Contaminants:
Trichloroethene, cis-1,2-Dichloroethene, Vinyl chloride

Reactive Media:
Fe⁰

Construction:
Continuous trench

Point of Contact:
Steven Schroeder
RMT, Inc.
Tel: 864-281-0030
Fax: 864-287-0288
Email: steve.schroeder@
rmtinc.com
100 Verdae Blvd.
P.O. Box 16778
Greensville , SC 29606-6778

Phase 1 of a full-scale permeable reactive barrier (PRB) was installed at a former industrial site in Manning, SC, in November 1997. Trichloroethylene (TCE), cis-1,2-dichloroethylene (cDCE), and vinyl chloride (VC) have been detected in two aquifers that underlie the site at concentrations of 25 mg/L, 3.5 mg/L, and 0.9 mg/L, respectively. TCE concentrations in the lower of the two contaminated aquifers are generally one order of magnitude less than those in the upper aquifer.

The upper aquifer is 5-15 ft below ground surface (bgs). It is composed primarily of sandy to silty fill material with a hydraulic conductivity of 2 ft/day. A clay unit forms the lower boundary of this aquifer. The intermediate aquifer (18-27 ft bgs) is composed of fine silt laminae and very fine sand layers within the clay unit and has a hydraulic conductivity of 2.6 ft/day. The lower portion of this clay unit forms a boundary between the intermediate and lower aquifers. Monitoring wells did not detect any volatile organic compounds (VOCs) in the lower aquifer. The hydraulic conductivity of the upper aquifer is reported to be 2 ft/day while the intermediate aquifer's hydraulic conductivity is 2.6 ft/day. No information is provided on the lower aquifer.

The PRB was installed to the base of the intermediate aquifer. It is a 1-ft-wide continuous trench composed of 50% sand and 50% zero-valent iron by volume in the form of iron filings. The 400 tons of zero-valent iron was homogeneously distributed throughout the sand using cement-mixing equipment. A one-pass trenching technique was used from a surface bench 4-6 ft bgs. This surface bench allowed the trenching equipment to reach the final depth of 29 ft bgs. Phase 1 of the installation called for a 325-ft section to address the highest concentrations of VOCs and mitigate suspected offsite migration. Phase I construction—including mobilization, benching, installation, and demobilization—was completed in 4 weeks.

Design for this PRB system was $50,000. The total installation cost for both phases will be approximately $350,000. This includes construction, materials, and the cost of the reactive media.

Cleanup goals for the site are 0.005 mg/L for TCE, 0.070 mg/L for cDCE, and 0.002 mg/L for VC. Quarterly ground-water sampling and analysis from wells both upgradient and downgradient of the wall is continuing. While VOC concentrations in the upgradient monitoring wells remain highly variable, wells installed on the downgradient side of the wall have shown generally consistent downward trends and lower VOC concentrations than the upgradient wells.

Minor problems were encountered at the start of Phase 1 installation, with some material cave-in occurring at the top 3-4 ft of the trench sidewalls. This problem was alleviated by reconfiguring the location of the feed hopper on top of the boot and by adding steel plates to the top portion of the boot, to improve material flow. Installation through the two aquifers has affected ground-water flow in the vicinity of the treatment wall. By providing a greater connection between the two aquifers, ground-water velocities have been reduced and ground-water flowpaths modified slightly. The reduction in ground-water velocities and modified flowpaths should not affect the capability of the treatment wall to intercept and adequately treat VOCs at the site. Increased residence time for treatment will improve the long term treatment efficacy.

Modifications to the ground-water monitoring schedule were also necessary to take into account differences in ground-water flow rates.

Lessons Learned

Compared with other methods, continuous trenching provided cost-effective installation and a high degree of confidence that materials would be placed according to the design, to create a continuous treatment wall with equal distribution of the zero-valent iron.


Because of the reduced ground-water flow velocity at the site, more time than originally estimated will be required to complete an initial flushing of VOCs in downgradient ground water.

Site managers have found that the presence of chloride is not a good indicator of the effectiveness of the dechlorination process for this site.

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