Groundwater is water that is found underground, where it fills spaces between rocks and soil particles. Though the ground may appear dry, it actually conceals large amounts of water, both near the surface and much deeper. Groundwater saturates loose soil or rocks, forming bodies of water called aquifers. It can also form underground rivers and lakes, though this is less common. Groundwater is a vital source of freshwater for agriculture, industry, and home use.
When rain and snow fall to the ground, the water keeps on moving. It may run into a river, stream, or lake. Plants suck up some of it through their roots. Some evaporates, heading back into the atmosphere. However, some water sinks into the ground. Soil, rocks, and clay are all full of pores, spaces wide enough for air and water to seep in. Rocks can have holes that are quite large, but even the tiny particles that form clay have space between them. Wet sand at a beach illustrates how how water can move through rocky particles.
An aquifer is an underground layer of sand or rock that can hold water. Aquifers can be shallow or deep. The shallower the aquifer, the more easily it is replenished by rainwater and the easier it is to drill into it. An artesian aquifer contains groundwater confined in layers of rock and under pressure, which allows water to easily be extracted from it. A well drilled into an artesian aquifer is called an artesian well.
Humans have made use of aquifers for thousands of years, digging down into them to make wells. As of 2015, about half of all the drinking water in the United States came from aquifers. Almost all rural homes use well water from aquifers. The Ogallala Aquifer, which runs from South Dakota to Texas, contains water that melted after the last glacial period, around 15,000 years ago; it provides water for much of the central United Page 373 | Top of ArticleStates. The Great Artesian Basin in Australia provides water for most of Queensland.
Some important sources of groundwater were compromised during the twentieth century. Groundwater can easily be contaminated by chemicals that are allowed to seep into the water; this contamination can poison wells and make them unusable. The Ogallala Aquifer has been depleted by cities and farms that have drawn on its water for decades; moreover, it has been heavily used by the oil and gas industry. Waiting for rain to refill it is not practical, so farmers in the area have implemented some conservation programs.