Installation Date:
08/2000

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

Reactive Media:
Fe⁰ and sand

Cost:
$2,200,000

Construction:
Continuous Wall

Point of Contact:
Tom Krug
GeoSyntec Consultants
Tel: 519-822-2230
Email: tkrug@
geosyntec.com
160 Research Lane
Suite 206

A full-scale permeable reactive barrier (PRB) system was installed in 2000 at the Somersworth Sanitary Landfill Superfund Site in Somersworth, NH. The site is a 26-acre landfill that was constructed in the early 1930s on the site of a former sand and gravel quarry. The landfill was used to dispose of household trash, business refuse, and industrial wastes. Waste was burned at the landfill until 1958. From 1958 to 1981, the waste material was placed in excavated areas, compacted, and covered with soil. In 1981, use of the landfill stopped when the City of Somersworth began disposing of its municipal waste at a regional incinerator. Also in 1981, the City implemented a closure plan for the landfill that involved the covering of a portion of the landfill with clean fill. Volatile organic compounds (VOCs), including tetrachloroethylene (PCE), trichloroethylene (TCE), 1,2-dichloroethylene (1,2-DCE), and vinyl chloride (VC) were found to be present in the ground water. Initial concentrations encountered were up to 410 μg/L for PCE, up to 370 μg/L for TCE, up to 530 μg/L for 1,2-DCE, up to 1,900 μg/L for (VC).

The site is characterized by sands and gravels having a hydraulic conductivity in the range of 0.02 cm/second. The hydraulic gradient varies from 0.01-0.004 ft/ft near the edge of the waste. The top of the water table ranges from less than 2-20 ft below ground surface. As much as 10% of the waste is located below the water table. The aquifer is 30-40 ft thick.

The PRB is a continuous wall 915 ft long and extending to a depth of 26-47 ft below the ground surface. The PRB was designed with 8 sections, each approximately 100 ft long, with differing amounts of zero-valent iron (ZVI) corresponding to the ZVI required to treat the specific concentration of VOCs entering each section of the PRB. The vertical interval of the PRB (interval containing ZVI) ranges from 20-40 ft. A continuous wall was used for the PRB as opposed to a funnel-and-gate system to reduce the impacts on the existing ground-water flow conditions at the site and to reduce the potential for mounding of ground water on the upgradient side of the PRB. The cleanup goals for VOC-contaminants are: 5 μg/L for PCE, 5 μg/L for TCE, 70 μg/L for cis-1,2-DCE, and 2 μg/L for VC.

The PRB was installed using an open trench supported by a biodegradable bio-polymer slurry. This installation method was used because of its low cost and suitability for site conditions.

The construction method dictated that the PRB have a minimum thickness of 30 in, corresponding to the width of the excavator bucket. Inert, coarse washed sand was mixed with the ZVI before being placed in the trench. The different sections of the PRB contained ZVI-sand mixtures with between 40% and 100% ZVI by weight. Total design cost for the system amounted to $200,000; construction/installation cost, including the cost of ZVI, totaled $2,000,000.

Some difficulties were encountered during installation. The PRB was divided into 23 separate panels, each typically 30-50 ft long. The contractor initially installed alternating panels (primary panels) along the length of the PRB, then installed secondary panels between the primary panels. Typically, primary panels were excavated in one day. During the installation of the primary panels, the bio-polymer remained stable (i.e., maintained sufficient viscosity to support the trench) overnight or in some cases for several days. During the installation of the first two secondary panels, the contractor had difficulties maintaining the stability of the bio-polymer and some sand and silt settled out into the bottom of the trench before the ZVI was placed in the trench. Following these difficulties, the contractor excavated and backfilled secondary panels in a single day and difficulties with subsequent panels were averted. The area affected by the silt and sand represents approximately 1% of the PRB. The difficulties during construction have not had any measurable effect on the performance of the PRB. Sampling is being conducted three times per year to monitor PRB performance.

Ground-water monitoring conducted during the first year following installation shows that the PRB is operating as designed. Ongoing ground-water monitoring required by the Consent Decree for the Site is being conducted three times per year.

Lessons Learned

The approach to the design and the bio-polymer construction method were demonstrated to be effective and economical for a large ZVI PRB. Difficulties during construction highlighted the need to monitor the stability of the bio-polymer slurry during construction.

SITE-SPECIFIC REFERENCES

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