An Evaluation of Compound-Specific Isotope Analyses for Assessing the Biodegradation of MTBE at Port Hueneme, CA
Bibliographic record
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.001 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".