Daniel Bouchard

Multiple lines of evidence to demonstrate vinyl chloride aerobic biodegradation in the vadose zone, and factors controlling rates

A field-based investigation was conducted at a contaminated site where the vadose zone was contaminated with a range of chlorinated hydrocarbons. The investigation consisted of groundwater and multilevel soil-gas monitoring of a range of contaminants and gases, along with isotope measurements and microbiology studies. The investigation provided multiple lines of evidence that demonstrated aerobic biodegradation of vinyl chloride (VC) was occurring in the vadose zone (i) above the on-site source zone, and (ii) above the downgradient off-site groundwater plume location. Data from both the on-site and off-site locations were consistent in showing substantially greater (an order of magnitude greater) rates of VC removal from the aerobic vadose zone compared to more recalcitrant contaminants trichloroethene (TCE) and tetrachloroethene (PCE). Soil gas VC isotope analysis showed substantial isotopic enrichment of VC (δ¹³C -5.2 to -10.9‰) compared to groundwater (δ¹³C -39.5‰) at the on-site location. Soil gas CO₂ isotope analysis at both locations showed that CO₂ was highly isotopically depleted (δ¹³C -28.8 to -33.3‰), compared to soil gas CO₂ data originating from natural sediment organic matter (δ¹³C= -14.7 to -21.3‰). The soil gas CO2 δ¹³C values were consistent with near-water table VC groundwater δ¹³C values (-36.8 to -39.5‰), suggesting CO₂ originating from aerobic biodegradation of VC. Bacteria that had functional genes (ethene monooxygenase (etnC) and epoxyalkane transferase (etnE)) involved in ethene metabolism and VC oxidation were more abundant at the source zone where oxygen co-existed with VC. The distribution of VC and oxygen vadose zone vapour plumes, together with long-term changes in soil gas CO₂ concentrations and temperature, provided information to elucidate the factors controlling aerobic biodegradation of VC in the vadose zone. Based on the overlapping VC and oxygen vadose zone vapour plumes, aerobic vapour biodegradation rates were independent of substrate (VC and/or oxygen) concentration. The high correlation (R=0.962 to 0.975) between CO₂ concentrations and temperature suggested that aerobic biodegradation of VC was controlled by bacterial activity that was regulated by the temperature within the vadose zone. When assessing a contaminated site for possible vapour intrusion into buildings, accounting for environmental conditions for aerobic biodegradation of VC in the vadose zone should improve the assessment of environmental risk of VC intrusion into buildings, enabling better identification and prioritisation of contaminated sites to be remediated.

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

Analytical models were developed that simulate stable isotope ratios of volatile organic compounds (VOCs) near a point source contamination in the unsaturated zone. The models describe diffusive transport of VOCs, biodegradation and source ageing. The mass transport is governed by Ficks law for diffusion. The equation for reactive transport of VOCs in the soil gas phase was solved for different source geometries and for different boundary conditions. Model results were compared to experimental data from a one-dimensional laboratory column and a radial-symmetric field experiment. The comparison yielded a satisfying agreement. The model results clearly illustrate the significant isotope fractionation by gas phase diffusion under transient state conditions. This leads to an initial depletion of heavy isotopes with increasing distance from the source. The isotope evolution of the source is governed by the combined effects of isotope fractionation due to vaporisation, diffusion and biodegradation. The net effect can lead to an enrichment or depletion of the heavy isotope in the remaining organic phase, depending on the compound and element considered. Finally, the isotope evolution of molecules migrating away from the source and undergoing degradation is governed by a combined degradation and diffusion isotope effect. This suggests that, in the unsaturated zone, the interpretation of biodegradation of VOC based on isotopic data must always be based on a model combining gas phase diffusion and degradation.

Use of stable isotope analysis to assess biodegradation of petroleum hydrocarbons in the unsaturated zone. Laboratory studies, field studies, and mathematical simulations

