Confined Aquifers | Civil Engineering Optional Notes for UPSC PDF Download

Confined Aquifers

Key Features:

• Aquitard: A confined aquifer is characterized by the presence of an aquitard (or confining bed) both above and below it. These aquitards act as barriers, restricting water movement into or out of the aquifer.
• Water Level in Wells: In tightly cased wells within a confined aquifer, the water level rises above the top elevation of the aquifer itself. This occurs due to pressure exerted by the confined water.
• Hydraulic Heads and Recharge: Groundwater recharge in confined aquifers typically occurs at outcrop areas where hydraulic heads are higher than the elevation of the confined portion of the groundwater system. Water flows into the aquifer, and hydraulic heads exceed the top elevation of the fully saturated confined zone.
• Flow Direction: The confining units above and below the aquifer direct flow parallel to the aquifer boundaries.
• Potentiometric Surface: The surface to which water rises in wells and piezometers penetrating a confined aquifer is known as the potentiometric surface. It represents the hydraulic head distribution within the aquifer.
• Water Movement: Water moves from areas where the potentiometric surface is at a higher elevation to areas where it is at a lower elevation.
• Importance: Understanding confined aquifers is crucial for assessing groundwater flow patterns and hydraulic gradients.

Unconfined vs. Confined Aquifers

Remember, the potentiometric surface provides valuable insights into the behavior of groundwater flow in both confined and unconfined aquifers. Whether water flows freely or is confined, aquifers play a vital role in our hydrological systems

Schematic of a Confined Aquifer

Garden Hose Analogy for Confined Aquifers

• A garden hose (representing the confined aquifer)
• Sediment (to fill the hose)
• A cap or plug (to seal one end of the hose)
• Water (to simulate groundwater)
• A well (a hole drilled into the hose)
• Piezometers (hollow tubes within the hose)

Steps:
Setting Up the Hose:

• Fill the garden hose with sediment (sand or gravel) to represent the confined aquifer.
• Seal one end of the hose with a cap or plug to mimic the confining bed.

Creating the Recharge Area:

• Raise the open end of the hose (the recharge area) to increase the potential energy of the system.
• This elevation change corresponds to the hydraulic head increase observed when water is introduced into a real aquifer.

Introducing Water:

• Open the capped end of the hose and introduce water into the upper end.
• Water flows into the confined aquifer, similar to groundwater recharge.

Well Installation:

• Drill a hole into the hose at some distance from the upper end to simulate a well.
• Water squirts out of the hole, representing groundwater extraction.

Piezometers:

• Install tall piezometers (hollow tubes) within the hose.
• Measure water elevation from a horizontal datum.
• These measurements help construct the potentiometric surface, showing hydraulic head distribution within the confined aquifer.

Observations:

• The total head observed in the piezometers exceeds the height of the confining bed (hose walls).
• This mirrors real-world conditions where hydraulic head values in confined aquifers surpass the elevation of the confining layers.

Example of Sediment Filled Garden Hose

Confined Wells (Artesian Wells)

Definition: Confined wells, commonly referred to as artesian wells, penetrate confined aquifers. These wells access groundwater under confined or artesian conditions.

• In some cases, the hydraulic head within a confined aquifer surpasses the elevation of the surrounding land surface.
• As a result, the water level in the well casing rises higher than the ground surface.

• If the top of the well casing is positioned lower than the elevation of the potentiometric surface (the confined aquifer’s water level), water naturally flows out of the well.
• No external pumping is required; the pressure within the confined aquifer drives the flow.

• The term “artesian well” traces back to wells constructed in the medieval Roman city of Artesium (now Artois, France).
• These wells tapped into confined aquifers and produced free-flowing water at the surface.

• While scientifically, any well penetrating a confined aquifer is an artesian well, the term is often used colloquially to describe any flowing well, regardless of whether it accesses a confined aquifer.
• Flowing wells may also occur in discharge areas of unconfined aquifers with significant upward hydraulic gradients.

Importance: Understanding confined wells helps us appreciate the dynamics of groundwater flow and the role of potentiometric surfaces.