Thermally-enhanced bioremediation is a promising treatment approach for petroleum contamination; however, studies examining temperature effects on anaerobic biodegradation in zones containing light non-aqueous phase liquids (LNAPLs) are lacking. Herein, laboratory microcosm studies were conducted for a former refinery to evaluate LNAPL transformation, sulfate reduction, and methane generation over a one-year period for temperatures ranging from 4 to 40 °C, and microbial community shifts were characterized. Temperatures of 22 and 30 °C significantly increased total biogas generation compared to lower (4 and 9 °C) and higher temperatures (35 and 40 °C; p < 0.1). Additionally, at 22 and 30 °C methane generation commenced ~6 months earlier than for 35 and 40 °C. Statistically significant biodegradation of benzene, toluene and xylenes was observed at elevated temperatures but not at lower temperatures (p < 0.1). Additionally, a novel differential chromatogram approach was developed to overcome challenges associated with resolving losses in complex mixtures of hydrocarbons, and application of this method revealed greater losses of hydrocarbons at 22 and 30 °C as compared to lower and higher temperatures. Finally, molecular biology assays revealed that the composition and activity of microbial communities shifted in a temperature-dependent manner. Collectively, results demonstrated that anaerobic biodegradation processes can be enhanced by increasing the temperature of LNAPL-containing soils, but biodegradation does not simply increase as temperature increases likely due to a lack of microorganisms that thrive at temperatures well above the historical high temperatures for a site. Rather, optimal degradation is achieved by holding soils at the high end of, or slightly higher than, their natural range.
Advances in our understanding of the microbial ecology at sites impacted by light non-aqueous phase liquids (LNAPLs) are needed to drive development of optimized bioremediation technologies, support longevity models, and develop culture-independent molecular tools. In this study, depth-resolved characterization of geochemical parameters and microbial communities was conducted for a shallow hydrocarbon-impacted aquifer. Four distinct zones were identified based on microbial community structure and geochemical data: (i) an aerobic, low-contaminant mass zone at the top of the vadose zone; (ii) a moderate to high-contaminant mass, low-oxygen to anaerobic transition zone in the middle of the vadose zone; (iii) an anaerobic, high-contaminant mass zone spanning the bottom of the vadose zone and saturated zone; and (iv) an anaerobic, low-contaminant mass zone below the LNAPL body. Evidence suggested that hydrocarbon degradation is mediated by syntrophic fermenters and methanogens in zone III. Upward flux of methane likely contributes to promoting anaerobic conditions in zone II by limiting downward flux of oxygen as methane and oxygen fronts converge at the top of this zone. Observed sulfate gradients and microbial communities suggested that sulfate reduction and methanogenesis both contribute to hydrocarbon degradation in zone IV. Pyrosequencing revealed that Syntrophus- and Methanosaeta-related species dominate bacterial and archaeal communities, respectively, in the LNAPL body below the water table. Observed phylotypes were linked with in situ anaerobic hydrocarbon degradation in LNAPL-impacted soils.
ZVI‐Clay is an emerging remediation approach that combines zero‐valent iron (ZVI)‐mediated degradation and in situ stabilization of chlorinated solvents. Through use of in situ soil mixing to deliver reagents, reagent‐contaminant contact issues associated with natural subsurface heterogeneity are overcome. This article describes implementation, treatment performance, and reaction kinetics during the first year after application of the ZVI‐Clay remediation approach at Marine Corps Base Camp Lejeune, North Carolina. Primary contaminants included trichloroethylene, 1,1,2,2‐tetrachloroethane, and related natural degradation products. For the field application, 22,900 m3 of soils were treated to an average depth of 7.6 m with 2% ZVI and 3% sodium bentonite (dry weight basis). Performance monitoring included analysis of soil and water samples. After 1 year, total concentrations of chlorinated volatile organic compounds (CVOCs) in soil samples were decreased by site‐wide average and median values of 97% and >99%, respectively. Total CVOC concentrations in groundwater were reduced by average and median values of 81% and >99%, respectively. In several of the soil and groundwater monitoring locations, reductions in total CVOC concentrations of greater than 99.9% were apparent. Further reduction in concentrations of chlorinated solvents is expected with time. Pre‐ and post‐mixing average hydraulic conductivity values were 1.7 × 10−5 and 5.2 × 10−8 m/s, respectively, indicating a reduction of about 2.5 orders of magnitude. By achieving simultaneous contaminant mass depletion and hydraulic conductivity reduction, contaminant flux reductions of several orders of magnitude are predicted.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.