Enhanced Bioremediation of Chlorinated Solvents in Wetland Seep Areas, Aberdeen Proving Ground: WBC-2 Dechlorinating Culture and Reactive Mat Pilot Test

Introduction

Approximate Locations of Proposed Seep Sampling Sites


Click image to view full-sized version.

Groundwater contaminated with chlorinated VOCs discharges from a 30-50-ft thick aquifer to wetlands and a tidal creek in the West Branch area at APG, Maryland. A previous USGS study in the wetland showed that natural attenuation (primarily biodegradation) in the wetland sediments efficiently removed the contaminants before they reached the creek.

However, subsequent USGS study identified DNAPL sources in the wetland area and localized seep areas where natural attenuation was not occurring in the wetland sediments. Chlorinated solvents are discharging to the surface water unattenuated. Targeted, enhanced bioremediation methods were developed and field tested for these seeps.

Zone 1: compost/peat mixture, bioaugmented with WBC-2
Zone 2: ZVI with the compost/peat mixture

The USGS designed a reactive mat to maximize natural processes observed in nearby wetland sediments. The mat is composed primarily of a commercial compost and peat bioaugmented with a microbial culture, WBC-2, derived from nearby non-seep wetland sediments where natural attenuation is efficient. Thus, the mat mimics the natural wetland processes and could be self-sustaining.

Reactive Mat Design Goals

With a mat thickness of 22 inches:

• Achieve 90 percent removal of VOCs in mat
• Establish and maintain conditions conducive to WBC-2 survival (redox, pH)
• Achieve hydraulic compatibility
• Achieve geotechnical stability (settlement 0.4-1 ft)
• Cause no adverse effects to wetland water quality from nutrients or metals mobilization

Reactive Mat Installation and Performance at Seep 3-4w Pilot Test

Seep Pilot Test Site

Thermal infared (TIR) imaging used to locate seep areas. Discrete, thermally-warm discharge areas were visible in winter 2002 and 2003. Example of TIR-image (left) and digital image (right) of seep area 3-4W, selected for the reactive mat pilot test.

Contours micrograms per liter

Seep Background Concentrations

Major VOCs at seep 3-4W include chloroform (CF), carbon tetrachloride (CT), tetrachloroethene (PCE), 1,1,2,2-tetrachloroethane (TeCA), hexachloroethane, and pentachloroethane. Total VOC concentrations in the shallow groundwater were greater than 50,000 micrograms per liter. Plan view of concentration contours show seep is localized in an area of about 30 ft x 40 ft.

Unlike non-seep areas where natural attenuation is effective, VOC concentrations do not decrease as groundwater flows upward through wetland sediments in seep area. Background sampling showed that biodegradation is insignificant.

Reactive Mat Installation

Removed top 1 ft of wetland sediment with vacuum truck, and then added mixtures for reactive mat (22 inches total). Mat initially extended above wetland surface to allow for settling.

Monitoring network installed in and around mat.

Reactive Mat Performance

Monitoring by the USGS over a 1-year period showed consistent mass removal in the mat, even during the cold winter months.

Click the image to view
the full-sized version.

Decrease in total chloromethanes (in micromoles) as plume moves from base of mat and 1.35 ft upward toward mat surface. Below detection at mat surface. Mass removal 95 to 99.99 %.

Click the image to view
the full-sized version.

Chlorinated ethenes and ethanes (in micromoles) maintain similar footprint to the chloromethanes; less overall mass. Mass removal 96-99%.

Example vertical profile at one monitoring site in mat. Decrease in concentration of all chlorinated VOCs in mat (top 2 ft). Increase in the non-toxic, non-chlorinated product, ethene, shows biodegradation is occurring.

WBC-2 Culture

USGS derived WBC-2 by enrichment of wetland sediments in non-seep area of West Branch Canal Creek area.

WBC-2 is unique in the broad range of chlorinated solvents that it can completely and rapidly degrade, including 1122TeCA, 112TCA, PCE, TCE, DCE, vinyl chloride, CT, and CF.

Anaerobic degradation pathways for VOCs in West Branch area:

WBC-2 grown to large quantities by GeoSyntec/SIRem. Currently in 100 liter vessel.

WBC-2 is an anaerobic culture but has surprising resistance to oxygen. Complete degradation of TeCA to ethene was observed after 60 minutes of bubbling with oxygen. This is important for ease and success in field application.

USGS and GeoSyntec formed a CRADA (Cooperative Research and Development Agreement) to further develop WBC-2 and the reactive mat. These technologies are available royalty free to government agencies.