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How Groundwater Moves Underground
Slow flow through pores and fractures
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Groundwater is water that soaks into the ground after rain, melting snow, or water from rivers and lakes. It matters because many communities use groundwater for drinking water, farming, and industry. Even though it is hidden underground, groundwater is part of the water cycle and can connect to streams, lakes, and wetlands. Understanding how it moves helps people protect water supplies from pollution and overuse.
After water infiltrates the soil, gravity pulls it downward through tiny spaces called pores and through cracks in rock. When all the pore spaces are filled with water, the water is in the saturated zone below the water table. Groundwater usually moves slowly sideways from higher pressure or higher elevation toward lower pressure or lower elevation. Aquifers are underground layers that store and transmit water, and they can be unconfined near the surface or confined between less permeable rock layers.
Key Facts
- Infiltration is the process of water entering the ground from the surface.
- The water table is the top of the saturated zone, where pore spaces are completely filled with water.
- Groundwater flows from higher hydraulic head to lower hydraulic head.
- Flow speed depends on permeability, porosity, and slope of the water table.
- Darcy's law: Q = K A (Δh / L), where Q is flow rate, K is hydraulic conductivity, A is area, and Δh / L is hydraulic gradient.
- Unconfined aquifers are open to recharge from above, while confined aquifers are trapped between less permeable layers.
Vocabulary
- Groundwater
- Water stored and moving beneath Earth's surface in soil, sediment, and rock.
- Aquifer
- An underground layer of permeable sediment or rock that can store and transmit usable amounts of water.
- Water table
- The upper surface of the saturated zone where underground spaces are filled with water.
- Porosity
- The percentage of a material's volume that is made of open spaces or pores.
- Permeability
- A measure of how easily water can move through connected pores or fractures in a material.
Common Mistakes to Avoid
- Thinking groundwater flows in large underground rivers is wrong because most groundwater moves slowly through tiny connected pores and cracks.
- Confusing porosity with permeability is wrong because a material can have many pore spaces but still not let water pass through easily if the pores are not connected.
- Assuming groundwater always moves straight downward is wrong because after reaching the saturated zone it often moves sideways along hydraulic gradients.
- Calling every underground layer an aquifer is wrong because an aquifer must both store water and allow enough water to flow for wells or springs.
Practice Questions
- 1 A rainstorm drops 30 mm of rain on a field. If 40 percent infiltrates into the ground, how many millimeters of water enter the soil?
- 2 Groundwater moves 12 meters in 4 years through a sandy aquifer. What is its average speed in meters per year?
- 3 A well is drilled into an unconfined aquifer near a stream. Explain how heavy pumping from the well could change the direction of groundwater flow and affect the stream.