Installation Date:
1998

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

Reactive Media:
Fe⁰

Cost:
$590,000

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.

Between April 1954 and September 1958, about 1,400 intact drums were temporarily stored at Rocky Flats’ Mound Site. Contaminants from the drum disposal area leaked and created a VOC ground-water plume. For many years, this ground water was collected from a discharge seep, stored in a holding tank, and then trucked to a wastewater treatment plant. This approach was expensive, and the PRB was installed in 1998 as an effective, long-term, passive, cost-effective alternative. Initial contaminant concentrations at the Mound Site Plume were: Trichloroethylene (TCE), 67-160 µg/L; perchloroethylene (PCE), 43-130 µg/L; carbon tetrachloride (CCl₄), 54-130 µg/L; chloroform (CCl₃), 12-26 µg/L; cis-1,2-dichloroethylene (cis-1,2-DCE), 21-62 µg/L; 1,1-dichloroethylene (1,1-DCE), 5-12 µg/L; and total uranium (U), 0-9.5 pCi/L.

The objective of installing a subsurface ground-water collection and treatment system for the Mound Site plume was to capture contaminated ground water at the distal end of the plume, and as a result, to minimize contaminant impacts to surface water quality. The goal of treatment at the site was 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; CCl₃ to 100 µg/L; cis-1,2-DCE to 70 µg/L; 1,1-DCE to 7 µg/L; and U to 10 pCi/L.

The Mound Site Plume System is completely passive and consists of a below-grade collection system along with two treatment cells. The collection system consists of a 230-ft long sand-filled collection trench with a downgradient impermeable-barrier. Ground water in the distal end of the plume is intercepted by and collected in the trench, then flows by gravity to the two subsurface treatment cells. Each treatment cell contains 300 ft³ of Fe⁰. After treatment, the water flows by gravity to the discharge gallery. Flow rate through the vessels is 0.1 to 2 gpm, and retention time is estimated at 20 hours at 1 gpm. Total installation cost for the system was $590,000.

Results have shown that concentrations of VOCs are routinely non-detectable in the effluent samples, and uranium concentrations are consistently below stream standards. The system treats contaminated ground water to levels consistent with unrestricted discharge, minimizes the generation of low level mixed waste, and has been demonstrated to be a low cost and effective technology.

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