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 item, signaling that the exam tests more than Darcy arithmetic. You must identify the aquifer setting, choose the right saturated thickness, and interpret 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 aquitards. The water is under pressure. A well screened in it rises above the top of the aquifer to the potentiometric surface, and if that surface is above ground the well flows under artesian pressure. The aquifer stays fully saturated even as the measured head drops.

An unconfined aquifer (water-table aquifer) has the water table as its upper boundary. Its saturated thickness changes as the water table rises or falls. When pumping lowers the water table, part of the pore space physically drains, which changes both storage behavior and the form of some flow equations.

Transmissivity

For horizontal flow in a confined aquifer of uniform properties, transmissivity is T = K b, where b is the aquifer thickness. Units are area per time, such as ft^2/day. High transmissivity means a given gradient moves more water and a well produces more flow for less drawdown. Transmissivity is the single property most pump tests are designed to estimate.

For an unconfined aquifer the saturated thickness varies, so simple horizontal Dupuit flow per unit width uses 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 exactly why unconfined problems must be read slowly.

Storage Terms

Storativity S is the volume of water released from storage per unit aquifer area per unit decline in head. For confined aquifers S is small because the aquifer stays saturated; water is released mainly because the skeleton compresses and the water expands slightly as pressure falls. Specific yield Sy is the drainable fraction of pore volume in an unconfined aquifer and is far larger because gravity empties part of the pore space as the water table falls. Specific retention is the water held by capillary forces that does not drain freely; porosity n equals Sy plus specific retention.

Calculation Workflow

1. Decide whether the screened interval is confined or unconfined.
2. Identify the relevant head surface: potentiometric surface or water table.
3. For confined horizontal flow, compute T = K b when needed.
4. For unconfined flow, measure h from the aquifer base to the water table.
5. For storage volume, use S x area x head decline (confined) or Sy x area x water-table decline (unconfined).
6. Reconcile units: acre-ft, ft^3, gal, and MGD must not be mixed without conversion (1 acre-ft = 43,560 ft^3 = 325,851 gal).

Leaky and Perched Settings

Real stratigraphy is rarely ideal. A leaky (semiconfined) aquifer has an aquitard above or below that transmits some vertical flow, so steady pumping eventually draws recharge through the leaky layer and drawdown stabilizes earlier than the confined theory predicts. A perched aquifer is a small saturated zone sitting on a low-permeability lens above the regional water table; it can mislead a site investigation if a shallow boring stops in perched water and the regional table is much deeper. The exam expects you to recognize these from the description rather than to solve detailed leaky-aquifer equations.

Worked Example

An unconfined aquifer sits on a clay base at elevation 480.0 ft. The water table is at 512.0 ft on the upgradient side and 504.0 ft at a stream 2,000 ft away, and K = 45 ft/day. Saturated thicknesses are h1 = 512.0 - 480.0 = 32.0 ft and h2 = 504.0 - 480.0 = 24.0 ft. Per unit width q = K (h1^2 - h2^2) / (2 L) = 45 (32^2 - 24^2) / (2 x 2,000) = 45 (1,024 - 576) / 4,000 = 45 x 448 / 4,000 = 5.04 ft^2/day per ft of width. The trap is measuring h from the datum rather than from the clay base at 480.0 ft.

Scenario Clues and Traps

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

• If both ground-surface and aquifer-base elevations are given, do not assume depth below grade equals saturated thickness; convert elevations first.
• Do not apply the squared-head Dupuit form to a confined aquifer, where thickness is fixed.
• Watch for storativity and specific yield being swapped; using a confined S where Sy belongs underestimates available water by two or more orders of magnitude.
• Do not confuse a perched zone with the regional water table when reading boring logs.

Many distractors are built around the wrong reference datum or the wrong storage parameter, so confirm both before computing.