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.