The location of old groundwater in hydrogeologic basins and layered aquifer systems
Bibliographic record
Abstract
The age of groundwater, the time since the water recharged the subsurface, is a fundamental characteristic of groundwater that impacts diverse geologic processes and practical applications. The distribution of groundwater age depends on many factors including permeability, recharge rate, aquifer geometry, and topography. Seminal work simulated topography‐driven regional groundwater flow with various topographies, localized high‐permeability zones, and more recently with permeability decreasing with depth, but the role of layered aquifer systems which are common in both consolidated and unconsolidated sediments has not been systematically explored. Here we show that high age zones with predictable locations occur in layered geologic systems across a wide range of hydraulic gradients, basin geometries, and permeabilities. Numerical simulations of a generic three‐layer aquifer system indicate that high age zones consistently form in the low‐permeability layer near the middle of the basin. The zones of older groundwater result from low groundwater velocities in the low‐permeability layer and the rejuvenation of the groundwater through mixing of different flow paths near discharge zones. The high age zones are not hydraulic stagnation points but are associated with areas of low velocity. Formation and location of zones of high groundwater ages in low‐permeability units are important as these units are targeted for radioactive waste disposal and shale gas extraction. High age zones are also likely to affect geologic processes that depend on groundwater or solute fluxes and may serve as archives of past hydrological or climatological conditions.
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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 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 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".