Passively Enhanced in Situ Biodegradation of Chlorinated Solvents.

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INTRODUCTION

Harding Lawson Associates (HLA) conducted a field demonstration to evaluate in situ enhanced anaerobic biodegradation of chlorinated solvents using Hydrogen Release Compound (HRC). The technology is designed to enhance the natural biodegradation process for a more rapid and complete degradation of the organic contaminants to non-toxic compounds (e.g., ethylene). This paper presents the results from a field demonstration that was conducted over an eleven month period.

Technical Approach. Anaerobic biodegradation of chlorinated solvents such as perchloroethene (PCE) and trichloroethene (TCE) require highly reducing conditions to stimulate anaerobic bacteria to dechlorinate the solvents. The technical approach was designed to provide a carbon or electron donor source to create the conditions necessary to enhance anaerobic biodegradation.

Hydrogen Release Compound (HRC) is a lactic acid ester that is manufactured by Regenesis. When delivered to the subsurface, lactic acid, which has been shown to be an effective electron donor, is released continuously into groundwater. Because this material can be injected directly into the subsurface it can be used to deliver nutrients passively and enhance active biodegradation of chlorinated volatile organic compounds (cVOCs).

HLA conducted a field demonstration to evaluate the performance of HRC using a system designed to recirculate groundwater and create a treatment cell. The objectives of this test were to evaluate the rate and extent of chlorinated solvent biodegradation using HRC as the electron donor source and also to gather design data such as how long HRC would last and the extent of biodegradation along the groundwater flowpath.

CASE STUDY: WATERTOWN, MA

Site Description. The site is situated in a historically industrial section of Watertown, MA. The general soil profile consists of approximately 13 feet of sand and gravel over approximately 7 feet of silty sand; then glacial till (an aquitard) is encountered. Groundwater occurs at approximately 8 feet below land surface and is contaminated with chlorinated solvents, including PCE, TCE and degradation products characteristic of natural biological reductive dechlorination.

Design, Construction and Operation. In the field demonstration, groundwater is extracted from three downgradient wells, and injected into three wells 17 feet upgradient. Figures 1 and 2 show the design of the recirculating system. Five 2-inch PVC monitoring wells are positioned between the injection and extraction wells to monitor the progress of the biodegradation process: a groundwater recirculation rate of 0.25 gallons per minute (gpm) establishes a single recirculation cell of about 30 feet in diameter (Figure 2).

lons per minute (gpm) establishes a single recirculation cell of about 30 feet in diameter (Figure 2).

FIGURE 2

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