---
title: "Confined versus unconfined aquifer"
subject: "Engineering Geology I"
chapter: 6
key: "semester_3/engineering_geology_i:confined-versus-unconfined-aquifer"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/confined-versus-unconfined-aquifer
site: BCT Engineering
---

# Confined versus unconfined aquifer

Engineering Geology I · Chapter 6: Physical Geology

- Asked in: 2066 Jestha (old course), 4 marks; 2062 Baisakh (old course), 2 marks (asked 2 times)
- Video: [Unconfined Aquifers vs. Confined Aquifers](https://www.youtube.com/watch?v=wx0w-Az5JOY) — Dane Andersen
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/confined-versus-unconfined-aquifer
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-6

## Question

Differentiate between confined and unconfined aquifer.

## Short answer

In an unconfined aquifer, the water table forms the upper boundary, pressure is atmospheric, recharge occurs over the entire area, and it is prone to pollution. A confined aquifer is between impermeable layers, water is under pressure above atmospheric, recharge is limited to outcrops, and it has lower pollution risk.

## Answer

| Point | Unconfined (water-table) aquifer | Confined (artesian) aquifer |
|---|---|---|
| Boundary | Permeable above; open to the surface | Between two impermeable layers (aquicludes) |
| Upper surface | Water table, free surface | Piezometric surface above the aquifer top |
| Pressure | Atmospheric | Greater than atmospheric |
| Water in well | Stands at the water table | Rises above the top of the aquifer, may flow (artesian) |
| Recharge | Direct from rain over the whole area | From the outcrop area only |
| Pollution risk | High | Low |
| Level fluctuation | Large | Small |
| Storage | Specific yield | Storage coefficient (small) |

```text
 ground  ~~~~~~~~~~~~~~~~
 unconfined: water table .  (sand)
 --------------------------
 clay (aquiclude)
 confined: sand under pressure
 clay (aquiclude)
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which describes a confined aquifer?
   A. Lies between two impermeable layers
   B. Open to surface with a water table
   C. Always dry
   D. Contains only clay
   Answer: A. Unconfined aquifers are permeable above and open to the surface. (Key point: Boundary)

2. What is the water pressure in a confined aquifer?
   A. Atmospheric
   B. Greater than atmospheric
   C. Zero
   D. Negative
   Answer: B. The piezometric surface lies above the aquifer top. (Key point: Pressure)

3. How is a confined aquifer recharged?
   A. Directly from rain over the whole area
   B. From the water table
   C. From the outcrop area only
   D. Not at all
   Answer: C. Unconfined aquifers recharge over the whole area. (Key point: Recharge)

4. Which aquifer has higher pollution risk and larger level fluctuation?
   A. Confined
   B. Both equally
   C. Neither
   D. Unconfined
   Answer: D. Confined aquifers are protected by overlying aquicludes. (Key point: Pollution)

## Related in Engineering Geology I

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## Related in other subjects

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- [Confined and unconfined aquifers](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aconfined-unconfined-aquifers-k) — Soil Mechanics, chapter 6
- [Confined and unconfined flow](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aconfined-unconfined-seepage-flow) — Soil Mechanics, chapter 7
- [Aquifer versus confining bed](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aaquifer-vs-confining-bed) — Engineering Geology II, chapter 3, asked 2×