An evaluation of compound-specific isotope analyses for assessing the biodegradation of MTBE at Port Hueneme, CA

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‰ over a two-order of magnitude concentration decrease, with an average isotope enrichment factor (ε_c) of -1.4%, 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 δ¹³C 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.