Question

What is the example of an aquitard?

• A. Zeolite
• B. Gravel
• C. Sand
• D. Shale

Hint:

• Aquifer: Stores and transmits significant water (e.g., sand, gravel).
• Aquitard: Low permeability, transmits water slowly, can confine an aquifer (e.g., clay, shale, siltstone).
• Aquiclude: Essentially impermeable, acts as a barrier (often used interchangeably with aquitard for very low permeability layers).
• Shale, being rich in clay, has low permeability and is a common example of an aquitard.

Answer 1

The Correct Option is D

In hydrogeology, geological formations are classified based on their ability to store and transmit groundwater:

• Aquifer: A saturated geological formation that can store and transmit significant quantities of water under ordinary hydraulic gradients. Examples: Sand, Gravel, Sandstone, fractured limestone.
• Aquitard (Leaky Confining Layer): A saturated or unsaturated geological formation that is of low permeability and transmits water at a very slow rate compared to an aquifer. It can store groundwater and slowly release it to adjacent aquifers or streams, but it does not yield water readily to wells. It can form a confining layer but allows some leakage.
• Aquiclude (Confining Layer): A saturated geological formation that is essentially impermeable and does not transmit significant quantities of water. It acts as a barrier to groundwater flow. (Often, the term aquitard is used more broadly to include aquicludes, or a distinction is made based on the degree of impermeability).
• Aquifuge: An impermeable geological formation that neither stores nor transmits water.

Let's look at the options:

• Zeolite (option a): Zeolites are microporous aluminosilicate minerals. While they have high porosity and can adsorb water, their permeability can vary. Some altered volcanic tuffs rich in zeolites can act as aquifers or aquitards depending on their specific properties. Not a classic example of an aquitard without more context.
• Gravel (option b): Gravel is typically highly porous and highly permeable, making it an excellent aquifer material.
• Sand (option c): Sand is also porous and permeable, commonly forming good aquifers.
• Shale (option d): Shale is a fine-grained sedimentary rock composed mainly of clay minerals. Clay particles are very small, and while shales can have high porosity (due to small interparticle spaces), these pores are poorly connected, resulting in very low permeability. Therefore, shale is a classic example of an aquitard or aquiclude, acting as a confining layer that restricts groundwater flow.

Thus, shale is the best example of an aquitard among the given options. \[ \boxed{\text{Shale}} \]