In Situ Gaseous Reduction System Demonstrated at White Sands Missile Range, New Mexico

Site Name:

White Sands Missile Range

Location:

New Mexico

Period of
Operation:

April - June 1998

Cleanup
Type:

Field demonstration

Technology:
In Situ Gaseous Reduction (ISGR)
- ISGR involves injection of a low concentration H2S gas mixture (100-200 ppmv) into soils, where it reacts with oxidized metals such as Cr6+ and uranium, followed by extraction of gas containing reduced metals, such as Cr3+
- System included an injection pump, extraction pump, water knockout tank, scrubber, one central injection well, and six extraction wells; wells were completed to approximately 20 ft bgs
- Treatment progress was measured by breakthrough of H2S at the extraction wells

Cleanup Authority:
Not identified

Technical Contacts:
Ed Thornton
Pacific Northwest National Laboratory (PNNL)
(509) 373-0358
DOE Contacts:
Jim Hanson
DOE EM50
(509) 372-4503

James A. Wright
DOE SRS
(803) 725-5608

Contaminants:
Heavy metals
- Cr6+

Waste Source:
Spills

Type/Quantity of Media Treated:
Soil (in situ)

Purpose/Significance of Application:
Demonstrate use of injection of H2S for in situ reduction of hexavalent chromium

Regulatory Requirements/Cleanup Goals:
- Objectives of demonstration were to provide technical and cost information about ISGR; obtain operational information; and determine site air flow characteristics
- No specific cleanup goals were identified

Results:
- After completion of the demonstration, soil samples were collected from nine boreholes; these results showed that nearly all the Cr6+ in the interval from 4 - 10 ft bgs was reduced - this zone contained clean white gypsum sand that initially contained the highest concentrations of Cr6+
- The mass of Cr6+ did not change appreciably in the 10-16 ft bgs interval, which contained a brownish sand containing gypsum plus clay
- These results suggested that the effectiveness of ISGR is limited by subsurface heterogeneities, with channeling of the injected gases in the most permeable white sand
- Comparison of pre- and post-demonstration soil samples showed that >70% of the Cr6+ mass was reduced, and all post-treatment samples had <30 mg/kg of Cr6+

Cost Factors:
- Projected costs for a full-scale application of ISGR were a total cost of $798,163, or $43/yd3
- Projected unit costs for ISGR were estimated to range as high as $100/yd3, depending on the size of the waste site

Description:
The White Sands Missile Range lies within the Mexican Highland Section of the Basin and Range Province. Contamination was discovered at SWMU 143 in January 1990 when greenish-yellow soil was found in a corner of the equipment yard. A review of facility records indicated that several 55-gallon drums of Entec 300 had spilled directly onto the ground in 1982 or 1983.

In a cooperative effort between DOE and DoD, ISGR was demonstrated at this site in the spring and summer of 1998. The technology involved injecting 200 ppm H2S mixture into chromate-contaminated soils. Results showed that >70% of the Cr6+ in the soil was reduced to Cr3+ during the demonstration, and that all post-treatment soil samples had <30 mg/kg of Cr6+. The amount of H2S consumed during the test was greater than the amount predicted in laboratory studies, and is likely due to interfering reactions in the field or slower reaction kinetics. A life-cycle cost analysis suggested that ISGR should be a less expensive remedy than excavation, especially for sites where the depth of contamination is more than 15 - 20 ft bgs. During FY 1999-2000, a deployment is planned at the DOE Hanford site to remediate Cr6+-contaminated soils in the 100 Area.