Contaminants:
Chromium

Reactive Media:
Sodium dithionite

Construction:
Injection

Point of Contact:
Jonathan S. Fruchter
Batelle Pacific Northwest National Laboratory
Tel: 509-376-3937
Fax: 509-372-1704
Email: john.fruchter@
pnl.gov
P.O. Box 999 (K6-96)
Richland , WA 99352

A large-scale treatability test of an in situ Redox Manipulation (ISRM) method was conducted successfully at the 100D Area of the U.S. Department of Energy (DOE) Hanford Site in Washington in 1997-1998. As a result, the technology is currently being deployed for full-scale remediation of a ground-water hexavalent chromium plume at the site.

The cost of design, construction, materials, and the reactive material is estimated to be $480,000. Cost for the 2,000 ft wall will be approximately $5,000,000. Hexavalent chromium concentrations of up to 2 mg/L have been detected within the 100D Area. Contamination resulted from the use of chromium-bearing anti-corrosion agents in onsite reactors.

The 100D Area is underlain by both glacial and fluvial sediments, predominantly sands and gravels. Hydraulic conductivity is approximately 100 ft/day. The upper surface of the contaminated aquifer is approximately 85 ft below ground surface and is approximately 15 ft thick, constrained at its lower boundary by an aquitard.

ISRM involves injection of a chemical reducing agent in the contaminant plume downgradient from the source area. This agent alters the chemical redox potential of aquifer fluids and sediments. Redox-sensitive metals migrating through the treatment zone are immobilized. The treatability test at Hanford's 100D Area began in September 1997 and consists of injecting sodium dithionite into a series of five existing wells to a depth of 100 ft below ground surface. Treated zones for each well overlap, creating a 150-ft-long barrier that is approximately 50 ft wide.

The upcoming deployment will consist of about 40 wells to form a barrier approximately 2,000 ft long.

Sodium dithionite was injected into the first of the five wells in 1997 and 1998. As a result, aqueous chromate concentrations have been reduced below 8 µg/L. After the completion of a gas tracer test studying rates of reoxygenation in the treated plume, plans called for sodium dithionite to be injected into the remaining four wells in mid-1998, followed by a bromide tracer test to determine the effect of the treatability test on ground-water flow within the aquifer. Performance monitoring is expected to continue through the end of 1999 followed by emplacement of the full-scale barrier.

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