11.2 Confined and Unconfined Aquifers and Storage

Aquifer Types and Storage

The Groundwater and Wells topic in the April 2024 PE Civil WRE specification names aquifers as a separate knowledge area. That is a signal that the exam can test more than Darcy arithmetic. You need to identify the aquifer setting, choose the right saturated thickness, and understand what a drawdown or water-level change means physically.

Confined Versus Unconfined

A confined aquifer is a permeable water-bearing layer bounded above and below by lower-permeability material. The water is under pressure. If a well is screened in a confined aquifer, the water level in the casing rises above the top of the aquifer to the potentiometric surface. The aquifer can remain fully saturated even when the measured head drops.

An unconfined aquifer has the water table as its upper boundary. The saturated thickness changes as the water table rises or falls. If pumping lowers the water table, part of the pore space actually drains. That difference changes both storage behavior and some flow equations.

Transmissivity

For horizontal flow in a confined aquifer with uniform properties:

T = K b

T is transmissivity, K is hydraulic conductivity, and b is aquifer thickness. Units are area per time, such as ft^2/day. High transmissivity means a given gradient can move more water and a pumping well may produce more flow for less drawdown.

For an unconfined aquifer, saturated thickness varies with the water table. In simple horizontal Dupuit flow per unit width, a common relationship is:

q = K (h1^2 - h2^2) / (2 L)

Here h1 and h2 are saturated thicknesses measured above the impermeable base, not water surface elevations above an arbitrary datum unless the datum is the base. This squared-head form is why unconfined problems must be read carefully.

Storage Terms

Storativity, S, is the volume of water released from storage per unit aquifer area per unit decline in hydraulic head. For confined aquifers, S is commonly small because the aquifer remains saturated. Water is released mostly because the aquifer skeleton compresses and the water expands slightly as pressure drops.

Specific yield, Sy, is the drainable fraction of pore volume in an unconfined aquifer. It is much larger than confined storativity because gravity drainage empties part of the pore space as the water table falls. Specific retention is the water that remains held by capillary forces and does not drain freely.

Calculation Workflow

1. Decide whether the well is screened in a confined or unconfined aquifer.
2. Identify the relevant head surface: potentiometric surface or water table.
3. For confined horizontal flow, compute T = K b if transmissivity is needed.
4. For unconfined flow, measure h from the aquifer base to the water table.
5. For storage volume, use volume = S x area x head decline for confined storage or Sy x area x water-table decline for unconfined drainage.
6. Check units. Acre-ft, ft^3, gal, and MGD must not be combined without conversion.

PE WRE Scenario Clues

A confined problem may mention clay aquitards, artesian pressure, a potentiometric surface, or water rising above the screened interval. An unconfined problem may mention shallow groundwater, water-table drawdown, infiltration basins, recharge, wetlands, basements, or dewatering by lowering the water table.

If the prompt gives both ground surface elevation and aquifer base elevation, do not assume the depth below grade is the saturated thickness. Convert elevations first. Many exam choices are built around using the wrong reference datum.