What is Groundwater?

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Groundwater is the water beneath the surface of the ground in the zone of saturation where every pore space between rock and soil particles is saturated with water. Above the zone of saturation is an area where both air and moisture are found in the spaces between soil and rock particles. This is called the zone of aeration. Water percolates (moves downward) through this zone until it reaches the zone of saturation. The water table is the top of the saturated zone.

Where does it come from?
It's not as mysterious as it seems. The real sources of groundwater are rain and snow. Rain and melting snow percolate into the ground and saturate the pores between rock and soil particles. Geologists call this process groundwater recharge and, the places where it occurs, recharge areas.

Where does it go?
Once it reaches the zone of saturation under the ground, groundwater begins to move slowly by the force of gravity through the interconnecting pore spaces until it reaches a discharge area, where it seeps or flows out into a wetland, spring, river, or pond to become part of the surface water.

Water evaporates from surface water bodies and from land surfaces and returns to the atmosphere. Plants transpire water into the atmosphere. Water in the atmosphere condenses into rain. Some of the rain recharges the groundwater, and the cycle keeps repeating. Groundwater, in other words, is part of the hydrologic cycle. Groundwater and surface water are interconnected; groundwater becomes surface water when it discharges to surface water bodies. Most streams keep flowing during the dry summer months because groundwater discharges into them from the zone of saturation - this flow is called baseflow. Under certain conditions the flow may be reversed and the surface water may recharge the groundwater. Only a portion of the water that falls as rain or snow in Massachusetts actually recharges the groundwater. The rest runs off into surface water bodies, is taken up by plants and transpired, or evaporates.

Groundwater movement
Groundwater is always moving from higher recharge areas to lower discharge areas; however, it moves slowly. Groundwater movement is measured in feet per day or, in some cases, in feet per year. In contrast, surface water movement is measured in feet per second. The speed at which groundwater moves is determined by the types of material it must flow through and the steepness of the gradient from the recharge area to discharge area. Water moves more easily through the large pores of sand and gravel, for example, than through material that contains fine silt and clay.

The water table is at the top of the zone of saturation, but it doesn't remain at one level all the time. The rise and fall of the water table is a natural part of the groundwater system. It occurs seasonally each year. In the late winter and early spring (February, March, April), melting snow and rain percolate into the ground to raise the water table to its annual high level. During the growing season, rainwater is used by plants for transpiration or it evaporates. As a result, little or no groundwater recharge occurs during the late spring and summer months. During that time, however, groundwater continues to discharge into streams, lakes, and wetlands, so the water table drops. By fall (October and November), the water table has dropped as much as fifteen feet to its lowest annual level. The groundwater is recharged again by rain that falls after the growing season. There is no recharge in the winter when the ground is frozen, but recharge can occur during midwinter thaws. During the winter, water is stored in the snow pack. In the spring, the melting snow recharges the groundwater, raising the water table to its annual high level again.

The water table also responds to cyclical periods of drought and heavy precipitation that last for several years. For example, starting in August and over the winter of 2001/ 2002 Massachusetts has suffered drought conditions so that by April groundwater levels at the monitoring well in Acton (USGS real-time groundwater conditions: http://waterdata.usgs.gov/nwis/gw) were more than 2 feet below normal for the month.


Adapted from:
Massachusetts Audubon Society: Groundwater Information Flyer #1 "An Introduction to Groundwater and Aquifers". November/December 1983; Revised January 1993.

Massachusetts Audubon Society: Groundwater Information Flyer #2 "Groundwater and Contamination from the Watershed into the Well". January/February 1984; Reprinted May 1985.

Download a PDF version of this document: Groundwater.pdf

Or jump to the USGS publication "Ground Water and Surface Water: A Single Resource"