the micro-cell as the mobilized PCE migrated downward, leaving a less permeable
region at the bottom of the cell. Because the PCE pool was less viscous than the aqueous phase,
the pool moved faster than the aqueous phase and began climbing up the surfactant front. As the
PCE climbed up the surfactant front, a bank of PCE formed at this front, i.e., a DNAPL bank
formed. Figure 2 is a representative set of images taken from a surfactant experiment with
"DNAPL bank formation."
Figure 2. Distinct DNAPL bank formation (Refer to internet source to view)
In four experiments, less distinct DNAPL banks were formed. PCE was mobilized upon contact with
the surfactant front, forming a DNAPL pool at the surfactant/clean water front. As the surfactant
mobilized DNAPL, some of the DNAPL moved ahead of the surfactant front, where the interfacial
tension increased, retrapping the DNAPL until the front reached it once again. This process
resulted in the formation of a DNAPL bank at the leading edge of the surfactant front. This process
has been reported elsewhere, by Constantinides and Payatakes (5) and Pennell et al. (6). As
shown in the series of images in Figure 3 below, these banks were not as distinct as the banks
formed in the two experiments depicted in Figure 2.
Figure 3. Diffuse DNAPL bank formation (Refer to internet source to view)
In the experiments depicted in Figures 2 and 3, PCE was mobilized in the directions of flow, not in
the direction of gravity (as in the downward mobilization experiments depicted in Figure 1). This
behavior was expected because of the dominant viscous forces (compared to the buoyancy forces)
and because the displaced fluid was less viscous than the flushing fluid.
At bank numbers less than 0.8 and greater than 2, the macroscopic flow behavior (i.e., whether
downward mobilization or DNAPL bank formation occurs) is easily explained by examination of the
relative magnitude and direction of the viscous and buoyancy forces. However, the data suggest
that a DNAPL bank may or may not form around a bank number of 1. This is a critical region,
where the DNAPL must "climb" up the surfactant/clean water front to form a bank. If the surfactant
solution lowers the IFT sufficiently on the upgradient side of the front, then the DNAPL moves
down this front and a DNAPL bank does not form. At bank numbers of 2 and greater, though, the
force acting on the DNAPL in the direction of flow is sufficiently large that a DNAPL bank always
forms. Because the bank number relies on general physical and chemical properties, it should be
applicable to flushing of any homogeneous porous media. Further testing is required to verify this
hypothesis.
References
1. Patrick, R., Ford, E., and Quarles, J., Groundwater Contamination in the United States, Second
Edition, University of Pennsylvania Press, 1987.
2. Mackay, D. M. and Cherry, J. A., Groundwater contamination: pump-and-treat remediation,
Environmental Science and Technology, 23(6), 630-636, 1989.
Ground-Water Remediation Technologies Analysis Center
Operated by Concurrent Technologies Corporation
Appendix - Page 158 of 164
Copyright GWRTAC 1998
Revision 1
Tuesday, November 17, 1998