| This thesis explores the feasibility of treating chlorinated solvents stored in low permeability zones through three activities: a modeling study addressing evolution of a chlorinated solvent release, a laboratory study involving treatment of contaminants using an alkaline persulfate solution, and laboratory testing of three innovative treatment technologies.;The hypothesis of the modeling study was that subsurface releases of chlorinated solvents evolve through time from a problem of dense non-aqueous phase liquids (DNAPLs) in transmissive zones, to a problem of dissolved and sorbed phases in low permeability zones. This hypothesis was tested using analytical solutions for a two-layer system involving a horizontal transmissive layer situated above a low permeability layer and a DNAPL-like perchloroethene source at the contact between the two layers. The source was active with a constant strength for 1,000 days. Subsequently, the source was shut off and contaminant distribution through the domain of interest was evaluated for an additional 2,000 days. All calculations were carried out using Mathcad™ 14.;The hypothesis of the alkaline persulfate laboratory study was that flushing an alkaline persulfate solution (a chemical oxidant) through a transmissive zone provides a means of significantly reducing future releases of contaminants from low permeability zones. A conceptual model and a laboratory-scale sand tank experiment were employed. The experiment involved a continuous transmissive sand layer with interbedded layers of kaolin-bentonite clay. The tank was flushed with water spiked with 100 mg/L fluorescein and 67 mg/L bromide for 92 days. During this period contaminants are attenuated by the clay layers. Fluorescein is the targeted contaminant and bromide is a conservative tracer. After 92 days, the tank was flushed with water without fluorescein and bromide for 38 days. This illustrates how release of contaminant stored in low permeability zones effects downgradient water quality. Next, fluorescein was treated by flushing the tank with 40,000 mg/L alkaline persulfate solution at pH 11 for eight days. The concentration and pH of persulfate were resolved through batch flask studies. Lastly, the tank was flushed with water with no additives for 69 days to evaluate post-treatment rebound of contaminant levels in the transmissive zones.;Results from the tank study may not be representative of performance under common field conditions. Constraints at field sites could include effective delivery of the high-density alkaline solution to a desired target, high soil oxidant demands in natural porous media that limit treatment of the targeted compounds, and uncertainties regarding treatment of compounds that are more recalcitrant than fluorescein. In addition, secondary water quality issues might arise from the use of a high pH and high total dissolved solids (TDS) alkaline persulfate solution.;The hypothesis of laboratory testing of innovative treatment technologies was that carbon sequestration, sonication, and calcium polysulfide (CPS) were promising technologies for treatment of chlorinated solvents in low permeability zones. Carbon sequestration involves emplacement of solid phase carbon in transmissive zones with the benefits of (1) adsorbing contaminants and (2) creating a redox poise that favors reductive dechlorination, and/or (3) providing a favorable substrate for microbes that facilitates in situ treatment. Column studies were performed to investigate deliverability of three carbon types (activated carbon, carbon black and charcoal) in a porous medium. In addition, vial studies were conducted to determine if conditions favoring reductive dechlorination were imposed. Delivery of carbon in sand columns resulted in clogging the influent pores precluding effective delivery. Furthermore, no evidence was developed to indicate that carbon could impose redox conditions that would drive reductive dechlorination of TCE. (Abstract shortened by UMI.). |
