Nonaqueous Phase Liquid Source Zone Research Articles

EVALUATION OF REMEDIATION PERFORMANCE AND COST FOR FIELD-SCALE SINGLE-PHASE MICROEMULSION (SPME) FLUSHING

Pump-and-treat strategies employed to contain contaminant plumes have been demonstrated to be inefficient, and often ineffective, for remediation of aquifers contaminated by nonaqueous phase liquids (NAPLs). This has prompted the development of alternative technologies to provide enhanced remediation of NAPL source zones. The results reported here are from a solubilization study wherein the objective was to achieve high NAPL remediation efficiencies using a low concentration of chemical additives in the flushing solution. A surfactant/alcohol mixture was used to generate a Winsor Type I system, where the NAPL was solubilized and transported as a single-phase microemulsion (SPME). For the SPME process, only 5.5 wt% of the flushing solution comprised chemical additives. The costs associated with this project are discussed and approaches aimed at reducing costs are suggested.

Biologically-enhanced removal of PCE from NAPL source zones.

The influence of dechlorinating microorganisms on PCE and its reduction products in a residual nonaqueous phase liquid (NAPL) source zone was investigated. Experiments were conducted in upflow columns containing glass beads (diameters 500-750 microns) contaminated with a residual NAPL consisting of tridecane and labeled 14C-PCE. Three columns were inoculated with a mixed PCE-dechlorinating culture, that was fed electron donor (pyruvate) at concentrations of 25, 100, and 250 mM. Pyruvate was fermented in all columns with essentially no methanogenic activity. Comparisons between actively dechlorinating columns and abiotic-PCE columns demonstrated that dechlorination resulted in an increase in total PCE removal, up to a factor of 16 over dissolution. PCE was sequentially reduced to trichloroethene, cis-dichloroethene, and vinyl chloride without ethene formation over the experimental period in the two columns operated at the lower electron donor levels. Total chlorinated ethenes removal for the columns that retained dechlorinating populations was enhanced from 5.0 to 6.5 times over the removal that would have resulted from dissolution alone. The system fed the highest pyruvate levels, interestingly, lost dechlorinating activity early in the experiment.

Reaction-enhanced mass transfer and transport from non-aqueous phase liquid source zones

The objective of the following research is to theoretically quantify the enhancement of interphase mass transfer of dissolved non-aqueous phase liquid (NAPL) compounds from the non-aqueous phase to the aqueous phase and the enhancement of dispersive mass transport from a NAPL zone due to destruction of dissolved NAPL compounds. For relatively slow reaction rates, such as for permanganate and perchloroethene (PCE), local-scale mass transfer enhancement is expected to be small. Dispersive mass transport with reaction from a horizontal NAPL zone can be quantified using equations derived for a mathematically equivalent falling film reactor system. In contrast to local-scale interphase mass transfer, dispersive mass transport from NAPL zones may be significantly increased by reaction. Enhancement factors due to destruction of the NAPL compound(s) are mainly dependent on NAPL solubility and oxidant concentration and to a lesser extent on reaction rate, stoichiometry, and transverse dispersion coefficients. Higher NAPL solubility and/or lower oxidant concentration reduces the maximum expected enhancement factor. Reaction enhancement factors for mass transport from NAPL zones are expected to be in the range of 5–50 for permanganate and chlorinated solvents. Theoretical results suggest that assuming instantaneous reaction rates may be appropriate for dispersive mass transport from NAPL zones.

NAPL source zone characterization and remediation technology performance assessment: recent developments and applications of tracer techniques

Several innovative tracer techniques have been introduced during the past 5 years for enhanced characterization of the “source zones” at sites contaminated with non-aqueous phase liquid (NAPL) wastes. These tracer techniques allow for an in situ estimation of domain-averaged values and spatial patterns for NAPL saturation ( S n), NAPL–water interfacial area ( a nw), and bio-geochemical reactivity ( k s) within the target test zone. The tracer tests can be used to evaluate the spatial patterns in these parameters, both before and after implementing some in situ technique for site cleanup, in order to evaluate the effectiveness of remediation achieved and the possible impacts of the cleanup technology on hydrodynamic and bio-geochemical processes. Here, we review the theoretical and experimental basis for these tracer methods, present selected examples of recent field-scale applications, and examine their reliability.