Compound-specific isotope analysis is increasingly used to demonstrate contaminant degradation in groundwater. The method relies on the frequent occurrence of characteristic shifts in the isotope ratios of contaminants due to the faster degradation of molecules with light isotopes compared to those with heavy isotopes. The goal of the study was to evaluate if the method can also be used to assess biodegradation of petroleum hydrocarbons in the unsaturated zone. The study included laboratory experiments to determine isotopic enrichment for biodegradation of petroleum hydrocarbon compounds under unsaturated conditions and a field experiment. The field experiment consisted of burying an artificial fuel source in the unsaturated zone at a site in Denmark. Concentration and isotope ratios of individual compounds were monitored using a dense network of sampling points. Significant shifts of the isotope ratios of most of the compounds occurred. Initially, a depletion in 13C with distance was observed likely due to the faster diffusion of molecules with 12C only. Later, most of the compounds became in enriched in 13C compared to the source due to biodegradation. To evaluate the relative contribution of diffusion and biodegradation in more detail, the concentration and isotope ratio evolution was simulated using an analytical model. The calculations confirmed that diffusion can lead to significant isotope fractionation at early times. For hydrogen isotopes, the effect of diffusion relative to biodegradation is expected to be smaller due to the large hydrogen isotope fractionation frequently observed during biodegradation of organic compounds. In conclusion, the study demonstrates that especially at early times after spills, the effect of diffusion has to be taken into account in the interpretation of isotope data from the unsaturated zone. Furthermore, it demonstrates that for many compounds, hydrogen isotopes are likely the more sensitive indicator of biodegradation.

Solvent-based dissolution method to sample gas-phase volatile organic compounds for compound-specific isotope analysis

An investigation was carried out to develop a simple and efficient method to collect vapour samples for compound specific isotope analysis (CSIA) by bubbling vapours through an organic solvent (methanol or ethanol). The compounds tested were benzene and trichloroethylene (TCE). The dissolution efficiency was tested for different air volume injections, using flow rates ranging from 25ml/min to 150ml/min and injection periods varying between 10 and 40min. Based on the results, complete mass recovery for benzene and TCE in both solvents was observed for the flow rates of 25 and 50ml/min. However, small mass loss was observed at increased flow rate. At 150ml/min, recovery was on average 80±17% for benzene and 84±10% for TCE, respectively in methanol and ethanol. The δ(13)C data measured for benzene and TCE dissolved in both solvents were reproducible and were stable independently of the volume of air injected (up to 6L) or the flow rate used. The stability of δ(13)C values hence underlines no isotopic fractionation due to compound-solvent interaction or mass loss. The development of a novel and simple field sampling technique undertaken in this study will facilitate the application of CSIA to diverse gas-phase volatile organic compound studies, such as atmospheric emissions, soil gas or vapour intrusion.

Documentation of time-scales for onset of natural attenuation in an aquifer treated by a crude-oil recovery system.

A pipeline transporting crude-oil broke in a nature reserve in 2009 and spilled 5100 m(3) of oil that partly reached the aquifer and formed progressively a floating oil lens. Groundwater monitoring started immediately after the spill and crude-oil recovery by dual pump-and-skim technology was operated after oil lens formation. This study aimed at documenting the implementation of redox-specific natural attenuation processes in the saturated zone and at assessing whether dissolved compounds were degraded. Seven targeted water sampling campaigns were done during four years in addition to a routine monitoring of hydrocarbon concentrations. Liquid oil reached the aquifer within 2.5 months, and anaerobic processes, from denitrification to reduction of sulfate, were observable after 8 months. Methanogenesis appeared on site after 28 months. Stable carbon isotope analyses after 16 months showed maximum shifts in δ(13)C of +4.9±0.22‰ for toluene, +2.4±0.19‰ for benzene and +0.9±0.51‰ for ethylbenzene, suggesting anaerobic degradation of these compounds in the source zone. Estimations of fluxes of inorganic carbon produced by biodegradation revealed that, in average, 60% of inorganic carbon production was attributable to sulfate reduction. This percentage tended to decrease with time while the production of carbon attributable to methanogenesis was increasing. Within the investigation time frame, mass balance estimations showed that biodegradation is a more efficient process for control of dissolved concentrations compared to pumping and filtration on an activated charcoal filter.

Diagnostic Tools to Assess Mass Removal Processes During Pulsed Air Sparging of a Petroleum Hydrocarbon Source Zone

During remediation of contaminated aquifers, diagnostic tools can help evaluate whether an intended mass removal process was successfully initiated and acted on specific contaminants of concern. In this study, several diagnostic tools were tested in a controlled-release in situ air sparging experiment that focused on the treatment of target hydrocarbons (e.g., benzene, toluene, ethylbenzene, and xylenes). The tools included compound-specific isotope analysis (CSIA), expression of functional genes (mRNA), and metabolites characteristic of aerobic and anaerobic biodegradation. Total and compound-specific mass balances were established and used, along with traditional monitoring parameters, to validate the results from the various tools. CSIA results indicated biodegradation as the main process contributing to benzene and toluene removal. Removal process-specific isotope shifts were detected in groundwater as well as in the system effluent gas. CSIA, metabolite, and mRNA biomarkers consistently indicated that both aerobic and anaerobic biodegradation of benzene and toluene occurred, but that their relative importance evolved over time and were related to the treatment system operation. While the indicators do not allow quantif cation of the mass removed, they are particularly useful to identify if a removal process has been initiated, and to track relative changes in the predominance of in situ contaminant attenuation processes resulting from remediation efforts.

