Effects of Water Table Fluctuations on Natural Source Zone Depletion of Petroleum Hydrocarbons in Contaminated Soils
Bibliographic record
Abstract
In soils contaminated with petroleum hydrocarbons (PHCs), water table fluctuations (WTFs) affect the kinetics of PHC biodegradation and the generation and transport of carbon dioxide (CO2) and methane (CH4). Thus, understanding the impacts of WTFs on natural source zone depletion processes is critical for environmental risk assessment and the design of soil remediation strategies. In this study, a 300 day-long column experiment was conducted to simulate the effects of water table fluctuations on the aerobic and anaerobic PHC biodegradation pathways and rates. Eight columns were each filled with 45 cm of soil from undisturbed cores collected at a site formerly contaminated with PHCs. Four columns simulating fluctuating water table conditions were subjected to three successive 6-week cycles of drainage and imbibition. The remaining four columns remained fully saturated over the period of the experiment, simulating a static water table. Except for the controls, the columns received injections of ethanol or ethanol plus naphthalene after 111 days of pre-equilibration. Over the duration of the experiment, soil moisture, soil surface CO2 and CH4 effluxes, dissolved CO2 and CH4 concentrations, δ13C compositions of CO2 and CH4, dissolved naphthalene concentrations, and ancillary geochemical parameters were monitored. The remaining naphthalene depth distributions in the soil columns were also measured at the end of the experiment. A reactive transport model representing 13 biogeochemical reaction pathways was verified against the acquired data. The experimental and modeling results confirmed that the prevailing pathway generating CH4 shifted from hydrogen-based to acetate-based methanogenesis in the ethanol and ethanol plus naphthalene spiked columns, while CH4 oxidation played a key role in controlling the CH4 efflux during the drainage periods. Compared to the static water table columns, the WTF columns exhibited significantly faster naphthalene attenuation while the cumulative CO2 and CH4 effluxes were about twice as high. These observations were attributed to the periodic incursion of air during the WTFs, which increased the porewater-air interface area for gas transfer while also accelerating the aerobic degradation of soil organic matter and naphthalene. Overall, our study advances the quantitative modeling of the biogeochemical reaction network in PHC contaminated soils under WTFs, including the role of methanogenic pathways.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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".