Investigations of HRC®-Stimulated Bioreduction of Cr(VI) at Hanford 100H
Hypothesis: Lactate (Hydrogen Release Compound-HRC{trademark}) injection into chromium contaminated groundwater through an injection well will cause indirect or direct bioreduction of chromate [Cr(VI)] and precipitation of insoluble species of [Cr(III)] on soil particles, probably catalyzed at oxide surfaces, at the field scale. Objective: Assess the potential for immobilizing and detoxifying chromium-contaminated groundwater using lactate-stimulated bioreduction of Cr(VI) to Cr(III) at the Hanford Site's 100-H Area field site. Types of Research: A three-well system (injection well and upgradient and downgradient monitoring wells) was used for conducting the in situ biostimulation and monitoring. To assess the pre- and post-injection test groundwater conditions, we used an integrated monitoring approach, involving hydraulic, geochemical, microbial, and geophysical techniques and analytical methods, as well as conducted five Br-tracer injection tests and four pumping tests (concurrently with the Br-tracer tests). Groundwater biostimulation was conducted by injection of 40 lbs of {sup 13}C-labeled HRC into the injection well (over the depth interval from 44-50 ft) on 8/3/2004, followed by low-flow pumping (1.2 to 2.5 l/min) through the downgradient well (to ensure capture of groundwater flow lines passing through the injection well) for 27 days. Main Results: Although the total microbial population in sediments is relatively low (<10{sup 5} cells g-1) under background conditions, which is likely insufficient for direct enzymatic Cr(VI) reduction, several types of bacteria, e.g., Bacillus/Arthrobacter and Geobacter, are present in the Hanford sediments, which are known to reduce or sorb hexavalent chromium. The HRC injection stimulated microbial cell counts to reach the maximum of 2 x 10{sup 7} cells g{sup -1} 13-17 days after the injection, and generated highly reducing conditions. Geochemical and isotopic observations confirmed microbial metabolism of HRC. The Cr(VI) concentration in the monitoring and pumping wells decreased below drinking water minimum contaminant limits and remained below background concentrations even after 1.5 years, when redox conditions and microbial densities had returned to background levels. Fe(II) levels have remained high and may account for the continued reduction of Cr(VI).
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- OSTI ID:
- 894998
- Report Number(s):
- CONF-NABIR2005-2; TRN: US200702%%539
- Resource Relation:
- Conference: Annual NABIR PI Meeting, April 18, 2005, Warrenton, VA
- Country of Publication:
- United States
- Language:
- English
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