Environmental Engineering Reference
In-Depth Information
A number of laboratory studies have examined the dissolution and
mobilization of NAPL by cosolvents (Luthy et al., 1992; Brandes and Farley,
1993; Peters and Luthy, 1993, 1994; Milazzo, 1993; Imhoff et al., 1995; Roy et
al., 1995; Lunn and Kueper, 1996, 1997). These studies have shown that NAPL
may be effectively removed using cosolvents such as alcohols. The laboratory
studies have also shown that cosolvent floods can be designed to favor either
enhanced dissolution or mobilization by judicious selection of the organic
cosolvent. For systems designed to promote NAPL dissolution (as with the
study reported herein), the solubility enhancement as a result of the addition
of the organic cosolvent can be estimated using the log-linear cosolvency
model (Yalkowsky and Roseman, 1981; Fu and Luthy, 1986; Morris et al.,
1988). The solubility of a nonpolar organic solute in a binary solvent mixture
(S
m
) increases in a log-linear manner with increasing volume fraction of
cosolvent (f
c
):
log S
m
= log S
w
+ β σ f
c
where σ is the cosolvency power, β is an empirical coefficient that accounts
for water-cosolvent interactions, S
c
is solubility in a neat solvent, and S
w
is
solubility in water.
5.1.2 Technology status: cosolvent flushing
A limited number of field-scale, cosolvent flushing demonstrations have
been conducted. Two cosolvent flushing demonstrations were conducted at
Hill Air Force Base (AFB), UT, in isolated test cells installed in a
sand-and-gravel aquifer contaminated with a multicomponent NAPL (Rao
et al., 1997; Sillan et al., 1998; Falta et al., 1999). Rao et al. (1997) demonstrated
NAPL remediation by enhanced dissolution. The test cell was approximately
4.3 m long by 3.6 m wide, and the clay-confining unit was 6 m below grade.
A total of 40,000 l of a ternary cosolvent mixture (70% ethanol, 12% pentanol,
and 18% water) was injected into the cell over a 10-day period. Based on
several remediation performance measures (target contaminant concentra-
tions in soil cores, target contaminant mass removed at extraction wells, and
pre- and postflushing target contaminant groundwater concentrations), the
cell-averaged reduction in contaminant mass was reported as >85%. They
also reported an approximate 81% reduction in NAPL saturation based on
pre- and postflushing partitioning interwell tracer tests (PITTs). Falta et al.
(1999) presented results from a second cosolvent flushing study at Hill AFB,
wherein the remedial mechanisms were NAPL mobilization and enhanced
dissolution. Their test cell was approximately 5 m long by 3 m wide, and
the clay-confining unit was 9 m below grade. They injected 28,000 l of a
ternary cosolvent mixture (80% tert-butanol, 15% n-hexanol, and 5% water)
over a 7-day period. Reductions in target contaminant concentrations mea-
sured from pre- and postflushing soil cores were reported to range from 70