δ13C and δ37Cl on Gas-Phase TCE for Source Identification Investigation - Innovative Solvent-Based Sampling Method

The use of stable isotopes (13C, 2H and 37Cl) has shown to be a reliable method to establish a link between the improper presence of a volatile organic compound (VOC) and the suspected emitting source, or to distinguish compound sources on the same contaminated site. While the isotopic tool is commonly applied to contaminated groundwater studies, the tool is considerably less applied for gas-phase studies such as indoor air or atmospheric investigations. The main reason is low VOC concentrations which impede collecting enough VOC mass to perform reliable isotope analysis. Accordingly, a successful isotopic investigation conducted on gas-phase VOC relies on a sampling technique that can sufficiently accumulate the compound during the sampling. Thus far, the most common sampling technique used for atmospheric or indoor air studies is the adsorption tubes. Although the latter method provides reliable isotopic measurements, the sampling method is time consuming and the results require elaborate validation for each sample. A simple and innovative solvent-based sampling method to collect VOC from the gas phase is presented. By analogy to the sorption tube method, the proposed sampling method uses an organic solvent as a sink to accumulate the VOC during the sampling process. Laboratory experiments were first carried out to evaluate the VOC dissolution efficiency in solvent during constant air flow injection, to identify suitable solvents (solvent volatility, VOC solubility) and to confirm reproducible isotopic measurements on selected VOCs (benzene and trichloroethene). After successful preliminary results, the performance of the sampling method was evaluated during an experiment carried out in a former industrial building. A TCE liquid source with known isotopic composition (δ13C and δ37Cl) was used to create a gas-phase plume inside an isolated room. The room air was sampled by using a device developed for the solvent-based method. Summa canisters were used in parallel as the reference method. The δ13C value for TCE showed excellent agreement between the two sampling methods. Furthermore, the δ13C and δ37Cl values measured for the TCE sampled with the solvent method were similar to the source values. The latter results suggested an absence of physical processes causing an isotope fractionation and consequently allowed establishing a direct link between the gas-phase TCE and the emitting source. The combination of the laboratory and field tests underlines a promising sampling method to perform CSIA measurements on gas-phase VOCs. Advantages compared to the use of sorbent tubes, method restrictions and the detection limits will also be covered.

Optimization of the solvent-based dissolution method to sample volatile organic compound vapors for compound-specific isotope analysis

The methodology of the solvent-based dissolution method used to sample gas phase volatile organic compounds (VOC) for compound-specific isotope analysis (CSIA) was optimized to lower the method detection limits for TCE and benzene. The sampling methodology previously evaluated by [1] consists in pulling the air through a solvent to dissolve and accumulate the gaseous VOC. After the sampling process, the solvent can then be treated similarly as groundwater samples to perform routine CSIA by diluting an aliquot of the solvent into water to reach the required concentration of the targeted contaminant. Among solvents tested, tetraethylene glycol dimethyl ether (TGDE) showed the best aptitude for the method. TGDE has a great affinity with TCE and benzene, hence efficiently dissolving the compounds during their transition through the solvent. The method detection limit for TCE (5±1μg/m3) and benzene (1.7±0.5μg/m3) is lower when using TGDE compared to methanol, which was previously used (385μg/m3 for TCE and 130μg/m3 for benzene) [2]. The method detection limit refers to the minimal gas phase concentration in ambient air required to load sufficient VOC mass into TGDE to perform δ13C analysis. Due to a different analytical procedure, the method detection limit associated with δ37Cl analysis was found to be 156±6μg/m3 for TCE. Furthermore, the experimental results validated the relationship between the gas phase TCE and the progressive accumulation of dissolved TCE in the solvent during the sampling process. Accordingly, based on the air-solvent partitioning coefficient, the sampling methodology (e.g. sampling rate, sampling duration, amount of solvent) and the final TCE concentration in the solvent, the concentration of TCE in the gas phase prevailing during the sampling event can be determined. Moreover, the possibility to analyse for TCE concentration in the solvent after sampling (or other targeted VOCs) allows the field deployment of the sampling method without the need to determine the initial gas phase TCE concentration. The simplified field deployment approach of the solvent-based dissolution method combined with the conventional analytical procedure used for groundwater samples substantially facilitates the application of CSIA to gas phase studies.