Groundwater Flow

Science background gives teachers more detailed information on the phenomena students explore in this unit. Below is an excerpt from the science background section on groundwater flow.

Earth’s Systems

Earth’s surface is constantly changing. Some of these changes are gradual, taking place over hundreds or thousands of years, such as the slow weathering and erosion of rock. Other changes seem to occur in an instant, such as when the ground suddenly falls away in a 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.

All of these changes take place because of interactions among Earth’s four primary systems: the hydrosphere, the geosphere, the atmosphere, and the biosphere.

Weathering, erosion, and the formation of sinkholes occur primarily because of interactions among these systems as energy flows through them. In fact, all geoscience processes that occur on Earth result from energy flowing and matter cycling within and between Earth’s systems.

Sinkholes form because of processes that occur underground. Sinkholes are common in areas where the land sits on top of rock that can naturally be dissolved by groundwater—the supply of fresh water found beneath Earth’s surface in the pores of soil, sand, and rock.

Groundwater and Aquifers

Groundwater is distributed around the planet according to one underlying scientific principle—the more porous the rock is, the more water it can hold. Porosity refers to the amount of space between particles in a substance. Rocks that have a lower density are more likely to contain water because they are often porous. Density is the mass of a substance in a given volume. Sedimentary rocks are often porous and found in lakes, rivers, and oceans.

Groundwater fills the spaces between porous soil particles and fractured rock underground. In addition to porosity, the presence of groundwater also depends on the permeability of the subsurface rock. Permeability refers to the ease with which substances such as water move through a material. If the rock has characteristics that allow water to move relatively freely through it, then groundwater can move a significant distance in a number of days. But groundwater can also sink into deep aquifers where it takes thousands of years to move back into the environment, or even go into deep groundwater storage, where it might stay for millions of years.

An aquifer is an underground layer of rock, sand, or gravel that holds groundwater. The word aquifer comes from two Latin words: “aqua,” which means water, and “ferre,” which means to bear or carry. Aquifers vary greatly in size and distance beneath the surface. Some aquifers are no more than a few feet thick, while others are hundreds of feet thick. Some are a few meters below the surface, and others are hundreds of meters underground. The water present in a shallow aquifer often is just a few hours old, while the water in the deepest aquifers may be several thousands of years old and take a million years to complete the water cycle and return to the oceans.

How Sinkholes Form

Understanding how groundwater moves through and interacts with the soil and rocks of Earth’s crust is essential to understanding how and why sinkholes occur. One such rock is limestone, a kind of porous sedimentary rock that is formed primarily from marine fossils. As groundwater seeps into the ground, it absorbs carbon dioxide and reacts with decaying vegetation. As a result, the water becomes slightly acidic. As the water moves into the pores of limestone, it dissolves the rock, leaving behind caverns and empty spaces within the rock. Over time, the limestone erodes to the point where it can no longer support the land above it.

In addition to naturally occurring sinkholes, sinkholes can also occur because of human actions, including digging wells and drawing too much water from aquifers. When people withdraw too much water from an aquifer, they can increase the risk of a sinkhole forming. When too much groundwater is withdrawn from an aquifer, the pores that were once filled with water become filled with air. Because air provides less support than water, the land surface becomes less stable. When large amounts of precipitation fall, the underground rock can no longer support the added weight of the rain, and so the land collapses, forming a sinkhole.