T213 Quantifying Controls on Microbial Reaction Rates in Subsurface Environments I

Wednesday, 19 October 2005
8:00 AM - 12:00 PM

QUANTIFICATION OF ENHANCED BIOREDUCTION OF CR(VI) USING 13C-PLFA MONITORING COUPLED WITH ADVANCED MICROBIOLOGICAL TECHNIQUES

Paper number: T213-5
Program: Technical Sessions
Time: 8:55 AM – 9:10 AM

Authors

Abstract

Cr(VI) is a widespread groundwater contaminant. Under appropriate geochemical conditions, it can be biologically reduced to Cr(III), a much less toxic/mobile form of Cr. To stimulate Cr(VI) bioreduction in groundwater at the Hanford 100H field site, 18 kg of HRC® was injected into the aquifer. HRC® consists of polylactate esterified to a glycerol backbone that slowly releases lactic acid, providing a carbon source to support microbial activity. 10 g of 13C-labeled polylactate was added to the HRC® giving it a δ13C value of ~40‰ (vs. an unlabeled value of -15‰). The response of the subsurface microbial communities to the HRC® injection is being monitored in water samples collected from 4 depth intervals in the injection well and in an extraction well located ~5 m down-gradient of the injection well. Standard geochemical parameters (e.g., dissolved O2, Eh, anion chemistry) and the δ13C values of dissolved inorganic carbon (DIC), dissolved organic carbon, and phospholipid fatty acid (PLFA) extracts of biomass in the samples are being analyzed. Cell counts, PLFA and 16s rDNA analyses are also being monitored.

Microbial populations increased from background levels of <104 cells/ml to 2x107 cells/ml 13-17 days after the injection of the HRC®. 16S rDNA microarray analysis showed enrichment of specific bacterial populations (e.g., Pseudomonas, Acidovorax, Thauera, Flavobacteria, Desulfovibrio) and archael (only non-methanogenic Crenarchaeotes) populations. This corresponds with the onset of reducing conditions, disappearance of NO3, decrease in SO4 and removal of Cr(VI) from the groundwater. At the same time, the δ13C of DIC increased from -15‰ to greater than 50‰, then began to decrease. The δ13C values of PLFA peaks common to many organisms showed a similar pattern of 13C-enrichment, but reached values (>200‰) much higher the bulk δ13C of the labeled HRC®, reflecting the faster dissolution rate for the 13C-labeled polylactate relative to the HRC®. After the δ13C of the PLFA peaked, it quickly returned to background values (-15‰) indicating rapid turnover of biomass in the system. Several PLFA peaks specific to organisms known to metabolize glycerol (e.g., Flavobacteria) increased, but did not show the same increase in δ13C. These data are being used to model changes in biological activity resulting from HRC® injection.