Installation Date:
10/1995

Contaminants:
Arsenic, Molybdenum, Selenium, Uranium, Vanadium, Zinc

Reactive Media:
Fe⁰, Copper Wool, Steel Wool

Cost:
$380,000

Construction:
Collection Drain Piped to Underground Treatment System

Point of Contact:
Don Metzler
U.S. Department of Energy
Tel: 970-248-7612
Fax: 970-248-6040
Email: donald.metzler@
gjo.doe.gov
2597 B 3/4 Road
Grand Junction , CO 81503

A pilot-scale demonstration of permeable reactive barriers (PRBs) was installed at Bodo Canyon in Durango, CO in 1995. The demonstration was conducted to help treat contaminated water seeping from a tailings disposal cell and test the efficiency of PRBs for remediation of uranium and metals.

Operation of PRB E began in the Spring 2000. Installation costs for the four PRBs, including construction and materials, was approximately $255,000. Design costs were about $125,000 Contaminants of concern at the site are arsenic (As), molybdenum (Mo), selenium (Se), uranium (U), vanadium (V), and zinc (Zn). Concentration of contaminants in the untreated water are 186 µg/L for As, 1180 µg/L for Mo, 337 µg/L for Se, 5540 µg/L for U, 8800 µg/L for V, and 1600 µg/L for Zn.

A total of 2.5 million yd³ of uranium mill tailings were relocated to the Bodo Canyon disposal cell in the fall of 1990. Contaminated seeps developed along the downgradient slope of the disposal cell shortly after construction. The seep water was collected by a collection drain and piped to a retention pond, where it was regularly treated and discharged to a nearby wash.

In order to be able to compare different designs, four PRBs were installed near the retention pond. Contaminated ground water in the collection drain (328 ft long, 4 ft wide, and 3 ft high) is diverted to a holding tank. Water from the holding tank flows to a manifold that distributes it to the PRB. Because of the limited water flow in the area, only one PRB operates at a time and the system is shut down during the winter. Flow rates vary from 0.3-2 gpm.

Zero valent iron (Fe⁰) in a variety of forms was used in each of the PRBs. Two of the PRBs were constructed similarly to septic leach fields, one containing steel wool (PRB A) and the second containing steel wool and copper wool (PRB B). Each of these systems is 20 ft wide, 3 ft long, and 7 ft high. The reactive media is about 12 in thick. The remaining two PRBs were constructed in steel tanks with baffles that forced the water to flow up and down through the PRB. One tank contained Fe⁰ foam plates (PRB C) and the other contained steel wool (PRB D). The foam plates were manufactured by binding fine-grained Fe⁰ with aluminosilicate. Each of these tanks is 6 ft long, 3 ft wide, and 4.2 ft deep. Approximately 70 ft³ of reactive media was used in PRBs C and D. The baffled tanks were used because the reactive media could be easily changed.

Because of the limited flow out of the disposal cell and the fact that this was developed as a pilot demonstration project, three of the four PRBs were shut down. PRB A never ran at all; B ran for approximately one year, and D for two months. In July 1999, PRB C was excavated and 72 samples of the solid foam plates were collected. After complete removal of the plates, PRB C was refilled with granular Fe⁰ (mesh size
-8+20) and renamed PRB E. The change to granular Fe⁰ was made in order to test the effectiveness of a more commonly used reactive media.

The baffled tank with Fe⁰ foam plates (PRB C) operated the longest. Effluent concentrations in May 1999 were 2.2 µg/L for As, 359 µg/L for Mo, 5.9 µg/L for Se, 1.2 µg/L for U, <6 µg/L for V, and <4 µg/L for Zn. After 3 years, flow ceased in PRB C probably due to mineral plugging. Water samples taken at PRB D indicated significant reductions in U concentrations from 7,430 µg/L to 738 µg/L with a flow rate of 0.66 gpm. Early in the project, samples were taken almost monthly. They are currently being taken semi-annually.

Lessons Learned

The PRB reduced concentrations of a wide variety of constituents. The hydraulic head is limited by the elevation between the PRBs and the holding tank. Additional head would be useful to keep the flow from stopping due to small decreases in hydraulic conductivity in the PRB. Gasses (H₂ and CH₄) that built up in the PRB required venting before accessing. The gasses may also have contributed to flow stoppage. The collection drain/PRB system is useful because it can easily be replaced and the flux of water is well known.

SITE-SPECIFIC REFERENCES

MORE INFO