An Evaluation of Compound-Specific Isotope Analyses for Assessing the Biodegradation of MTBE at Port Hueneme, CA

Abstract

The use of compound-specific isotope analysis (CSIA) as a diagnostic tool for MTBE biodegradation in aquifers was tested at the Port Hueneme, CA site. There, a 1500-m long dissolved MTBE plume and associated engineered aerobic flow-through biobarrier have been well-studied, leading to delineation of regions of known significant and limited bioattenuation. This allowed comparison of field-scale CSIA results with a priori knowledge of aerobic MTBE biodegradation, leading to conclusions concerning the utility of CSIA as a diagnostic tool for other aerobic biodegradation sites. Groundwater samples were collected and analyzed for both 13C and 2H (D) in MTBE through the bioactive treatment zone and within the larger MTBE plume. For reference, the 13C enrichment factor for MTBE biodegradation in laboratory-scale microcosms using site groundwater and sediments was also quantified. Aerobic microcosms showed a 13C enrichment of 5.5 to 6.4 +/- 0.2 per thousand over a two-order of magnitude concentration decrease, with an average isotope enrichment factor (epsilon(c)) of -1.4 per thousand, in agreement with other aerobic microcosm studies. Less 13C enrichment (about 25%) was observed for similar MTBE concentration reductions in groundwater samples collected within the aerobic biotreatment zone, and this enrichment was comparable to the scatter in delta13C values within the source zone. Increasing enrichment with decreasing MTBE concentration seen in microcosm data was not evident in either the 13C or D field data. The discrepancy between field and laboratory data may reflect small-scale (<1 m) spatial heterogeneity in MTBE biodegradation activity and the mixing of water from adjacent strata during groundwater sampling; for example, relatively nonattenuated MTBE-impacted water from one stratum could be mixed with highly attenuated/low-MTBE concentration from another, and this could produce a sample with both reduced MTBE concentration and low enrichment. Overall, the results suggest that 13C data alone may produce inconclusive results at sites where MTBE undergoes aerobic biodegradation, and that even with two-dimensional CSIA (13C and D), an increase in the confidence of data interpretation may only be possible with data sets larger than those typically collected in practice.

Linking Low-Level Stable Isotope Fractionation to Expression of the Cytochrome P450 Monooxygenase-Encoding ethB Gene for Elucidation of Methyl tert -Butyl Ether Biodegradation in Aerated Treatment Pond Systems

“…Some of these MTBE-contaminated sites have been characterized recently by compound-specific stable isotope analysis (CSIA) (19,22,26,27,32,34,57). This tool allows for a qualitative and/or quantitative integrative assessment of biological transformation of organic contaminants over a flow path in environmental systems (36,46), thanks to the reaction-dependent compound-specific isotope enrichment factors (ε) obtained under different controlled laboratory conditions employing a modified form of the Rayleigh distillation equation.…”

&lt;sup&gt;13&lt;/sup&gt;C/&lt;sup&gt;12&lt;/sup&gt;C Isotope Fractionation during Aerobic and Anaerobic Biodegradation of Naphthalene

“…The isotopic enrichment observed during aerobic and anaerobic biodegradation of naphthalene, although small, is statistically significant. It has been shown that some compounds that exhibit isotopic fractionation during biodegradation under laboratory conditions may only show a limited fractionation in field samples [23]. Therefore, the isotopic enrichment detected in this study for naphthalene may not be sufficient to demonstrate its biodegradation in field situations where many other processes such as advection, dispersion and adsorption, also help to reduce the concentrations of contaminants.…”

“…However, the BTEX compounds produce very small stable carbon isotopic fractionation during biodegradation [20][21][22]. The gasoline additive, MTBE, also produces a very small stable carbon isotopic fractionation during biodegradation [23]. Richnow et al [24] found a very low carbon isotopic fractionation on naphthalene in a field study.…”

Field applicability of Compound-Specific Isotope Analysis (CSIA) for characterization and quantification of in situ contaminant degradation in aquifers

“…The overall isotope signature in the mixture will be dominated by the less-degraded contaminant fractions with higher remaining concentrations ). The isotope shift of the mixture is then lower than expected for this degree of biodegradation, which may lead to an underestimation of degradation (Mariotti et al 1988;Kopinke et al 2005b;Lesser et al 2008;Green et al 2010).…”