Installation Date:
1998

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

Reactive Media:
Fe⁰

Cost:
$235,000

Construction:
Continuous Trench

Point of Contact:
Peter Kjeldsen
Technical University of Denmark
Tel: +45 45251561
Fax: +45 45932850
Email: pk@er.dtu.dk
Environmental & Resources DTU
Building 115
DTU, DK-2800
Kgs. Lyngby Denmark

A full-scale permeable reactive barrier (PRB) system was installed in 1998 to remediate chlorinated aliphatics in a shallow aquifer at the Copenhagen Freight Yard, Denmark. Degreasing agents, including trichloroethylene (TCE) and possibly perchloroethylene (PCE), were used in train repair operations on the site. The upper aquifer contains chlorinated aliphatics in concentrations up to 4,000 µg/L. Although cis- and trans-1,2-dichloroethylene (DCE) are the dominant contaminants due to dehalogenation in the aquifer, PCE, TCE, and vinyl chloride (VC) also are found. Up to 3,000 µg/L of cis-1,2-DCE and up to 700 µg/L of trans-1,2-DCE are present.

The local geology consists of a top layer of 6.5 ft of sandy fill underlaid by 3.3-6.6 ft of clayey fill mixed with peat. Below that is 6.5-10 ft of marine coastal sand, which acts as a local upper ground-water aquifer with a piezometric head about 8 ft below ground level. The sand is underlaid by 3.3-6.6 ft of clayey till followed by 20 ft of silty, fluvial sand in hydraulic contact with Danien limestone, which acts as a regional ground-water aquifer. The permeability of the upper aquifer is 0.00019 ft/s, and the average linear velocity of the ground water is about 0.36 ft/day.

The PRB system is about 50 ft long, 20 ft deep, and 3 ft thick. During construction of the PRB, sheet pilings were installed first, and the area inside was excavated to form a continuous trench that was backfilled with 83 short tons of Fe⁰. The top of the PRB is 8.2 ft below ground and is covered with a layer of geotextile, compacted clay, gravel, and pavement on top. The PRB was installed to cut off that part of the plume where the total concentration of chlorinated aliphatics exceeds 100 µg/L. The PRB design was chosen due to cost and availability of construction techniques in Denmark. Design and installation costs are not available at this time.

Sampling has been completed. To date, the PRB has effectively treated upgradient concentrations of chlorinated aliphatics above 650 µg/L, bringing them below 10 µg/L downstream. Total chlorinated solvents reduced in the PRB from greater than 1,000 µg/L to less than 50 µg/L, which represents about a 95% reduction.

Detailed hydraulic studies of ground-water levels and contaminant concentration distributions revealed that as much as one-fifth of the plume is migrating around the barrier and escaping the PRB’s capture zone. The reason why water is flowing around the barrier may be due to a combination of factors. The PRB may not be in an ideal location to capture the plume that is flowing more to the northeast than the initial site investigation indicated. Also, hydraulic conductivity of the Fe⁰ in the PRB has decreased with time. The loss of conductivity is probably due to precipitation of iron hydroxides, carbonates, and calcium carbonates. Slug tests in the PRB revealed conductivity of 0.00004-0.00021 ft/s, compared to the value of 0.0016 ft/s that the manufacturer provided. An estimated 2,200 lb of iron hydroxide (Fe[OH]₂), 440 lb of calcium carbonate (CaCO₃), 440 lb of iron carbonate (FeCO₃), and 130 lb of iron sulfide (FeS) precipitate in the PRB each year.

Lessons Learned

Precipitation leading to decreased permeability of the PRB may occur in ground water containing high concentrations of inorganic compounds. In the Freight Yard PRB, the total dissolved solids decreased with about 600 mg/L passing though the wall. If the permeability of the Freight Yard PRB continues to decrease, the efficiency of the PRB also may decrease with time.

SITE-SPECIFIC REFERENCES

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