Origin, fate, and transport of dissolved organic gases in bedrock aquitards; Saskatchewan, Canada
Bibliographic record
Abstract
Dissolved gases including methane (CH4), ethane (C2H6), and propane (C3H8) are natural components of groundwater. Knowledge of the origin, distribution, and fate of these gases is crucial for assessing the impacts of oil and gas development on shallow groundwater systems. This study provides one of the first suites of baseline profiles of dissolved CH4, C2H6, and C3H8 through a bedrock aquitard and provides insight into their origin (using carbon (C) and hydrogen (H) stable isotopes), fate, and transport. Core samples were collected over 240 m of continuously-cored Quaternary-aged Battleford till (80 m) and underlying Cretaceous units (160 m) from an aquitard research site 140 km southwest of Saskatoon, SK, Canada. Physical properties (moisture content, density, grain size and carbon forms), stable H and oxygen isotopes of pore water (d2H and d18O), pore water chemistry, and gas concentrations (CH4, C2H6, and C3H8) and their stable C and H isotope values (d13CCH4, d2HCH4, d13CC2H6, and d13CC3H8) were measured on these cores. Results of physical parameters and d2H and d18O measurements agreed with previously reported data. The high-resolution profiles extend existing data sets into the Ardkenneth aquifer, the upper layer of which contains high percentages of sand and abrupt increases in values of d2H, and chloride (Cl-) and CH4 concentrations; these peaks were likely introduced laterally from an alternate source. One-dimensional numerical modeling of the d2H confirms that transport across the till-shale interface occurred over ~20-30 ka Before Present (BP), and from the base of the oxidized till (~5 m below ground surface, BGS) into the till over ~10 ka BP. CH4 concentrations through the till were below detection limit (BDL; < 5 ppmv); concentrations increase from 0.04 mg L-1 below the till-clay interface (at ~85 m BGS), to 49 mg L-1 at 240 m BGS. One-dimensional numerical modeling of the CH4 concentration profile suggests that the timing of deposition of the till cannot be constrained using CH4, and that a geochemical reaction impacts its transport to ~30 m below the till-clay interface. Concentrations of C2H6 and C3H8 are minor throughout the profile. Depleted 13C and 2H values of CH4, ranging between –52 and –92‰ VPDB and –173 to –269‰ VSMOW, respectively, and the 13C-depleted nature of d13CC2H6 (–45 to –85‰ VPDB) support a bacterial origin for the gases.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.003 |
| Science and technology studies | 0.003 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| 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".