Contaminants:
Trichloroethene, cis-1,2-Dichloroethene

Reactive Media:
Fe⁰

Construction:
Slurry Wall Funnel in situ reaction vessel

Point of Contact:
Dale Haig
Goders Associates (UK), Ltd.
Tel: 44-115-9456544
Fax: 44-115-9456540
Email: DHaigh@
GOLDER.com
Landmere Lane
Edwalton , Nottingham NG124DG United Kingdom

A full-scale field test of a permeable reactive barrier (PRB) system was conducted at an industrial facility in Belfast, Ireland. A circular in situ reaction vessel filled with iron was installed to a depth of about 40 ft in December 1995, to treat up to 390 mg/L of trichloroethylene (TCE) and related breakdown products. Previous owners of the site had used chlorinated solvents while manufacturing electronic components. Years of spillage resulted in an intense but localized plume close to the current site boundary.

The TCE plume at this site is located in an area characterized by a thick deposit of till (up to 78 ft) underlain by Mercia mudstones. The till has silt, sand, and gravel lenses that allow contaminants to migrate from the source; however, migration is constrained by the specific orientation of the permeable lenses that contain discrete clay or clayey silt lenses. The depth of the barrier was chosen to intercept the horizon of low permeability that is present at a depth of around 33 ft. The site is characterized as having a water table approximately 20 ft below ground surface, and an underlying aquifer about 40 ft in depth. No information is provided on porosity, transmissivity, hydraulic gradient, etc.

Two 100-ft bentonite cement slurry walls directed water to the inlet of the steel reaction vessel, which was 4 ft in diameter and contained a 16-ft vertical thickness of zero-valent iron. Ground water flowed by gravity through the iron zone and discharged through a piped outlet on the downgradient side of the slurry wall. The vessel was equipped with a manhole to access the top of the iron zone, in the event that periodic scarification of the iron surface proved access was necessary. The system was designed to provide residence time of about 5 days.

The total cost of the system, including slurry walls, granular iron, reaction vessel, and engineering was about $375,000.

The system was designed to meet ground-water quality criteria of 500 µg/L for TCE, which apply to ground water beneath industrial land slated for redevelopment. Flow rates through the reactor have varied substantially since its installation, but data have shown an overall 99.7% reduction in TCE and cis-1,2-dichloroethylene (cDCE) levels through the reaction vessel. Both increased and decreased levels of cDCE resulting from reductive dehalogenation have been identified. TCE levels in the system have been decreasing in the effluent sample ports. Only low levels (in the range of 100 µg/L) of cDCE have been detected. Vinyl chloride, a common breakdown product of this process, has not appeared in appreciable quantities.

SITE-SPECIFIC REFERENCES