Impact/Purpose:

Progress of the first year of the project will be discussed using the framework of objectives defined in the proposal.

1. Verification of the hypothesis that degradation is the primary factor leading to isotopic fractionation in the field.
2. Verification of the hypothesis that stable isotopes provide both positive and negative evidence of biodegradation.
3. A database of isotope fractionation by available microbial cultures, primarily of MTBE degraders (and as applicable – TBA and other gasoline compounds).
4. A database of stable isotopic composition (carbon and hydrogen) of MTBE, TBA and other gasoline compounds (reference for data interpretation).
5. A set of case studies of contaminated sites, primarily on MTBE and TBA, but ultimately on other gasoline-range contaminants.
6. Development of a commercially viable method for site characterization based on the stable isotope values and trends observed in the results obtained from this study.

Description:

Isotope effects resulting from biodegradation of MTBE
To conduct the microcosm biodegradation study, sediment samples were collected from sites offering high potential of MTBE biodegradation. Sites where sediment samples were collected for the MTBE microcosm construction were selected based on the criteria of historical MTBE concentration data (decrease of MTBE corresponding to a transient accumulation of TBA) and on standard geochemical biodegradation footprints. Additionally, CSIA was performed on groundwater samples prior to sediment sample collection. Only the sites where the traditional biodegradation criteria were met and the isotope data were indicative of anaerobic biodegradation (cf. Kuder and Philp, 2008) were selected for sediment sampling. The sediment samples were collected from the areas of the historical transition from peak MTBE to peak TBA concentrations, typically away from the present plume maximum concentrations. In addition, two sites were selected for TBA-only experiment. Both sites had previously-determined isotope ratios of TBA in groundwater, that were more positive than the typical field values observed in various plumes. In one case samples of water with sediment suspension were collected, in the other case, two sediment samples were collected from the area of TBA plume.

All microcosms were set for anaerobic conditions, including live and control replicates. The electron acceptor conditions were chosen on the basis of the actual concentrations of sulfate and methane at the time of sampling. The two subsets of microcosms will be referred to as methanogenic (low initial sulfate concentration and methane accumulation over the duration of experiment) and sulfate reducing or SR (high initial sulfate, no accumulation of methane). The presence of alternative electron acceptors, in particular Fe³⁺ in the SR samples cannot be excluded. The microcosm bottles were amended with MTBE (up to 10-20 ppm) or with TBA (up to 40-60 ppm) and periodically monitored (MTBE, TBA, sulfate and methane concentration). Once there was evidence of apparent reduction of substrate (MTBE only, to date none of the microcosms exhibited TBA biodegradation activity) concentrations, time-series of samples were collected for CSIA. Active microcosms were re-amended several times to collect mode data points. Eventually, each active microcosm bottle was sampled multiple times at various stages of attenuation, allowing high-precision determination of isotope effects. Carbon and hydrogen CSIA data were collected.

For all of the studied MTBE-amended sediments, at least some of the replicates developed anaerobic MTBE-degrading activity, permitting measurement of isotope enrichment factors (ε). The values of ε for MTBE degradation calculated for the data sets from all of the active microcosms are virtually identical and cluster around -18. This value implies even stronger isotope effect for this type of reaction than the values reported earlier. The difference is likely due to different experimental designs. Unlike in the previous studies, all of the microcosms discussed here were kept homogenous (culture bottles set on a shaker or turned over daily) to minimize heterogeneity of the medium and avoid underestimation of isotope effects. The consistency of the value between the individual data sets from different sediment samples, identical for the methanogenic and the sulfate reducing conditions, gives strong support for using the obtained value of ε in evaluation of MTBE degradation in-situ to calculate the amount of degraded MTBE.

