Passive Reactive Barrier at Oak Ridge National Laboratory, Oak Ridge, Tennessee

Site Name:

Oak Ridge National Laboratory

Location:

Oak Ridge, Tennessee

Period of
Operation:

August 1997 to Ongoing
(data available through August 1999)

Cleanup
Type:

Field demonstration

Technology:
PRBs
- Two PRBs installed to intercept two pathways of the shallow groundwater contaminant plume - funnel-and-gate PRB (FGPRB) and trench PRB (TPRB)

FGPRB
- Sand-filled collection trench that is 220 ft long and 25 ft deep- two wing walls used to funnel groundwater to treatment zone; collection side and discharge side separated by a HDPE impermeable barrier installed vertically in middle of the trench
- Two treatment systems demonstrated; (1) treatment train of three 55-gal canisters run in series - pH adjustment using magnesium hydroxide, iron and gravel; ZVI for uranium removal; iron and peat mixture for nitrate removal; (2) electrochemical cell - one 55-gal drum equipped with electrodes at the top and bottom and filled with ZVI; applied current used to increase groundwater pH to increase reductive capacity of the iron - Buildup of hydraulic head in discharge portion of PRB required installation of pumps in the treatment area to move groundwater through the system

TPRB
- 225 long and 30 ft deep groundwater capture trench installed subparallel to groundwater flow with an impermeable barrier on downgradient side of the trench; groundwater flows through a section of reactive iron media in the middle of the trench (26ft long by 2 ft wide by 30 ft deep) then discharged through gravel backfill - Buildup of hydraulic head in discharge portion of PRB required the trench to be extended and an enhancement zone to be constructed to provide sufficient gradient to overcome the hydraulic buildup

Cleanup Authority:
Not identified

Contacts:
Technical Contact:
William Goldberg
MSE Technology Applications
Telephone: (406) 494-7330

DOE Contract:
Scott McMullin
DOE/OST/Savanah River Operations Office
Telephone: (803) 725-9596

Contaminants:
Radioactivity
- Uranium concentrations in groundwater ranging as high as 1.7 mg/L to 2.6 mg/L

Waste Source:
Storage of uranium-contaminated liquid waste in ponds

Type/Quantity of Media Treated:
Groundwater
- Shallow groundwater - unconsolidated zone; relatively low hydraulic gradient
- Deep groundwater - bedrock
- Soil is 20-30 ft thick; relatively low permeability except at transition zone (weathered and fractured bedrock between soil and competent bedrock)
FGPRB - 133,000 gal of groundwater treated
TPRB - 200,000 to 400,000 gal of groundwater treated

Purpose/Significance of Application:
Two demonstrations of PRB technology to treat groundwater contaminated with uranium

Regulatory Requirements/Cleanup Goals:
- The objective of the demonstrations were to reduce the amount of contaminant reaching Bear Creek through the two pathways in the shallow groundwater
- No specific cleanup levels identified for the demonstration

Results:
FGPRB
- Uranium concentrations in groundwater reduced 80 to 99.6%
- Reduction of secondary contaminants 75% for nitrate and 42% for sulfate

TPRB
- Uranium concentrations in groundwater reduced about 90%
- Reduction of secondary contaminants highly variable

Low hydraulic gradient and recharge from the deep aquifer affected the performance of the PRBs by causing buildup of hydraulic head on the downgradient side of the trenches adversely impacted the hydraulic operation and treatment effectiveness of the systems

Cost Factors:
- Actual installation costs for the FGPRB demonstration were $943,000; costs for long term O&M are under development
- Costs for the TPRB had not been fully developed and were therefore not included in the report

Description:
The S-3 Ponds at the Oak Ridge National Laboratory, Oak Ridge Reservation Y-12 Plant in Oak Ridge, Tennessee were used for the disposal of uranium-contaminated waste. These waste disposal activities resulted in three contaminant migration pathways at the site - two in the shallow groundwater and one in the deep groundwater. Demonstrations of two PRB systems were conducted at the site to treat the two contaminant pathways from the shallow groundwater. Uranium concentrations in the shallow groundwater ranged as high as 1.7 mg/L to 2.6 mg/L.

The two technologies demonstrated were FGPRB and TPRB, using reactive iron as the treatment media. Data from two years of operation (August 1997 to August 1999) show that uranium concentrations in groundwater were reduced by as much as 96.6% by the FGPRB and 90% by the TPRB. During this time, the FGPRB treated about 133,000 gallons of groundwater and the TPRB treated between 200,000 and 400,000 gallons of groundwater. During the demonstrations, buildup of hydraulic head on the downgradient side of the trenches adversely impacted the hydraulic operation and treatment effectiveness of the systems and system modifications were performed to address the problem. The systems are continuing to operate.