Installation Date:
06/30/1999

Contaminants:
Uranium, Arsenic, Manganese, Selenium, Vanadium

Reactive Media:
Fe⁰

Cost:
$800,000

Construction:
Funnel and Gate

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

Stan Morrison
MACTEC-ERS
Tel: 970-248-6373
Fax: 970-248-6040
Email: s.morrison@
gjo.doe.com
2597 B 3/4 Road
Grand Junction , CO 81503

A full-scale permeable reactive barrier (PRB) was installed in 1999 to remediate ground water at a Superfund site in Monticello, UT. Cleanup of this site is being managed by the U.S. Department of Energy (DOE) Grand Junction Project Office, in cooperation with U.S. EPA Region 8 and the State of Utah. The former Monticello mill site was built in 1942. It was operated as a uranium and vanadium ore-processing mill intermittently until 1960. Major ground-water contaminants of concern at the site are uranium (U), selenium (Se), vanadium (V), manganese (Mn), and arsenic (As). These contaminants were present at initial concentrations of 700 μg/L, 40 μg/L, 400 μg/L, and 10 μg/L, respectively.

The site occupies a 110-acre tract on the southeast edge of Monticello. When the mill was closed, approximately 2.3 million yd³ of low-level radioactive mill tailings, contaminated soil, and other miscellaneous debris remained on site.

The funnel-and-gate system includes slurry walls that funnel the contaminated plume through the gate, a PRB containing zero-valent iron (Fe⁰). The south slurry wall is 240 ft long and the north wall is 97 ft long. These funnel walls are composed of a bentonite and soil slurry mix. The barrier was built by driving steel sheet piling into the bedrock forming a rectangular box approximately 100  ft  long by 8 ft wide. The native soils inside the box were replaced with Fe⁰ and gravel packs upgradient and downgradient of the Fe⁰. The Fe⁰ and gravel packs extend more than 1 ft into the underlying bedrock aquiclude. The upgradient gravel pack is approximately 2 ft wide and is composed of 13% –4/+20 mesh Fe⁰ (by volume) mixed uniformly with 0.5-in gravel. The center section of the barrier contains 4 ft of 100% -8/+20 mesh Fe⁰. The downstream gravel pack is approximately 2 ft wide composed of 0.5-inch gravel and includes an air-sparging system constructed of perforated polyvinyl-chloride pipe. The air sparging system is being used to remove dissolved manganese and iron if the concentrations increase to unacceptable levels. Installation of the system cost $800,000.

An extensive monitoring network was installed to evaluate the performance of the PRB. To date, nine sampling rounds have been completed (September, October, and November 1999; and January, April, August, and October 2000; January and April 2001). Additional comprehensive sampling will be conducted in July 2001 and then a reduced number of wells and analytes will be evaluated on a quarterly basis in the future.

Overall, the PRB wall has been effective in reducing contaminant concentrations. Concentrations of, As, Se, U, and V have been reduced to non-detectable levels within the PRB. As expected, concentrations of iron increase as ground water passes through the PRB. Concentrations of iron exiting the wall are lower than expected (based on the treatability studies) and are well within acceptable risk ranges. Additional data evaluation is ongoing to make more accurate estimates of ground-water velocities and residence times in the gate.

Lessons Learned

An extensive tracer study, conducted in July 2000, showed that ground water moved through the reactive gate at a rate of 2.4 to 18 ft/day; the design velocity was 10 ft/day. While ground water generally moved in a straight path through the system, some ground water moved laterally through the Fe⁰ before it moved downgradient.

SITE-SPECIFIC REFERENCES

MORE INFO