Contaminants:
Uranium, Technetium, Nitric acid

Reactive Media:
Fe⁰

Construction:
Funnel and Gate, Continuous Trench

Point of Contact:
Baohua Gu
Oak Ridge National Laboratory
Tel: 423-574-7286
Fax: 423-576-8543
Email: b26@ornl.gov
Env. Sciences Division
Oak Ridge , TN 37831-6036

Permeable reactive barrier (PRB) systems have been constructed in two different ground-water pathways through the Y-12 site at the U.S. Department of Energy's (DOE) Oak Ridge National Laboratory, TN. Liquid wastes, including nitric acid (HNO₃) with uranium (U), and technetium (Tc), were placed in disposal ponds on the site from 1952 to 1981. The site was capped in 1983. Leached wastes have contaminated both ground water and surface water.

The site is underlain by unconsolidated clay and regolith overlying fractured shales. The permeability of the clay is very low (approximately 4 x 10^-7 in/sec), but the weathered bedrock above the shales generally has a higher permeability (locally as high as 4 x 10^-4 in/sec). The depth to ground water is 10-15 ft, and the shallow unconsolidated unit aquifer is 10-20 ft thick. The PRBs are focused on capturing ground water in this shallow unconsolidated zone.

Pathway 1 PRB

A funnel and gate system was installed in the area designated Pathway 1 in December 1997. It is approximately 220 ft long and consists of two wing walls designed to funnel ground water to a concrete vault containing treatment canisters for evaluating different treatment media. The treatment vault consists of five vertically stacked reactors. An advantage of vertical reactors is the ease of cleaning and replacing used or clogged iron. The wing walls were installed to a depth of approximately 25 ft. The natural ground-water gradient and permeability contrast between the gravel backfill in the trench and surrounding native silt and clay is designed to generate flow through the treatment zone. Barriers were installed using a guar gum slurry for support to reduce slumping in the trench. An enzyme breaker was used to digest the guar which was recycled down the trench as construction progressed.

Pathway 2 PRB

A continuous trench system was installed in the area designated Pathway 2 in November 1997. It is 225 ft long, 2 ft wide, 22-30 ft deep, and filled with gravel except for a 26-ft section in the middle that is filled with 80 tons of zero-valent iron (Fe⁰). Guar gum was added during excavation to keep the trench walls from collapsing. The trench was installed parallel to the direction of ground-water flow.

Although total iron and ferrous iron concentrations were initially high after installation, concentrations have decreased as the pH within the iron has increased over time (to as high as 9 or 10). This initial spike is likely a result of enhanced microbial activity from the guar used in the barrier installation. Due to the effect of the guar on ground-water chemistry, nitrate concentrations increased in the upgradient wells over time. Sulfate levels in the ground water have decreased as sulfate is reduced to sulfide. Additionally, a decreased concentration of calcium in ground water was observed and may be attributed to the precipitation of calcium carbonate within the iron barrier. Continued monitoring and performance evaluation is in progress to better understand the flow paths through the PRB, the potential for clogging due to mineral precipitation, and the long-term effectiveness for uranium removal.

The total cost for the two walls was approximately $1,000,000. This includes the cost of design, construction, materials, and the reactive material.

The goals of the project were to investigate the feasibility and effectiveness of passive in situ treatment systems to remove the contaminants in the ground water that are migrating to Bear Creek from the disposal ponds. Early results indicate that the Fe⁰ is an efficient and cost-effective method of simultaneously removing certain radionuclides, such as U and Tc, as well as HNO₃. Sampling to monitor performance is occurring on a monthly basis.

Lessons Learned

Pathway 1 PRB:

The use of guar increased biological activity in the system.

Pathway 2 PRB:

Preliminary evaluation of hydraulic and chemical data suggests that, under wet-season hydraulic conditions, contaminated ground water may migrate across the trench instead of down the trench as designed. Vertical gradients at the site appear to have a significant impact on ground-water flow and capture. The data suggest that to effectively operate passively in all hydraulic conditions, the trench needs to be longer and discharge at a lower hydraulic head downgradient. The following modifications are planned for the Pathway 2 PRB in fiscal year 1999 to enhance treatment efficiency:

1) The trench will be extended an additional 100 ft to increase the ground-water capture zone.

2) Guar will not be used to excavate the trench extension because of potential geochemical impacts on the iron, media, native soil, and ground water observed during initial trench construction.

3) Ground water from the trench extension will be siphoned approximately 800 ft to a second Fe⁰ treatment zone deployed in subsurface concrete boxes.

4) The treated water will flow into an infiltration trench downgradient of the second treatment zone.

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