Groundwater, 2010-2017

State environmental policy
U.S. environmental policy
Endangered species policy
State endangered species
Federal land policy
Environmental terms

Water located within cracks and spaces in sand, soil, and rocks below the Earth's surface is known as groundwater. This water seeps into the ground and accumulates in rock beneath the Earth's surface. Sources of groundwater include rainfall, snowmelt, and water from the bottom of lakes and rivers. The main uses for groundwater are irrigation and drinking water.

According to the U.S. Geological Survey in a November 2014 report, out of the 355 billion gallons of water withdrawn each day for use in the United States in 2010, approximately 76 billion gallons—21 percent—were fresh groundwater.

Background

After precipitation reaches the Earth's surface or snow melts into water, it flows into lakes, streams, and soils. Some of this water remains in the soil or attaches to the roots of plants, which use the water and moisture to grow. The remaining water seeps into the ground through the cracks and spaces of rocks, sand, and soil. This water eventually reaches rock layers it cannot easily pass through. Groundwater is the result of water that enters empty cracks and spaces above the impermeable rock layers.

The underground soil or rock through which groundwater can move is called an aquifer, which is composed of gravel, sandstone, sand, or limestone. Aquifers contain pores—large, interconnected spaces—through which water can flow. Without interconnected pores in the rock, groundwater cannot permeate an aquifer. For example, shale and clay contain pores, but groundwater cannot easily flow through these pores because they are not interconnected.

Groundwater supplies are naturally replenished by rainfall, snowmelt, and water from the bottom of lakes and rivers. The amount of rainfall or snowmelt that replenishes groundwater depends on local geology and climate, and water from rainfall or snowmelt is distributed in other ways. For example, some water from rainfall or snowmelt evaporates into air, which is taken in by plants through plant respiration. Rainfall or snowmelt can also be collected by streams.

Aquifers containing groundwater may be also artificially replenished. This process, known as artificial recharge, involves spreading treated surface water onto land so it seeps into aquifers or involves directly injecting treated water into aquifers through a well.

Extraction

Groundwater is brought to the surface naturally through springs or lakes and can also be extracted by digging or drilling wells into aquifers. A well, which is generally a single pipe, delivers groundwater to the surface through a pump. Another type of well, known as an artesian well, is drilled into artesian aquifers, which are found within two impermeable rock layers. An artesian well delivers groundwater to the surface using underground pressure that thrusts the water above the aquifer and to the surface.

Groundwater treatment

Several treatments processes remove particles, chemicals, microorganisms, and other potential contaminants from groundwater to make it suitable for drinking:

• Groundwater is first treated by adding chemicals with a positive charge, which overcome negatively charged dirt and dissolved particles in the water. The particles and positively charged chemicals together form larger particles.
• These larger particles settle at the bottom of the water, a process known as sedimentation. With the larger particles settled at the bottom, the clearer water above is filtered through gravel, sand, charcoal, or other materials to remove additional dissolved particles, including bacteria, viruses, chemicals, parasites, and dust.
• After filtration, chlorine or another disinfectant is added to the water to eliminate remaining viruses or bacteria and to ensure quality as the water is delivered to homes and businesses.

See also

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  Drinking water quality

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  Public water system

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  Wetland

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  Glossary of environmental terms