Installation Date:
1999

Contaminants:
Trichloroethene, Tetrachloroethene, Carbon tetrachloride, Chloroform, cis-1,2-Dichloroethene, Methylene chloride

Reactive Media:
Fe⁰ and Pea Gravel

Cost:
$1,300,000

Construction:
Reaction Vessels

Point of Contact:
Annette Primrose
Kaiser-Hill Co, LLC
Tel: 303- 966-4385
Fax: 303- 966-5180
Email: Annette.Primrose@
rfets.gov
Rocky Flats Environmental Technology Site
10808 Highway 93, Unit B, Building 116
Golden , CO 80403-8200

Full-scale permeable reactive barrier (PRB) systems have been installed to treat contaminated ground water at three sites at the U.S. Department of Energy's Rocky Flats Environmental Technology Site (RFETS) in Golden, CO. These projects were a cooperative effort between RFETS and the Department of Energy (DOE) Subsurface Contaminant Focus Area, with support from U.S. EPA's National Risk Management Research Laboratory.

The same type of PRB system was installed in 1999 to treat contaminated ground water at the East Trenches Plume. As a result of past waste storage practices, VOCs are present in excess of the Action Level Framework Tier I level ground-water concentrations defined in the Rocky Flats Cleanup Agreement. Sources were removed in 1996, but the contaminated ground water migrated away from source areas towards a nearby creek. Contaminant concentrations at the East Trenches plume are primarily: TCE, 2,700-4,500 µg/L; PCE, 250-490 µg/L; CCl₄, 130-240 µg/L; chloroform, 82-140 µg/L; cis-1,2-DCE, 23-38 µg/L; and methylene chloride 6-470 µg/L. The treatment goal at the site is to remove contaminants of concern (COCs) to below Action Levels specified in the Rocky Flats Cleanup Agreement. This involves removal of the volatile organic compounds (VOCs) as follows—TCE to 5 µg/L; PCE to 5 µg/L; CCl₄ to 5 µg/L; chloroform to 100 µg/L; cis-1,2-DCE to 70 µg/L; and methylene chloride 5 µg/L.

The ground-water collection system for the East Trenches Plume PRB system extends approximately 1,200 ft. To install the collection system, an excavation was dug at a variable depth of approximately 16-26 ft below ground surface, at least 6 in and an average of 3 ft into claystone. The barrier was installed on the downgradient side of the excavation and consists of 80-mil high-density polyethylene (HDPE) panels fitted with an interlocking strip on each side. The trench was backfilled with sand and includes a perforated HDPE ground-water collection line that was placed on the sand, and piped to a central collection sump. The collected ground water then flows by gravity from the collection sump through a 2-in, non-perforated HDPE line to the two treatment cells.

Each treatment cells is approximately 12 ft in diameter and 13 ft tall. Ground water enters the cells at the top and percolates through the 6.5 ft of iron. There is 1 ft of granular material on the bottom of each treatment cell to disperse the ground water. The upper foot of each cell is a 50/50 mixture of iron and pea gravel to simplify mechanical break-up of the expected crust formation.

Results have shown that, with the exception of methylene chloride, concentrations of VOCs are routinely non-detectable in the effluent samples.

Lessons Learned

Lessons learned to date from these installations include the following: (1) during construction, the length of trench open must be minimized to reduce slope failure, (2) equipment operations and stockpiling adjacent to open trenches must be minimized, (3) maintenance must be considered in a PRB's design, (4) backfill specifications must be rigidly followed, (5) gravity flow is most effective when the natural contours of a hillside can be utilized; and (6) cost estimated for this system are about one-fourth of the estimated baseline “pump-and-treat” costs.

SITE-SPECIFIC REFERENCES