Stable Isotope Evidence for Biodegradation of Chlorinated Ethenes at a Fractured Bedrock Site
Bibliographic record
Abstract
Stable carbon isotope analysis of chlorinated ethenes and ethene was performed at a site contaminated with trichloroethene (TCE), a dense non-aqueous phase liquid (DNAPL). The site is located in fractured bedrock and had variable groundwater hydraulic gradients during the study due to a local excavation project. Previous attempts to biostimulate a pilot treatment area at the site resulted in the production of cis-1,2-dichloroethene (cis-DCE), the first product of reductive dechlorination of TCE. Cis-DCE concentrations accumulated however, and there was no appreciable production of the breakdown products from further reductive dechlorination, vinyl chloride (VC) and ethene (ETH). Consequently, the pilot treatment area was bioaugmented with a culture of KB-1, a natural microbial consortium known to completely reduce TCE to nontoxic ETH. Due to ongoing dissolution of TCE from DNAPL in the fractured bedrock, and to variable hydraulic gradients, concentration profiles of dissolved TCE and its degradation products cis-DCE, VC, and ETH could not convincingly confirm biodegradation of the chlorinated ethenes. Isotopic analysis of cis-DCE and VC, however, demonstrated that biodegradation was occurring in the pilot treatment area. The isotope values of cis-DCE and VC became significantly more enriched in 13C over the last two sampling dates (in one well from -17.6%o to -12.8%o and from -22.5%o to -18.2%o for cis-DCE and VC, respectively). Quantification of the extent of biodegradation in the pilot treatment area using the Rayleigh model indicated that, depending on the well, between 21.3% and 40.7% of the decrease in cis-DCE and between 15.2% and 36.7% of the decrease in VC concentrations can be attributed to the effects of biodegradation during this time period. Within each well, the isotope profile of TCE remained relatively constant due to the continuous input of undegraded TCE due to DNAPL dissolution.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.002 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
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 teacher head, 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".