Environ. Sci. Technol. 2002, 36, 3176-3187
Density-Modified Displacement for DNAPL Source Zone Remediation: Density Conversion and Recovery in Heterogeneous Aquifer Cells C. ANDREW RAMSBURG AND KURT D. PENNELL* School of Civil and Environmental Engineering, Georgia Institute of Technology, 200 Bobby Dodd Way, Atlanta, Georgia 30332-0512
Low interfacial tension (IFT) displacement (mobilization) of nonaqueous phase liquids (NAPLs) offers potential as an efficient remediation technology for contaminated aquifer source zones. However, displacement of dense NAPLs (DNAPLs) is problematic due to the tendency for downward migration and redistribution of the mobilized DNAPL. To overcome this limitation, a density-modified displacement method (DMD) was developed, which couples in situ density conversion of DNAPLs via alcohol partitioning with low IFT NAPL displacement and recovery. The objective of this work was to evaluate the DMD method for two representative DNAPLs, chlorobenzene (CB) and trichloroethene (TCE). Laboratory-scale experiments were conducted in a two-dimensional (2-D) cell, configured to represent a heterogeneous unconfined aquifer system containing low permeability lenses. After release and redistribution of either CB- or TCE-NAPL, the 2-D aquifer cells were flushed with a 6% (wt) n-butanol aqueous solution to achieve DNAPL to light NAPL conversion, followed by a low IFT surfactant solution consisting of 4% (4:1) Aerosol MA/Aerosol OT + 20% n-butanol + 500 mg/L CaCl2. Visual observations and experimental measurements demonstrated that in situ density conversion and immiscible displacement of both CB and TCE were successful. Effluent NAPL densities ranged from 0.96 to 0.90 g/mL for CB and from 0.95 to 0.92 g/mL for TCE, while aqueous phase densities remained above 0.96 g/L. Density conversion of CB and TCE was achieved after flushing with 1.2 and 4.9 pore vol of 6% n-butanol solution, respectively. Recoveries of 90% CB and 85% TCE were realized after flushing with 1.2 pore vol of the low IFT surfactant solution, which was followed by a 1 pore vol posttreatment water flood. Surfactant and n-butanol recoveries ranged from 75 to 96% based on effluent concentration data. The observed minimal mobilization during the n-butanol density conversion preflood and near complete mobilization during the low IFT displacement flood were consistent with total trapping number calculations. The results reported herein demonstrate the potential efficiency of the DMD technology as a means of DNAPL source zone restoration.
Introduction Many advances in the immiscible displacement of nonaqueous phase liquids (NAPLs) from unconsolidated aquifer * Corresponding author telephone: (404)894-9365; fax: (404)8948266; e-mail:
[email protected]. 3176
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 14, 2002
materials are attributable to research in the field of enhanced oil recovery. One of the first applications of surfactant flooding techniques for NAPL recovery was designed to displace free phase gasoline in a laboratory soil column (1). Early surfactant enhanced aquifer remediation (SEAR) studies were cognizant of the potential for low interfacial tension (IFT) surfactant solutions to initiate downward migration of dense NAPLs (DNAPLs) (2, 3). Furthermore, experiments conducted by Pennell et al. (3) indicate that ultralow IFTs (
