This project evaluated the cost and performance of in situ surfactant flooding for DNAPL removal, and the feasibility and benefits of surfactant regeneration and reuse at a former dry cleaning facility located at the Marine Corps Base (MCB) Camp Lejeune, North Carolina. During the field demonstration, 110 yd3 of a low permeability (5 x 10-4 to 5 x 10-5 cm/sec) shallow aquifer contaminated by residual and free-phase tetrachloroethene (PCE) was treated using a surfactant formulation that was tailored for high PCE solubilization, injection into a high clay content aquifer, and amenability to surfactant recovery. The removal of PCE DNAPL in the bottom 5 ft of the shallow aquifer was targeted. A petroleum distillate, secondary contaminant dissolved in the PCE DNAPL, was removed incidentally with the PCE DNAPL. The extracted surfactant solution was treated at the surface using pervaporation to separate the organic contaminants and concentrated using micellar-enhanced ultrafiltration (MEUF) to reinject surfactants at their original concentration.
Using a well array consisting of a central row of 3 injection wells flanked by 2 rows of 3 extraction wells, and 2 hydraulic control wells, a total of 76 gallons of DNAPL was recovered during the surfactant flood and subsequent water flood. Approximately 32 gallons of PCE were recovered as solubilized DNAPL and 44 gallons were mobilized as free-phase DNAPL. Post-treatment soil samples showed the DNAPL removal efficiency to be approximately 72%. The cost of treating the remaining DNAPL contamination (460 yd3) by surfactant enhanced aquifer remediation (SEAR) at this site, omitting additional site characterization and performance assessment costs before or following treatment and assuming conventional wastewater treatment (i.e., no surfactant recycle), is estimated at $1,391,900. Present worth cost estimates for other treatment alternatives at a similar low permeability site (also excluding pre- and post-site characterization costs) include pump and treat at $1,535,000, steam injection at $1,195,000, and resistive heating at $639,000.
This in situ technology effectively removed DNAPL from the most mobile (high permeability) regions of the aquifer within a 4.5-month period. No PCE degradation by-products were generated because SEAR is not a destruction technology. The SEAR process can also remediate LNAPL, DNAPL, and other NAPLs of intermediate densities (such as mixed NAPLs). An experimental anionic surfactant was developed for the dual purpose of accomplishing low interfacial tensions and efficient surfactant recycle, that has applicability to surfactant remediation of other chlorinated solvents and petroleum hydrocarbon NAPLs.
The low permeability and order of magnitude permeability contrast in the contaminated zone significantly increased the project duration and costs. Shallow conditions prevented the use of mobility control fluids to improve the sweep of surfactant solution through lower permeability zones. Mobility control fluids should be used whenever site conditions are not limiting for optimum SEAR performance. Pore clogging of fine sediments was avoided by including alcohol as the cosolvent to prevent formation of liquid crystals and gels, and calcium as the electrolyte to prevent fine clay mobilization. Pervaporation achieved PCE removal from surfactant to reach regulatory standards for reinjection; however, during MEUF, contaminants and calcium became concentrated to unacceptable levels. Fresh surfactant was added to adjust calcium concentrations prior to reinjection. Currently, the feasibility of surfactant recycle is strongly dependent on site-specific regulations, as contaminant removal to maximum contaminant levels (MCLs) prior to surfactant reinjection is cost-prohibitive. Additional technical improvements are needed to realize the cost benefits of surfactant recovery for sites with high clay content necessitating the use of calcium as the sole electrolyte. (Project Completed - 2001)