Treatment of a Chromate-Contaminated Soil Site by In Situ Gaseous Reduction
Laboratory testing activities indicate that hexavalent chromium, a vadose zone contaminant at many waste sites owing to its mobility and toxicity, can be immobilized in place through chemical reduction to the nontoxic trivalent oxidation state using diluted hydrogen sulfide gas. Treating vadose zone contamination by in situ gaseous reduction thus may be potentially applied as part of an overall strategy for groundwater protection and remediation. A proof-of-concept field test has been undertaken by the U.S. Department of Energy and U.S. Department of Defense in a joint demonstration conducted at White Sands Missile Range, New Mexico, to evaluate this remedial approach. This test involved injecting hydrogen sulfide diluted in air into contaminated vadose zone sediments via a centrally located borehole over a 76-day period. The gas mixture was then directed through the sediments using a vacuum applied to six extraction boreholes at the site periphery. Comparison of soil samples taken before and after the test indicated that 70% of the total mass of hexavalent chromium originally present at the site was reduced and immobilized. The zone of highest Cr(VI) contamination was nearly completely treated, with Cr(VI) concentrations of soil samples decreasing from an average of 8.1 mg/kg before treatment to 1.14 mg/kg after treatment and a mass reduction of 88% achieved. Treatment was generally better in zones of higher permeability sand containing less silt and clay. However, all Cr(VI) concentrations measured in post-test samples were well below the EPA Region 9 Residential Preliminary Remediation Goal of 30 mg/kg, compared to a maximum pre-test concentration as high as 85 mg/kg, thus indicating the viability of the technology as a remediation approach.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 902955
- Report Number(s):
- PNNL-SA-45340; GWMREV; KP1302000; TRN: US200719%%89
- Journal Information:
- Ground Water Monitoring and Remediation, 27(1):56-64, Vol. 27, Issue 1; ISSN 1069-3629
- Country of Publication:
- United States
- Language:
- English
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