Installation Date:
1999

Contaminants:
Nitrate, Uranium

Reactive Media:
Fe⁰ and Wood Chips

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.

Another of these PRB systems was installed in 1999 to treat contaminated ground water at the Solar Ponds Plume where, as a result of past waste storage practices, nitrate (NO₃) and uranium (U) are present in excess of the Action Level Framework Tier I level ground-water concentrations defined in the RFCA. The Solar Ponds were drained and the sludges removed by 1995, but contaminated ground water migrated away from the source area to a nearby creek. Contaminant concentrations at the Solar Ponds Plume are: NO₃ at 140-170 mg/L and U at 20-28 pCi/L. The treatment goal is to meet surface water standards of 100 mg/L of nitrate and 10 pCi/L of uranium in the adjacent creek. These goals are currently being achieved.

The ground-water collection system at the Solar Ponds Plume extends approximately 1,100 ft. To install the collection system, an excavation was dug at a variable depth of approximately 20-30 ft bgs and approximately 10 ft into claystone. The barrier consists of the same HDPE panels as the barrier at the East Trenches Plume; it was installed and operates in the same way.

The below grade, concrete treatment cell is divided into two sections. Treatment media occupies the lower 10 ft of each section. The first cell is about 32 ft long and 17 ft wide and is filled with a mixture of sawdust and leaf mold with 10% zero-valent iron (Fe⁰) by weight to induce denitrification and to remove the uranium by chemical reduction. The media was selected on the basis of bench scale tests conducted at the University of Waterloo. The second cell is about 11 ft long and 17 ft wide and is filled with Fe⁰ to act as a final polisher. The two treatment cells can be run in series or in parallel.

Water exiting the treatment cell typically contains less than 5 mg/L nitrate and less than 1 pCi/L uranium. While the PRB system does not collect and treat all the ground water in this Plume, the surface water standards are consistently met in the nearby creek.

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