Groundwater Flow

Understanding the interactions of the hydrosphere and the geosphere allows scientists to study many Earth processes that shape the planet. When Earth materials are permeable and porous, they absorb and hold water. This groundwater is a part of the water cycle. Students use this knowledge to test which Earth materials can hold the most groundwater.

• Groundwater is the supply of fresh water found beneath Earth’s surface in the pores of soil, sand, and rock.
• Groundwater is held in aquifers, which are underground layers of rock, sand, or gravel that hold water.
• Different rocks hold water differently, so water does not move through the ground in the same way everywhere.
• Water on Earth’s surface seeps into the ground, just like water is slowly absorbed into a jar of sand. This is part of the water cycle.
• Groundwater is in constant motion, although it generally moves more slowly than water in a stream or river moves because it must pass through the interconnected pores of the rock.
• Sinkholes are holes in the ground formed when water has dissolved underground rock to the point where it can no longer support the land surface, and they form because of processes that occur underground, sometimes gradually and sometimes all of a sudden.
• Some regions are more prone to sinkholes than others because of the kind of rocks underground.

• How does groundwater get underground?
• What causes groundwater to move?
• Why can aquifers hold water?
• Why does the water in some aquifers move more slowly than in other aquifers?
• How does groundwater cause underground rock to erode?
• What causes a sinkhole to form?
• How do scientists know which parts of the country are most likely to experience sinkholes?
• Why can sinkholes be catastrophic?
• How are engineers working with scientists to address this problem?

Misconception: Groundwater is stored in underground lakes.

Fact: Groundwater is stored in the pores that make up rocks, soil, and sand.

Misconception: There is the same amount of groundwater everywhere on Earth.

Fact: Groundwater is distributed unevenly around the planet. Its presence depends on many factors, including the amount of precipitation that falls and the properties of the rock that make up a region—specifically whether the rocks are porous and permeable.

Aquifer : an underground layer of rock, sand, or gravel that holds groundwater; a source of well and spring water

Groundwater : the supply of fresh water found beneath Earth’s surface in the pores of soil, sand, and rock

Hydrogeology : the study of geology that is focused on the distribution and movement of groundwater in the soil and rocks of Earth’s crust

Permeability : the ease with which substances such as water move through a material

Porosity : the amount of space between particles in a substance

Sinkhole : a hole in the ground formed when water has dissolved underground rock to the point where it can no longer support the land surface

Sinkhole in Louisiana

One day in 2012, a massive hole opened in the ground in southern Louisiana. The force of this event was so strong that entire trees were flipped upside down. Their roots faced the sky. Beneath the water, the hole measured 117 meters (384 feet) deep. This is roughly the height of a 40-story building.

Almost a year later, the hole grew even larger. Within 30 seconds, a row of 40- foot-tall cypress trees was completely swallowed up by the water. This massive hole was the result of two interacting Earth systems: the hydrosphere and the geosphere. Remember that the hydrosphere is the Earth system that includes all of the water on the planet. The geosphere is the Earth system that includes the rocks, soil, landforms, and interior of the planet.

Water has been carving out holes in Earth’s rocky interior over millions of years. As water seeps into the ground, it erodes underground rock. Sometimes it erodes it so much that the rock can no longer support the land above it. When this happens, a sudden collapse of the land on the surface can occur. The result is a sinkhole. A sinkhole is a hole in the ground formed when water has dissolved underground rock to the point where it can no longer support the land surface. Most sinkholes are only 10 to 12 feet in diameter. Some, like the sinkhole in Louisiana, are hundreds of feet in diameter. This is large enough to swallow trees, cars, and even entire buildings.

A Changing Planet

Sinkholes form because of processes that occur underground. These processes can occur gradually or all of a sudden. To understand what leads to giant holes appearing on Earth’s surface, we need to begin with the rocks that make up Earth’s surface. Remember that these rocks do not stay the same. Energy is constantly breaking them down and reforming them through heat and pressure in the rock cycle.

Water and the Rock Cycle

The rock cycle refers to the processes that form, break down, and re-form rock from one category to another. Water plays a major role in the rock cycle, as well as in the formation of sinkholes. As water moves over rocks, it shapes those rocks through weathering and erosion, reshaping Earth’s surface.

Properties of Rocks

Groundwater generally moves more slowly than water in a stream or river. This is because it must pass through the interconnected pores of the rock.

The speed at which groundwater moves depends on both the rock’s porosity and its permeability. Porosity refers to the amount of space between particles in a substance. Permeability refers to the ease with which substances such as water move through a material. Porosity determines how much water a material can hold, while permeability determines whether water can move through the material. Rocks that are both porous and permeable are most likely to hold water.

Predicting Sinkholes

Small sinkholes happen all the time. Many of these sinkholes don’t cause a lot of damage. Sometimes, however, they can be catastrophic. Because of this, scientists and engineers are trying to develop technologies that help them predict where sinkholes are likely to occur.

Scientists believe that 10 percent of Earth’s surface can experience sinkholes. In the United States, sinkholes are most common in Alabama, Florida, Kentucky, Missouri, Pennsylvania, Tennessee, and Texas. However, scientists still can’t say exactly when a sinkhole will occur.

Developing Technologies

There are technologies that can determine if the rock underneath the ground is full of holes. This makes it more likely that a sinkhole will occur. It won’t necessarily indicate when a sinkhole might occur. This makes it hard for people to prepare.

In response, engineers from NASA are working on a new technology that can predict when sinkholes might form. This technology uses a plane that has radar technology and sends out electronic pulses. These pulses allow scientists to map out how Earth’s surface is shifting. For example, the technology picked up signals that the ground around the sinkhole in Louisiana was moving at least a month before it collapsed.

In this lesson students design an experiment to test the permeability of different Earth materials. Students gather data from their experiment and use it along with their knowledge of the rock cycle, the water cycle, and Earth’s interacting systems, to analyze the phenomena of Earth material permeability and decide which earth material is most permeable to groundwater.