In-situ biodegradation potential of 1,2-DCA and VC at sites with different hydrogeological settings.

This paper investigates the feasibility of applying in-situ Bioremediation (ISB) to three sites contaminated with vinyl chloride and/or chlorinated alkanes such as 1,2-DCA and 1,1,2-TCA, presenting distinct hydrogeological settings and history of contaminant loading. Biotransformation of these compounds is well established in laboratory studies and pure cultures. Due to confidential aspects, however, few field data are available to support real case studies to the predictability of their fate and lifetime in soil and groundwater. Bio-Trap® In Situ Microcosm (ISM) studies were performed in selected monitoring wells, and consisted of a control unit which simulated Monitored Natural Attenuation conditions and other units which were amended with either lactate, emulsified vegetable oil (EVO) or molasses as electron donors. For wells with moderate Dhc counts, the ISM study demonstrated that electron donor addition could stimulate further growth of Dhc and enhance reductive dechlorination. Conversely, for wells with high population counts, substrate addition did not alter results significantly. Site-specific determining factors that most influenced the biodegradation results were microbial activity, soil texture and presence of organic matter, site pH, redox conditions and presence of free phase.

Multilevel samplers as microcosms to assess microbial response to biostimulation.

Passive multilevel samplers (MLS) containing a solid matrix for microbial colonization were used as in situ microcosms in conjunction with a push-pull biostimulation experiment designed to promote biological U(VI) and Tc(VII) reduction. MLS were deployed at 24 elevations in the injection well and two downgradient wells to investigate the spatial variability in microbial community composition and growth prior to and following biostimulation. The microbial community was characterized by real-time quantitative polymerase chain reaction (Q-PCR) quantification of bacteria, NO(3)(-)-reducing bacteria (nirS and nirK), delta-proteobacteria, Geobacter sp., and methanogens (mcrA). Pretest cell densities were low overall but varied substantially with significantly greater bacterial populations detected at circumneutral pH (t-test, alpha= 0.05), suggesting carbon substrate and low pH limitations of microbial activity. Although pretest cell densities were low, denitrifying bacteria were dominant members of the microbial community. Biostimulation with an ethanol-amended ground water resulted in concurrent NO(3)(-) and Tc(VII) reduction, followed by U(VI) reduction. Q-PCR analysis of MLS revealed significant (1 to 2 orders of magnitude, Mann-Whitney U-test, alpha= 0.05) increases in cell densities of bacteria, denitrifiers, delta-proteobacteria, Geobacter sp., and methanogens in response to biostimulation. Traditionally, characterization of sediment samples has been used to investigate the microbial community response to biostimulation; however, collection of sediment samples is expensive and not conducive to deep aquifers or temporal studies. The results presented demonstrate that push-pull tests with passive MLS provide an inexpensive approach to determine the effect of biostimulation on contaminant concentrations, geochemical conditions, and the microbial community composition and function.