Percolating Water: The Movement of Water beneath the Earth's Surface
The amount of surface water that sinks into the ground depends on the permeability of the soil. Groundwater stops sinking when it reaches an impermeable layer.
In this project, you will demonstrate the percolation of groundwater. The effect of rainwater quantities on the level of the water table in a region is determined. You will model the upward movement of groundwater from the zone of saturation to the zone of aeration. You will also model how the pressure from an aquifer forces groundwater up through an artesian well.
Purpose: To model percolation of groundwater.
- Large spoon
- 2 cups (500 ml) of sand
- 2 cups (500 ml) of aquarium gravel or small pebbles
- 2-quart (2-liter) plastic or metal bowl
- 1 quart (1 liter) wide-mouthed transparent plastic jar
- One-half cup (125 ml) of tap water
- Use the spoon to mix the sand and gravel together in the bowl.
- Spoon the sand and gravel mixture into the jar.
- Slowly pour the water into the jar (see Figure 22.1).
- Observe the movement of the water through the sand and gravel mixture.
- Allow the jar to stand in a warm area, such as a window with direct sunlight, for two or three days.
- Observe the sand and gravel mixture, making note of its wetness.
The water first wets the sand and fills the spaces between the sand and gravel in the upper part of the jar. As the water moves toward the bottom of the jar, it moves out of the spaces in the upper layer and fills the spaces in the lower layer. The upper layer becomes dry, with a wet layer below it.
Water, such as rainfall, that sinks into the ground is known as groundwater. This water passes through permeable (capable of having substances, such as fluids, move through it) materials, like the sand used in this experiment. The passing or seeping of groundwater or any liquid through a permeable material is called percolation.
As groundwater percolates through the ground, gravity pulls the water downward until it reaches an impermeable layer. Impermeable means that a fluid cannot pass through it. When the water reaches a layer that is impermeable to liquids, it spreads out as far as possible, then rises. In the experiment, the impermeable layer was the bottom of the jar. When the water reached the bottom of the jar, it spread across the glass bottom and the water level in the glass rose. At the surface of the Earth is a thin layer of moist soil from which plants receive their water. Below this moist layer, the percolation of groundwater creates two distinct underground zones. The first is called the zone of aeration because the pore spaces in this zone are mostly filled with air, except immediately after a rain. Rainwater fills the spaces between particles of soil, pushing the air out. The rainwater, like the water in the experiment, is pulled down by gravity and the empty pore spaces again fill with air. The zone of saturation lies directly below the zone of aeration. In this zone, the rocks and mineral grains are saturated (soaked thoroughly so that all the pore spaces are filled) with water. The boundary line between the two zones is called the water table. Below the water table, the pore spaces are filled with water, and above the water table, the pore spaces are filled with air. In the model, the place where the top dry layer and lower wet layer of sand and gravel meet represents the water table.