Environmental Engineering Reference
In-Depth Information
possible) for residual phase materials were utilized to address this
problem. The limitations of these approaches in terms of both efficacy and
performance, as well as a more developed understanding of the nature of
dense nonaqueous phase liquid (DNAPL) source areas and plume dynamics,
precipitated the research and testing of second phase technologies, such as
source zone treatment by mobility agents (cosolvents, surfactants, steam),
reactants for source zones (permanganate) and dissolved phase (permeable
reactive barriers), and biological-based technologies focused on the dis-
solved phase. We are entering a third phase of technological development
driven by the twin forces of risk management and cost to restoration. This
third phase of technology development is characterized by multiple tech-
nology application and focused on comprehensive site restoration
approaches.
5.1.1
In situ
flushing technology
NAPLs, such as fuels, oils, and industrial solvents, may act as long-term
sources of groundwater pollution when released into aquifers because of
their low aqueous solubilities. DNAPLs are denser than water and are more
difficult to remediate because of their tendency to sink and pool in the
aquifer. Conventional remediation such as pump and treat can take many
decades to remove DNAPLs (MacKay and Cherry, 1989). Enhanced source
zone remediation can expedite the removal of contaminants. One enhanced
source zone remediation technique is
in situ
cosolvent flushing, which
involves the addition of miscible organic solvents to water to increase the
solubility or mobility of the NAPL (Imhoff et al., 1995; Roeder and Falta,
1998; Lunn and Kueper, 1997; Rao et al., 1997; Augustijn et al., 1997; Lowe
et al., 1999). In the case of DNAPLs, increased mobility can result in greater
contaminant risk due to the potential for downward migration, and density
modification of the NAPL has been proposed to prevent this risk (Roeder et
al., 1996; Lunn and Kueper, 1997, 1999). Alcohols have principally been used
as cosolvents for enhanced source zone remediation (Lowe et al., 1999).
In situ
cosolvent flushing provides four primary advantages over the
traditional pump-and-treat remediation approach (Augustijn et al., 1994a,
1994b; Augustijn et al., 1997):
•
Enhanced solubilization as a result of reduced polarity of the displac-
ing fluid
•
Enhanced mobilization of the NAPL as a result of reduced interfacial
tension between the NAPL and the cosolvent and because of swelling
of the NAPL ganglia by uptake of the cosolvent
•
Enhanced desorption and a concomitant reduction in retardation,
again due to an increase in solubility
•
Increased dissolution/desorption kinetics due to an increase in the
mass transfer rate constant, and an increase in the driving force (i.e.,
the concentration gradient)
