Impacts of Co-Solvent Flushing on Microbial Populations Capable of Degrading Trichloroethylene

Abstract

With increased application of co-solvent flushing technologies for removal of nonaqueous phase liquids from groundwater aquifers, concern over the effects of the solvent on native microorganisms and their ability to degrade residual contaminant has also arisen. This study assessed the impact of ethanol flushing on the numbers and activity potentials of trichloroethylene (TCE)-degrading microbial populations present in aquifer soils taken immediately after and 2 years after ethanol flushing of a former dry cleaners site. Polymerase chain reaction analysis revealed soluble methane monooxygenase genes in methanotrophic enrichments, and 16S rRNA analysis identified Methylocystis parvus with 98% similarity, further indicating the presence of a type II methanotroph. Dissimilatory sulfite reductase genes in sulfate-reducing enrichments prepared were also observed. Ethanol flushing was simulated in columns packed with uncontaminated soils from the dry cleaners site that were dosed with TCE at concentrations observed in the field; after flushing, the columns were subjected to a continuous flow of 500 pore volumes of groundwater per week. Total acridine orange direct cell counts of the flushed and nonflushed soils decreased over the 15-week testing period, but after 5 weeks, the flushed soils maintained higher cell counts than the nonflushed soils. Inhibition of methanogenesis by sulfate reduction was observed in all column soils, as was increasing removal of total methane by soils incubated under methanotrophic conditions. These results showed that impacts of ethanol were not as severe as anticipated and imply that ethanol may mitigate the toxicity of TCE to the microorganisms.

Interphase mass transfer between fluids in subsurface formations: A review

“…A simple model to predict the equilibrium solubility of organic compounds in aqueous phase in the presence of cosolvents was also developed by Yalkowsky [65] whereby the enhanced solubility is proportional to the octanol-water partition coefficient of the organic compounds. In NAPL remediation the use of cosolvents also influences the interfacial tension, viscosity, density and microbial activity [66][67][68]. As will be discussed in Section 3, alteration of these properties may also have a significant indirect impact on the interphase mass transfer process.…”

Sustained treatment: Implications for treatment timescales associated with source‐depletion technologies

“…For in situ chemical oxidation sites with sufficient data to evaluate, the reduction in parent CVOC concentration actually decreased from a median value of 90 percent to a median value of 78 percent at the end of the (available) monitoring period. The incidence of concentration rebound was relatively high (13 of 16 wells), arguing that at most chemical • Use of high concentrations of organic carbon-based compounds as surfactants/cosolvents provides a potential substrate to support microbial growth and activity (Christ et al, 2005) • Examples include ethanol (Mravik et al, 2003;Ramakrishnan et al, 2005), cyclodextrin (Hinrichs, 2004), and Tween-80 (Ramsburg et al, 2004) under anaerobic conditions • Fermented to H 2 and shorter-chain organics (Christ et al, 2005) • Potential enhanced solubilization of matrix-bound organic carbon to support near-term dechlorination • Most surfactants can sorb to soils (Paria, 2008;Smith et al, 1997), potentially increasing their persistence within subsurface for promoting microbial activity…”

Development of Assessment Tools for Evaluation of the Benefits of DNAPL Source Zone Treatment

“…In a staged treatment scenario, the physicochemical remedy removes significant contaminant mass, and, in case of surfactant or co-solvent flushing, delivers electron donors that may stimulate microbial reductive dechlorination activity (Mravik et al 2003;Ramakrishnan et al 2005). Hence, in this sequential approach, reductive dechlorination acts as a "polishing" step that detoxifies residual contaminants, thereby reducing contaminant mass flux and controlling long-term plume development.…”