Groundwater is the water stored underground in formations of saturated rock, sand, gravel, and soil. Unlike surface water, groundwater does not flow in a series of lakes and rivers. Instead, precipitation that seeps into the soil continues its downward journey and eventually fills the pores of rock formations similar to the way that water fills a sponge. This cycling of water is known as the hydrologic cycle.

Rock formations that contain enough water to feed springs (or wells) are called aquifers. Two factors determine the amount of water that an aquifer can provide: porosity and permeability.

Groundwater quality generally decreases with increased depth. Water with less salinity and and mineral content is normally located closer to the earth's surface. Deeper waters, into which underground injection normally takes place, are generally of limited quality and have higher concentrations of dissolved mineral solids. These deep waters, therefore, are not usually considered to be potential sources of drinking water. Despite this generalization, some deep aquifers do serve as sources of drinking water.

Groundwater is a very important source of fresh water in the United States. According to the USEPA:.

1. Fifty percent of the nation's drinking water comes from groundwater.

2. Seventy five (75) percent of US cities "derive all or part of their water from underground sources."

3. Daily use of groundwater in the US has nearly tripled, from 34 billion gallons/day to 90 billion gallons/day since 1950!

A typical American uses an average of 100 gallons of water each and every day. With such a high, and growing, demand for fresh water, protective measures are needed to ensure that a safe and adequate water supply will be available in the future.

For more information concerning groundwater, please refer to the Groundwater Education System.

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