36.5.3. Unconfined Aquifer

In unconfined aquifers, the saturated thickness changes with drawdown:

An unconfined aquifer is a type of aquifer that is not confined by a layer of impermeable rock or clay above it. This means that the water table, which is the upper surface of the saturated zone, can fluctuate and is directly affected by atmospheric pressure and rainfall. In these aquifers, when water is pumped out (drawdown), the saturation level changes, and the thickness of the saturated part of the aquifer reduces.

Think of an unconfined aquifer like a sponge that is sitting on a counter. When you pour water on the sponge, it absorbs the water until it reaches its saturation point. If you then start to squeeze the sponge (like pumping water from an aquifer), the water level in the sponge decreases and the amount of wet sponge (saturated thickness) decreases. If you stop squeezing, the sponge takes its time to regain its shape and absorb more water.

πk(h2−h2

) Q= 2 1 r ( ) ln 2 r 1 Where:

• h1, h2 = Water table elevations

This equation is used to calculate the discharge (Q) of water flowing into a well from an unconfined aquifer. Here, π (pi) is a constant, k represents the coefficient of permeability which indicates how easily water can flow through the aquifer material, and (h2 - h1) represents the difference in water table elevation between two points at radial distances r2 and r1 from the well. This formula shows how the flow rate increases with higher differences in water levels and more permeable materials.

Imagine a water fountain that is connected to a large bucket filled with water (the unconfined aquifer). If you poke a hole in the side of the bucket, water will flow out. The greater the difference between the water level in the bucket and the level outside (h2 - h1), the stronger and faster the water will flow out. If the hole is big (high permeability), even more water can gush out.

Assumptions for Steady Flow:

• Aquifer is homogeneous and isotropic.
• Flow is horizontal and radial.
• Well is fully penetrating the aquifer.
• Flow is steady (inflow = outflow).

These assumptions simplify the analysis of water flow from an unconfined aquifer. A homogeneous and isotropic aquifer means that the material is the same throughout, allowing for a uniform flow. Horizontal and radial flow indicates that water moves evenly in all directions from the well. The well fully penetrating the aquifer ensures that it can draw water from all available layers. Finally, assuming steady flow means that the amount of water flowing into the well matches the amount being pumped out, indicating a consistent state of flow.

Consider watering a circular garden from a central sprinkler. If the sprinkler emits water uniformly in all directions, you can relate that to horizontal and radial flow. If the area around the sprinkler is flat and consistent (homogeneous), water will spread evenly. If the water supply remains stable while the sprinkler is on and it puts out just the right amount of water to keep the garden lush, this is akin to a steady flow condition.

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