In Situ Redox Manipulation at U.S. DOE Hanford Site, 100-H and 100-D Areas

Site Name:

Hanford Site

Location:

Richland, WA

Period of
Operation:

September 1995 to September 1998

Cleanup
Type:

Field demonstration

Technology:
In Situ Redox Manipulation (ISRM)
- The field demonstration used 20,500 gallons of buffered sodium dithionite solution (Na2S2O4, also known as hydrosulfite) to react with natural iron in the subsurface and form reduced iron (Fe2+); the reduced iron reacts with chromate to form insoluble chromium oxides
- Dithionite solution was injected through one 8-inch diameter injection/extraction well, allowed to react for 18 hrs, and then withdrawn; this created a reduced zone 50 ft in diameter
- The withdrawal phase took 83 hrs and 4.8 injection volumes to remove unreacted reagent, buffer, reaction products, bromide tracer, and mobilized metals
- 16 two-inch monitoring wells were used to assess physical and chemical conditions after the test

Cleanup Authority:
Not identified

Technical Contacts:
John Fruchter
Pacific Northwest National Laboratory
(509) 376-3937

Wayne Martin
Pacific Northwest National Laboratory
(509) 372-4881
Management Contacts:
James A. Wright
DOE SR, Field Manager
(803) 725-5608

Contaminants:
Chromium
- Initial chromate concentrations 60 ug/L in 100-H area and 910 ug/L in 100-D area

Waste Source:
Nuclear processing operations

Type/Quantity of Media Treated:
Groundwater
- Depth to groundwater is 50 ft in 100-H area and 85 ft in 100-D area
- Aquifer is 15-20 ft thick

Purpose/Significance of Application:
Demonstrate in situ redox manipulation for treatment of hexavalent chromium

Regulatory Requirements/Cleanup Goals:
- Evaluate performance of ISRM for treating chromium in groundwater
- No specific cleanup goals were identified

Results:
- Concentrations of chromium in groundwater were reduced to less than 8 ug/L in one month
- 87-90% of the dithionite solution was recovered during the withdrawal phase, along with most of the mobilized metals (Fe, Mn, Zn)
- Within 25 ft of the injection well, 60-100% of the available iron was reduced; this zone was estimated to have a life of 7-13 yrs
- Two years after treatment was complete, the treatment zone remained anoxic and hexavalent chromium below detection limits

Cost Factors:
- Projected costs for use of ISRM in a full-scale deployment at this site were identified using two methodologies (one for a 200 ft barrier and one for a 1,400 ft barrier), both in comparison to projected costs for pump and treat; this analysis showed cost savings for use of ISRM of $4.6 to 16 million

Description:
The 100 Area of the Hanford site contains nine nuclear reactors, and is located in the north-central portion of the site near the Columbia River. During reactor operations, chromium was introduced to the soil and groundwater in this area.

A demonstration of in situ redox manipulation (ISRM) was conducted in the 100-H and 100-D areas at Hanford that consisted of field-scale demonstrations. ISRM is a passive barrier technique that uses injection of buffered sodium dithionite solution (Na2S2O4) to react with natural iron in the subsurface and form reduced iron (Fe2+); the reduced iron reacts with chromate to form insoluble chromium oxides. Results from the field demonstration test showed that initial chromate concentrations of 60 ug/L in the 100-H area and 910 ug/L in the 100-D area were reduced to less than 8 ug/L in a one month period. In addition, 87-90% of the dithionite solution was recovered during the withdrawal phase, along with most of the mobilized metals (Fe, Mn, Zn). A full-scale deployment for the Hanford 100-HR-3 operable unit is planned to begin in late 1999.