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.
Understanding How Groundwater Moves Underground
Water rarely moves underground as a wide, open river. In sand, soil, and many rocks, it follows a twisting route around mineral grains. The connected spaces may be extremely small.
Water sticks slightly to grain surfaces, so friction slows its movement. Clay can hold much water in tiny pores, yet transmit it very poorly because those pores are not well connected. Coarse sand and gravel often transmit water more easily.
Solid rock can behave in unexpected ways. A dense granite may block flow except where joints, faults, or weathered cracks create pathways. This is why the type and arrangement of underground material matter as much as the amount of empty space.
Groundwater movement is driven by differences in total water energy, called hydraulic head. Elevation is one part of this energy. Pressure is another part.
Water in a deep confined layer can be under high pressure even when that layer is low in the landscape. If a well reaches such a layer, water may rise inside the well without pumping. Scientists compare water levels in several wells to find the direction of movement.
They draw contour lines of equal water level on a map. Flow crosses these lines toward lower head.
The flow rate increases when the material conducts water well, the route is steeper, or the flow passes through a larger cross sectional area. This relationship is described by Darcy's law, which says flow rate equals hydraulic conductivity times area times hydraulic gradient.
Groundwater has places where it enters the subsurface and places where it leaves. Recharge is often greatest where rain can soak through exposed soil, sand, or fractured rock. Pavement, compacted ground, and steep slopes can reduce recharge because more water runs off at the surface.
Discharge can occur at springs, wetlands, streambeds, and coastal areas. During a dry period, groundwater may keep a stream flowing. This contribution is called baseflow.
A pumping well changes the natural pattern by lowering water levels nearby. The lowered area around the well is called a cone of depression. If pumping continues faster than recharge, nearby wells can dry up and streams may receive less baseflow.
Pollution underground is difficult to manage because movement can be slow and uneven. A spilled chemical may travel quickly through a gravel layer or a rock fracture, while moving very slowly through fine sediment. Some substances attach to soil particles, which delays them.
Others dissolve easily and travel with the water. Bacteria and some chemicals can break down over time, but many contaminants remain for years. When studying groundwater, pay close attention to diagrams that show layer boundaries, well water levels, and arrows of flow.
Do not assume water always moves straight down. Use evidence from several wells, since one well gives information about only one location.
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.