Isotope effects resulting from abiotic weathering of MTBE
As described in our proposal, the two non-degradative processes potentially leading to isotope effects are sorption and volatilization. Recent publications in the field provide a good basis to understand the significance of sorption in isotope fractionation of contaminants. The basic conclusion from their work is that the contribution of sorption of VOC to the net isotope effect as observed at field conditions is negligible and limited to a narrow front of an expanding contaminant plume. For MTBE, in particular, there was no measurable sorption-related isotope fractionation. A lab study on the volatilization-related isotope effects was initiated in the second year of the project. A conceptual model of volatilization of MTBE involves a cumulative effect of phase equilibria and of diffusive/advective venting of the gas phase. Volatilization-related isotope effects are similarly complex and experiments were set up to measure both the individual components and the net fractionation. Equilibrium fractionation of carbon and hydrogen isotopes was measured for simple 2-phase systems: water-air, water-NAPL and NAPL-air. Kinetic fractionation associated with diffusion of MTBE vapor through a porous medium was also determined. Sediment column experiments on passive and advective (such as in air sparging) volatilization of MTBE from aqueous and NAPL phase have been completed. The latter involve complex isotope effects, resulting from a combination of phase equilibria and vapor diffusion. The general conclusion is that the carbon isotope effects resulting from volatilization are low or absent. The largest value was observed in passive volatilization of MTBE, with a carbon isotope enrichment factor of approximately –1‰. In the advective volatilization, none or minor inverse effects (change of isotope ratios opposite to those resulting from biodegradation) were observed.

Net change of carbon isotope ratio due to volatilization of MTBE (even at sites undergoing vapor phase extraction or air sparging treatment) is not likely to exceed the analytical error of CSIA and therefore it is not a significant factor for site evaluation. On the other hand, hydrogen isotope fractionation resulting from volatilization is higher. The worst-case scenario occurs for air sparging. Enrichments of hydrogen isotopes resulting from air sparging may be mistaken for aerobic biodegradation signal. While MTBE in particular is not readily volatilizing from the aqueous medium, the contaminated sites are frequently subject to remediation techniques such as air sparging or soil vapor extraction. The potential for interferences from weathering is proportional to the extent of mass attenuation via the vapor phase and isotope effects resulting from air sparging or soil vapor extraction should be studied in more detail. Site evaluation relying on hydrogen isotope signatures (i.e., the situations where aerobic biodegradation signal is sought) should consider the potential interference from volatilization-related phenomena.

Isotope compositions of commercial gasolines
Evaluation of stable isotope data from biodegraded materials requires a benchmark of pre-degradation isotope ratios. Only if the pre-degradation isotope ratios are known, the isotope ratios measured in-situ can show that attenuation resulted with any change of δ13C or δD that can be interpreted as a signature of biodegradation. Several previously published studies show isotope ratios of various compounds in commercial gasolines. Data obtained in this project are intended to fill some gaps in the overall record.

Samples of gasoline have been analyzed by CSIA to obtain carbon and hydrogen isotope ratios of MTBE and the aromatic fraction (monoaromatics and naphthalene). Carbon isotope ratios only have been determined for TBA (due to a problematic accuracy of hydrogen isotope ratios measured for alcohols). While a standard approach for gasoline CSIA is a direct injection onto a GC-IRMS instrument, all of the reported data have been obtained by purge-and-trap-GC-IRMS analysis of aqueous phase equilibrated with gasoline. This approach improved GC resolution of the samples (oxygenates in particular). In the standard methodology, GC separation of MTBE, while adequate for the purpose of concentration determination, is not ideal and therefore the isotope ratio determination may be biased except in high-MTBE samples. TBA cannot be determined by direct injection due to major coelution with light gasoline fractions. An overall increase of GC separation quality for the aromatic fraction is also expected due to preferential partitioning of the aromatics into the aqueous phase.

Field applications – focus on MTBE and benzene
Groundwater samples from a number of contaminated sites have been analyzed by CSIA to detect changes in isotope ratios indicative of in-situ biodegradation. Attenuation of gasoline oxygenates and benzene was studied. For benzene, two-dimensional CSIA analysis (carbon and hydrogen) has the best potential to show a fractionation pattern consistent with benzene biodegradation. On the other hand, due to relatively low magnitude of isotope fractionation in the case of benzene, carbon CSIA alone seems to be insufficient to confirm in-situ biodegradation. The results on MTBE show carbon isotope effects due to anaerobic degradation. CSIA can be also used to verify the extent of biodegradation at sites with induced aerobic activity (e.g., aeration by air sparging).

URLs/Downloads:

Record Details:

Related Organizations:

Project Information:

Project IDs: