Artesian Aquifer Definition Explained

An artesian aquifer is a specific type of underground water reservoir. Let's break down why the correct option defines it perfectly.

Understanding Confined Water Pressure

The key characteristic of an artesian aquifer is that the groundwater is trapped or confined between two layers of impermeable material. These layers act like seals, preventing water from easily escaping upwards or downwards. Typically, these confining layers are made of dense materials like shale or granite.

Because the water is trapped, it exists under pressure. This pressure often comes from the weight of the water held in the aquifer at higher elevations (recharge areas). When a well is drilled into an artesian aquifer, this built-up pressure can cause the water level in the well to rise above the top of the aquifer itself. If the pressure is high enough, water might even flow out of the well without needing a pump – this is called a flowing artesian well.

Therefore, the description 'water is under pressure between two impervious strata' accurately captures the essence of an artesian aquifer.

Why Other Options Are Incorrect

Let's look at why the other choices don't describe an artesian aquifer:

• 'water surface under the ground is at atmospheric pressure': This describes an unconfined aquifer. In an unconfined aquifer, the upper boundary is the water table, which is directly connected to the atmosphere, meaning the water is not under significant pressure beyond atmospheric levels.
• 'water table serves as upper surface of zone of saturation': This is also a description of an unconfined aquifer. The water table is the top surface of the saturated zone in such aquifers, and it fluctuates with water availability, but the water isn't held under pressure by confining layers.
• 'None of the above': Since option 2 correctly defines an artesian aquifer, this option is incorrect.

Key Features of Artesian Aquifers

• Confined: Sandwiched between impermeable layers (aquitards or aquicludes).
• Pressurized: Water is stored under hydrostatic pressure.
• Recharge: Usually recharged in areas where the confining layers dip down and allow water to enter from surface water bodies or higher ground.
• Artesian Wells: Wells drilled into artesian aquifers often have naturally rising water levels, sometimes resulting in flowing wells.