Comparison of bacterial and archaeal communities in depth-resolved zones in an LNAPL body.

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.

Temperature impacts on anaerobic biotransformation of LNAPL and concurrent shifts in microbial community structure.

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.

Impacts of cryogenic sampling processes on iron mineral coatings in contaminated sediment.

This study focused on comparing iron mineral coatings found in contaminated sediments from a cryogenic (Cryo Core) core versus an Anaerobic Core (collected under oxygen-free and ambient conditions). After thawing the Cryo Core in an oxygen-free glovebox, a suite of analyses was applied on sediments from both cores: pH, redox potential, X-ray fluorescence (XRF), X-ray diffraction (XRD), and field-emission scanning electron microscopy (FESEM) with energy dispersive X-ray analyzer (EDX). Among the iron minerals identified, crystalline pyrite was found throughout the Cyro Core sediment samples, which is in contrast to that observed for the Anaerobic Core. Moreover, mackinawite and greigite that were ubiquitous in the Anaerobic Core were not observed in Cryo Core samples. To better understand why the metastable minerals were not present, a freeze/thaw process was simulated on Anaerobic Core samples using a liquid­nitrogen quench with surface coatings characterized by FESEM/EDX. In these quenched samples, mackinawite was no longer observed, and in its place was pyrite. In addition, both greigite and pyrite were found to be unique morphologically after quenching. Dissolution and re-precipitation of iron sulfide coatings during the freeze/thaw process appears to affect the geochemistry of the pore water through two main mechanisms of freeze-concentration and freezing potential.

Electrolytic trichloroethene degradation using mixed metal oxide coated titanium mesh electrodes.

Electrochemical systems provide a low cost, versatile, and controllable platform to potentially treat contaminants in water, including chlorinated solvents. Relative to bare metal or noble metal amended materials, dimensionally stable electrode materials such as mixed metal oxide coated titanium (Ti/MMO) have advantages in terms of stability and cost, important factors for sustainable remediation solutions. Here, we report the use of Ti/MMO as an effective cathode substrate for treatment of trichloroethene (TCE). TCE degradation in a batch reactor was measured as the decrease of TCE concentration over time and the corresponding evolution of chloride; notably, this occurred without the formation of commonly encountered chlorinated intermediates. The reaction was initiated when Ti/MMO cathode potentials were less than -0.8 V vs. the standard hydrogen electrode, and the rate of TCE degradation increased linearly with progressively more negative potentials. The maximum pseudo-first-order heterogeneous rate constant was approximately 0.05 cm min(-1), which is comparable to more commonly used cathode materials such as nickel. In laboratory-scale flow-though column reactors designed to simulate permeable reactive barriers (PRBs), TCE concentrations were reduced by 80-90%. The extent of TCE flux reduction increased with the applied potential difference across the electrodes and was largely insensitive to the spacing distance between the electrodes. This is the first report of the electrochemical reduction of a chlorinated organic contaminant at a Ti/MMO cathode, and these results support the use of this material in PRBs as a possible approach to manage TCE plume migration.

Estimating Aquifer Properties Using Derivative Analysis of Water Level Time Series from Active Well Fields.

Water level time series from groundwater production wells offer a transient dataset that can be used to estimate aquifer properties in areas with active groundwater development. This article describes a new parameter estimation method to infer aquifer properties from such datasets. Specifically, the method analyzes long-term water level measurements from multiple, interacting groundwater production wells and relies on temporal water level derivatives to estimate the aquifer transmissivity and storativity. Analytically modeled derivatives are compared to derivatives calculated directly from the observed water level data; an optimization technique is used to identify best-fitting transmissivity and storativity values that minimize the difference between modeled and observed derivatives. We demonstrate how the consideration of derivative (slope) behavior eliminates uncertainty associated with static water levels and well-loss coefficients, enabling effective use of water level data from groundwater production wells. The method is applied to time-series data collected over a period of 6 years from a municipal well field operating in the Denver Basin, Colorado (USA). The estimated aquifer properties are shown to be consistent with previously published values. The parameter estimation method is further tested using synthetic water level time series generated with a numerical model that incorporates the style of heterogeneity that occurs in the Denver Basin sandstone aquifers.

Relative contribution of DNAPL dissolution and matrix diffusion to the long-term persistence of chlorinated solvent source zones.

The relative contribution of dense non-aqueous phase liquid (DNAPL) dissolution versus matrix diffusion processes to the longevity of chlorinated source zones was investigated. Matrix diffusion is being increasingly recognized as an important non-DNAPL component of source behavior over time, and understanding the persistence of contaminants that have diffused into lower permeability units can impact remedial decision-making. In this study, a hypothetical DNAPL source zone architecture consisting of several different sized pools and fingers originally developed by Anderson et al. (1992) was adapted to include defined low permeability layers. A coupled dissolution-diffusion model was developed to allow diffusion into these layers while in contact with DNAPL, followed by diffusion out of these same layers after complete DNAPL dissolution. This exercise was performed for releases of equivalent masses (675 kg) of three different compounds, including chlorinated solvents with solubilities ranging from low (tetrachloroethene (PCE)), moderate (trichloroethene (TCE)) to high (dichloromethane (DCM)). The results of this simple modeling exercise demonstrate that matrix diffusion can be a critical component of source zone longevity and may represent a longer-term contributor to source longevity (i.e., longer time maintaining concentrations above MCLs) than DNAPL dissolution alone at many sites. For the hypothetical TCE release, the simulation indicated that dissolution of DNAPL would take approximately 38 years, while the back diffusion from low permeability zones could maintain the source for an additional 83 years. This effect was even more dramatic for the higher solubility DCM (97% of longevity due to matrix diffusion), while the lower solubility PCE showed a more equal contribution from DNAPL dissolution vs. matrix diffusion. Several methods were used to describe the resulting source attenuation curves, including a first-order decay model which showed that half-life of mass discharge from the matrix-diffusion dominated phase is in the range of 13 to 29 years for TCE. Because the mass discharge rate shifts significantly over time once DNAPL dissolution is complete, a Power-Law model was shown to be useful, especially at later stages when matrix diffusion dominates. An assessment of mass distribution showed that while relatively small percentages of the initial source mass diffused into the low permeability compartment, this mass was sufficient to sustain concentrations above drinking water standards for decades. These data show that relatively typical conditions (e.g., 50-year-old release, moderate to high solubility contaminant) are consistent with late stage sources, where mass in low permeability matrices serves as the primary source, and fit the conceptual model that mass in low permeability zones is important when evaluating source longevity.