Installation Date:
10/25/1999

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

Reactive Media:
Fe⁰, iron filings

Cost:
$350,000

Construction:
Funnel and Gate

Point of Contact:
Barry Kellems , P.E.
Hart Crowser, Inc.
Tel: 206-324-9530
Fax: 206-328-5581
Email: barry.kellems@
hartcrowser.com
1910 Fairview Ave. East
Seattle , WA 98102-3699

A full-scale permeable reactive barrier (PRB) system was installed at a former manufacturing site in Seattle, WA in 1999. The funnel and gate PRB was selected to accommodate the buildings, railroad tracks, and public walkways on site, as well as adjacent public areas and properties which precluded large excavations for remediation. The 200 ft wide ground-water plume consisted of tetrachloroethylene (PCE) and its degradation products trichloroethylene (TCE), cis-1,2-dichloroethylene (cis-DCE), and vinyl chloride (VC). Two potential source areas were identified with the maximum detected constituent concentrations in ground water of 50 mg/L PCE, 23 mg/L TCE, 8 mg/L cis-DCE, and 0.8 mg/L VC. Areas of high pH (up to pH 12) were also found in the ground water.

The site was used for metal anodizing and light manufacturing from the 1940s until 1989. It sits within 200 ft of the Lake Washington Ship Canal and adjacent to the third most popular tourist destination in the city. Upper soil layers consisted of heterogeneous fill material overlaying stratified estuarine deposits, forming two distinct water-bearing zones. Chlorinated solvents were found up to 32 ft deep beneath the surface where a dense, relatively impermeable till prevents vertical migration of contaminants.

The 330 ft long funnel and gate consists of three cement bentonite (CB) cutoff walls (funnels) and two 45 ft long permeable reactive gates. The 3 ft wide gates were designed with a 50/50 mixture of iron filings and sand. Approximately 180 tons of iron were used. The gates were constructed with biodegradable, guar-based slurry thus avoiding the cost of and need to drive sheet piling. The barrier was embedded 3 ft into the dense, impermeable till layer to prevent underflow. The total depth of the wall ranged from 24-35 ft.

Each of the funnels was constructed by excavating under CB slurry. Within days the CB hardened to a clay-like material that is relatively impermeable. It was able to withstand various obstructions including sewer pipes, a water line, and railroad tracks. The gate sections were constructed by excavating under a bio-polymer slurry composed of guar gum, additives and water. Two temporary wells were installed in each gate. An iron and sand mixture was piped directly into the trench under the slurry. After backfilling, the temporary wells were pumped while adding breaker enzymes, and recirculating the slurry over the top of the iron/sand. This caused the slurry to degrade without the need for disposal. After a few days of recirculating, the viscosity of the bio-polymer was reduced and the trench was ready for a final cap of compacted clay. The total installation cost including construction and materials was approximately $300,000. Design costs were $50,000 including the pilot test. An additional $50,000 was spent on site preparation and restoration and soil disposal.

Due to the proximity to the Ship Canal, the cleanup goals for treatment were based on protection of surface water, specifically 4.2 µg/L PCE, 56 µg/L TCE, 80 µg/L c-DCE, and 2.9 µg/L VC. Six months after installation, the measured treatment efficiencies for the wall were 65% for c-DCE, 95% for VC, and greater than 99% for both PCE and TCE. Natural attenuation processes down gradient of the wall, including intrinsic biodegredation, are reducing concentrations to below surface water cleanup standards before reaching the canal. Quarterly monitoring continues at the site.

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