Groundwater Basics

Science Center Objects

Groundwater is the largest source of fresh water on Earth - it's kind of a big deal. The USGS monitors, tests, and studies groundwater resources to assure one of our Nation's most precious resources remains viable for future generations.


In simplest terms groundwater is what its name implies: water in the ground that fully saturates pores or cracks in soils and rocks. Water underlies the Earth's surface almost everywhere – beneath oceans, hills, valleys, mountains, lakes, and deserts. It is not always easy to get to or clean enough for use without treatment, but it exists essentially everywhere if you dig deep enough. Groundwater may occur close to the surface or it may lie many hundreds of feet below. And not all groundwater is the same age... water at very shallow depths could be just a few hours old, whereas water at great depth may have been there for several thousands years.

Most of the groundwater we extract is stored in and moves slowly through permeable rocks called aquifers. An aquifer may be a layer of gravel or sand, a layer of sandstone or limestone, or a buried rubbly old lava flow - as long as material is saturated enough that it can yield significant quantities of water for extraction. Water is a solvent and dissolves minerals from the rocks with which it comes in contact, so groundwater may contain dissolved minerals and gases that give it a tangy taste that is familiar to many people. The most common dissolved minerals found in groundwater are sodium, calcium, magnesium, potassium, chloride, bicarbonate, and sulfate.

Diagram showing groundwater as saturated zones underground.

It is difficult to visualize water underground. Groundwater is simply the subsurface water that fully saturates pores or cracks in soils and rocks. Groundwater is replenished by precipitation and, depending on the local climate and geology, is unevenly distributed in both quantity and quality to nearby lakes, rivers, and wetlands.



Generally, the quality of water found in groundwater aquifers is acceptable for most common uses like drinking or irrigation. Groundwater is less susceptible to pollution than surface water because as water enters the ground, the soil and rocks naturally filter much of the water as it passes through the ground. However, the quality of groundwater continues to change as people modify landscapes and release pollutants into the environment. The growth of industry, technology, population, and water-use demand has increased the stress on both our land and water resources. Municipal and industrial wastes and chemical fertilizers, herbicides, and pesticides have infiltrated into the soil in certain areas, percolated into local and regional aquifers, and degraded groundwater quality in many parts of the US. Other pollution problems include sewer leakage, faulty septic-tank operation, and landfill leachates. In some areas, intensive pumping of groundwater has caused groundwater tables to drop, salt water to intrude into freshwater aquifers, and land to subside (sink).

For data and information on principal aquifers in the United States: